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Title
Author(s)
Multivalent carbohydrate-lectin interactions: the way synthetic
chemistry enables insights into nanometric recognition
Murphy, Paul V.
Publication
Date
2016-05-13
Publication
Information
R. Roy, P. V. Murphy, and H.-J. Gabius (2016) 'Multivalent
carbohydrate-lectin interactions: the way synthetic chemistry
enables insights into nanometric recognition'. Molecules
(Basel, Switzerland), 21 .
Publisher
MDPI Open Access
Link to
publisher's
version
http://dx.doi.org/10.3390/molecules21050629
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http://dx.doi.org/10.3390/molecules21050629
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molecules
Review
Multivalent Carbohydrate-Lectin Interactions:
How Synthetic Chemistry Enables Insights into
Nanometric Recognition
René Roy 1, *, Paul V. Murphy 2 and Hans-Joachim Gabius 3
1
2
3
*
Pharmaqam and Nanoqam, Department of Chemistry, University du Québec à Montréal, P. O. Box 8888,
Succ. Centre-Ville, Montréal, QC H3C 3P8, Canada
School of Chemistry, National University of Ireland Galway, University Road, Galway, Ireland;
[email protected]
Institute of Physiological Chemistry, Faculty of Veterinary Medicine, Ludwig-Maximilians-University Munich,
Veterinärstr. 13, München 80539, Germany; [email protected]
Correspondence: [email protected]; Tel.: +1-514-987-3000 (ext. 2546)
Academic Editor: Trinidad Velasco-Torrijos
Received: 7 April 2016; Accepted: 10 May 2016; Published: 13 May 2016
Abstract: Glycan recognition by sugar receptors (lectins) is intimately involved in many aspects of cell
physiology. However, the factors explaining the exquisite selectivity of their functional pairing are not
yet fully understood. Studies toward this aim will also help appraise the potential for lectin-directed
drug design. With the network of adhesion/growth-regulatory galectins as therapeutic targets, the
strategy to recruit synthetic chemistry to systematically elucidate structure-activity relationships
is outlined, from monovalent compounds to glyco-clusters and glycodendrimers to biomimetic
surfaces. The versatility of the synthetic procedures enables to take examining structural and spatial
parameters, alone and in combination, to its limits, for example with the aim to produce inhibitors
for distinct galectin(s) that exhibit minimal reactivity to other members of this group. Shaping spatial
architectures similar to glycoconjugate aggregates, microdomains or vesicles provides attractive tools
to disclose the often still hidden significance of nanometric aspects of the different modes of lectin
design (sequence divergence at the lectin site, differences of spatial type of lectin-site presentation).
Of note, testing the effectors alone or in combination simulating (patho)physiological conditions,
is sure to bring about new insights into the cooperation between lectins and the regulation of
their activity.
Keywords: agglutinin; galectin; glycocluster; glycodendrimer; glycophane; glycoprotein; liposomes;
oligosaccharides; sugar code
1. The Third Alphabet of Life
Synergy, by teaming up chemistry with biosciences, can be expected to arise when the topic on
which to apply complementarity of the disciplines is clearly defined. In principle, the chemical platform
of life includes molecular alphabets, whose ‘letters’ are a toolbox to form ‘messages’ (or ‘words’) in the
form of oligomers and polymers. “To an observer trying to obtain a bird’s eye view of the present state
of biochemistry, or what is sometimes referred to as molecular biology, life may until very recently
have seemed to depend on only two classes of compounds: nucleic acids and proteins” [1]. Their broad
natural occurrence is shared by a third group, i.e., glycans. Present in Nature as polysaccharides such
as chitin, or as part of cellular glycoconjugates, i.e., glycoproteins and glycolipids [2–13], their ubiquity
and abundance have attracted interest to characterize rules of structural design, starting with the
composition. This systematic work on material from pro- and eukaryotes step by step led to compiling
the list of all individual building blocks, which form cellular glycocompounds. With the analogy
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Molecules 2016, 21, 629
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Molecules 2016, 21, 629
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eukaryotes step by step led to compiling the list of all individual building blocks, which form
cellular glycocompounds. With the analogy to a molecular language in mind, this list constitutes the
to a molecular
language
in mind,
this list constitutes
the of
third
alphabet and
of life,
which
complements
third
alphabet of
life, which
complements
the alphabets
nucleotides
amino
acids
(Figure 1).
the
alphabets
of
nucleotides
and
amino
acids
(Figure
1).
As
is
common
for
amino
acids
in proteins,
As is common for amino acids in proteins, substitutions such as phosphorylation or sulfation
also
substitutions
such as
or This
sulfation
also
the range
letters for glycans.
This use
broaden
the range
ofphosphorylation
letters for glycans.
use of
thebroaden
same principle
ofofpost-synthetic
modifications
of the same principle
of post-synthetic
underscores
equivalence
of these
alphabets
underscores
the equivalence
of these modifications
alphabets with
respect to the
their
involvement
in the
flow of
with
respect
to
their
involvement
in
the
flow
of
information
from
genes
to
cellular
activities
[14,15].
information from genes to cellular activities [14,15].
HO
OH
O
HO
HO
OH
HO
D-Glucose (Glc)
OH
O
OH
D-Galactose (Gal)
OH
OH
OH
HO2C
HO2C
O
b
a OH
L-Iduronic acid (IdoA)
CO2H
OH
O
AcHN
-L-Rhamnose (Rha)
OH
HO HO
HO
OH
OH
D-Xylose (Xyl)
OH
HO
OH
O
HO
HO
OH
OH
OH
-L-Fucose (Fuc)
D-N-Acetylgalactosamine (GalNAc)
OH
O
O
Me
OH
AcHN
HO
D-Glucuronic acid (GlcA)
OH
O
HO
OH
OH
Me
HO
OH
D-Mannose (Man)
HO
D-N-Acetylglucosamine (GlcNAc)
HO
HO
OH
O
HO
AcHN
CO2H
O
HO
HO
HO
HO
OH
HO
HO
OH
O
OH
HO
5-N-Acetylneuraminic acid (Neu5Ac)
O
OH
HO
O
OH
OH
OH
L-Arabinose (Ara)
HO H
HO
HO
OH
O
CO2H
OH
2-Keto-3-deoxy-Dmanno-octulosonic acid (Kdo)
Figure
1. Illustration
alphabet of
of the
the sugar
sugar language.
language. Structural
and
Figure 1.
Illustration of
of the
the alphabet
Structural representation,
representation, name
name and
symbol
as
well
as
the
set
of
known
acceptor
positions
(arrows)
in
glycoconjugates
are
given
for
each
symbol as well as the set of known acceptor positions (arrows) in glycoconjugates are given for each
letter.
Four sugars
sugars have
have LL-configuration:
-configuration: fucose
fucose (6-deoxy(6-deoxy-LL-galactose),
(6-deoxy-LL-mannose)
letter. Four
-galactose), rhamnose
rhamnose (6-deoxy-mannose)
L-iduronic acid (IdoA) results from
and
arabinose
are
introduced
during
chain
elongation,
whereas
and arabinose are introduced during chain elongation, whereas L-iduronic acid (IdoA) results from
1
conformation
of IdoA (a)ofisIdoA
in equilibrium
post-synthetic epimerization
of glucuronic
acidacid
at C-5.
1 C conformation
post-synthetic
epimerization
of glucuronic
at The
C-5.C4The
(a) is in
4
2S0 form (b) in glycosaminoglycan chains, where this uronic acid can be 2-sulfated. All other
with
the
2
equilibrium with the S0 form (b) in glycosaminoglycan chains, where this uronic acid can be 2-sulfated.
letters
areletters
D-sugars.
Neu5Ac,
of theone
more
50 sialic
often
terminates
sugar chains
in
All
other
are D
-sugars. one
Neu5Ac,
of than
the more
thanacids,
50 sialic
acids,
often terminates
sugar
animal
glycoconjugates.
Kdo
is
a
constituent
of
lipopolysaccharides
in
the
cell
walls
of
Gram-negative
chains in animal glycoconjugates. Kdo is a constituent of lipopolysaccharides in the cell walls of
bacteria
and is bacteria
also found
polysaccharides
of green algae
higher
Foreign
to
Gram-negative
andinis cell
alsowall
found
in cell wall polysaccharides
of and
green
algae plants.
and higher
plants.
D
-galactose
mammalian
glycobiochemistry,
microbial
polysaccharides
contain
the
furanose
ring
form
of
Foreign to mammalian glycobiochemistry, microbial polysaccharides contain the furanose ring form of
and
also D/and
L-arabinose,
then indicated
an italic
derived
from the
heterocyclus
furan.furan.
The
D-galactose
also D/L-arabinose,
then by
indicated
by “f”
an italic
“f” derived
from
the heterocyclus
α-anomer
is prevalent
for the
arabinose,
e.g., e.g.,
in mycobacterial
cell wall
arabinogalactan
and
The
α-anomer
is prevalent
forpentose
the pentose
arabinose,
in mycobacterial
cell wall
arabinogalactan
lipoarabinomannan.
β1-5/6-Linked
galactofuranoside
is present
in in
the
and lipoarabinomannan.
β1-5/6-Linked
galactofuranoside
is present
thearabinogalactan
arabinogalactanand
and the
the
α1-3/6
linkage
in
lipopolysaccharides
(adapted
from
[15]).
α1-3/6 linkage in lipopolysaccharides (adapted from [15]).
Drawing on textbook knowledge, the consideration of availability of several hydroxyl groups
for building the glycosidic bond lets the special talent of carbohydrates to encode bioinformation
become obvious. In contrast to the fixed connecting points of nucleotides and amino acids, two
Molecules 2016, 21, 629
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monosaccharides can be covalently linked to yield a groups of isomers: the anomeric position and
linkage points are the sources of variability. What initially appeared as a deterrent to work with glycans,
that is the enormous structural variety unsurpassed among biopolymers that so fittingly is expressed
in the term ‘complex carbohydrates’, was then realized to be the basis for enabling high-density coding.
Ironically, what at first impeded smooth progress turned out to be a means to put signals for different
aspects of cell sociology into a minimum of space, an amazing functional dimension of glycans [14,15].
Due to the growing realization of this potential developing tools for chemical analysis of glycans on all
structural levels [16] is crucial to delineate structure-activity relationships. That “in many cases the real
role of biological glycoconjugate glycans is still unknown and, in many cases, we are unable to answer
the question: why are proteins and lipids glycosylated?” [17] and that “on the whole, the role of many
glycans remains obscure and the subject of speculation or controversy” [17] is no longer valid. In fact,
the sequence of glycans, as in proteins or nucleic acids, gives shape to these biomolecules, seemingly
minor alterations acting like switches between conformations [18–22]. Of note, the often encountered
limitations to intramolecular flexibility of biorelevant oligosaccharides, combined with their enormous
potential to engage in intermolecular recognition via hydrogen bonding, C-H/π-interaction and apolar
complementarity, make them well-suited as binding partners. They are like molecular keys fitting into
appropriate locks, and such receptors are present in Nature, as previously outlined in detail in this
journal for lectins [23,24].
2. Lectins: Translators of Sugar-Encoded Information
Given the realized potential of carbohydrates to serve as a chemical platform for generating
a large array of molecular signals, which defines the glycome, a functional implication could be
expected if emergence of sugar receptor(s) is not a singular event in evolution. Strongly supporting
the concept of significance of carbohydrate-protein binding, several families of proteins with this
ability have developed, among them figuring prominently the lectins. They are separated from
enzymes treating the bound glycan as substrate, immunoglobulins and transport proteins for free
mono- or oligosaccharides [23–25]. In the case of animals and man, more than a dozen types of
protein folds are capable to form sites to accommodate such ligands on the basis of mutual recognition
(for a gallery of these lectins presenting folds and a snapshot of the molecular rendez-vous with
a ligand, please see [26]), a sweet complementarity. Structurally, the architecture of the contact site
spans the range from shallow groves to deep pockets, in certain cases recruiting the assistance of
Ca2+ for structural organization or coordination bonds with hydroxyl groups of the sugar [26–30].
Physiologically, the architecture then determines the reactivity to glycans, generally with no or very
limited promiscuity. From the perspective of the glycan, the same sugar epitope can become the target
of several lectin types. As a means of implementing flexibility and responsiveness, these cellular signals
can undergo a recoding without requiring neosynthesis. Enzymatic remodeling in situ can drastically
alter a glycan’s reactivity its binding profile, opening the way to dynamic up- or down regulation of
binding capacity. A classical example is the conversion of the hexasaccharide of the disialo-ganglioside
GD1a to that of monosialo GM1 by a cell surface ganglioside sialidase (Neu3). Removal of one sialic
acid moiety (for its structure, please see Figure 1) from ganglioside GD1a generates a functional marker.
It signals differentiation of neuroblastoma cells in vitro and acquired responsiveness of activated
effector T cells to a control by regulatory T cells, in both cases sensed exerted by an endogenous
lectin [31,32]. The formation of a complex between the produced ligand, i.e., the pentasaccharide of
ganglioside GM1, and the lectin, i.e., the homodimeric galectin-1, is the molecular trigger for a signaling
cascade, which results in the final response (growth/activity inhibition, setting limits to tumor cell
proliferation and T cell-mediated activity causing auto-immune diseases). This connection between
the glycan and the cell sociology justifies to call the human lectin a translator of the sugar (ganglioside
GM1)-encoded message.
What is in general so intriguing about the recognition process with endogenous receptors is the
exquisite selectivity of a tissue lectin (receptor) for distinct cellular glycoconjugates (in terms of the
Molecules 2016, 21, 629
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scaffold and its glycosylation). As it turned out, often only few glycoproteins or glycolipids from
the complexity on cells are binders, and lectin reactivity is temporally and spatially tightly regulated.
Modulations of glycoconjugate synthesis, routing and/or degradation and of the structure of the
presented glycan(s) open ways to make cells specifically prepared for carbohydrate-protein recognition.
To give a further biomedically relevant example, the tumor suppressor p16INK4a is the master regulator
for a glycan-based effector route to drive pancreatic carcinoma cells into programmed cell death:
the underlying caspase-9 activation is achieved by clustering of α5 β1 -integrin and galectin-1 on the
cell surface, a process favored by their concerted upregulation and the increase of galectin-1-suited
integrin glycosylation through reducing α2,6-sialylation of its N-glycans [33,34]. That availability of
an antagonist of galectin-1 in this context, i.e., galectin-3, is downregulated [35] lays open intricacies of
fine-tuning this effector pathway. In addition to the direct contact possible and the complementarity
between the cognate glycan determinant and the lectin, factors of spatial presentation, too, will most
likely matter. Multivalency of glycans, achieved by branching or tight arrangement in a microdomain,
also artificially established by a lectin and synthetic ligands [36], facilitates fractional high-affinity
binding in the nM (and even below) range for galectins [37]. To give research to unravel the molecular
origin of the selectivity of matchmaking direction, affinity regulation has been formally assigned to
work on six levels [30,38]. After the target identification, the endogenous lectin in this case acts as
a cross-linker of a glycoprotein, thus turning attention to the modes of structural design of tissue lectins
are relevant for the outcome. As glycan structures vary in terms of branching and repeats, per glycan
and per glycoconjugate, various modes are realized to present a carbohydrate recognition domain
(CRD) (Figure 2). Looking at the mentioned galectin-1 (bottom, left), it is a homodimer of two identical
CRDs, ideal for cross-linking its counter receptor(s) to elicit the intracellular signaling cascade.
Looking through the different sections of Figure 2, and through its legend presenting further
details on the respective lectins, it becomes clear that the topology of CRD presentation has gained
a large degree of diversity, and there is more. Detailed analyses within lectin families based on database
mining have underscored that the course of evolution has led to complexity on several levels: gene
diversification in terms of variations of the number of genes in each family by duplications/losses
and of the individual sequences, even among related species [39–44]. In overview, covalent and
non-covalent modes of linking CRDs as well as the connection of CRDs with other types of modules
are frequent among lectins, signaling a fundamental functionality to be disclosed by thorough
structure-activity investigations. Here, using bioinspired synthetic ligands as sensors or glycoclusters
as molecular rulers offers a promising perspective. To depict principles of this line of research we here
describe instructive examples selected from work on a class of adhesion/growth-regulatory lectins.
How carbohydrate chemistry can advance our level of understanding of aspects of how selectivity is
achieved is thus examined for biomedically relevant effectors, two already mentioned above. These
proteins are referred to as galectins (please see Figure 2, left part and Figure 3 for their modes of
structural design in vertebrates).
Molecules 2016, 21, 629
Molecules 2016, 21, 629
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5 of 35
Figure 2. Illustration of the strategic ways that carbohydrate recognition domains (CRDs) in animal
Figure 2. Illustration of the strategic ways that carbohydrate recognition domains (CRDs) in
lectins are positioned to reach optimal ligand selection (for example, to separate self from non-self
animal lectins are positioned to reach optimal ligand selection (for example, to separate self from
glycan profiles in innate immunity) and topological complementarity. From left to right, the CRD
non-self
glycan profiles in innate immunity) and topological complementarity. From left to right,
display in the three subtypes within the galectin family (chimera-type (a), proto-type (b) and
the CRD
display in the(c)
three
subtypesbinding
withintothe
family
(chimera-type
tandem-repeat-type
arrangements
thegalectin
saccharide
portion
of a ganglioside(a);
or aproto-type
branched (b)
and tandem-repeat-type
(c)
arrangements
binding
to
the
saccharide
portion
of
a
ganglioside
complex-type N-glycan without or with terminal α2,3-sialylation; please see also Figure 3), the or
a branched
complex-type
N-glycan
without or with
terminal
α2,3-sialylation;
please
see or
also
Figure 3),
presentation
of CRDs (C-type
or fibrinogen-like
domain)
in serum
and surfactant
collectins
ficolins
(d) connectedofto
their (C-type
collagenous
stalks and the domain)
non-covalent
association
of binding collectins
sites in or
the presentation
CRDs
or fibrinogen-like
in serum
and surfactant
transmembrane
C-type
lectins
by α-helical
coiled-coil
stalks
(e) (for example
asialoglycoprotein
ficolins
(d) connected
to their
collagenous
stalks
and the
non-covalent
association
of bindingand
sites in
Kupffer cell receptors,
the scavenger
receptor
C-type lectin,
CD23,
DC-SIGN
or DC-SIGNR)
are given. and
transmembrane
C-type lectins
by α-helical
coiled-coil
stalks
(e) (for
example
asialoglycoprotein
Similar
tandem-repeat-type
galectins,
the C-type
family
of lectins
has a branch
of membersare
with
Kupffer
cell to
receptors,
the scavenger
receptor
C-type
lectin,
CD23,also
DC-SIGN
or DC-SIGNR)
given.
this design, i.e., immulectins-1, -2 and -3. Next, the tandem-repeat display in the mannose-specific
Similar to tandem-repeat-type galectins, the C-type family of lectins also has a branch of members with
macrophage receptor (f) (also found on dendritic cells, hepatic endothelial cells, kidney mesangial
this design, i.e., immulectins-1, -2 and -3. Next, the tandem-repeat display in the mannose-specific
cells, retinal pigment epithelial cells and tracheal smooth muscle cells) and the related C-type lectin
macrophage receptor (f) (also found on dendritic cells, hepatic endothelial cells, kidney mesangial cells,
Endo180 with eight domains as well as in the cation-independent P-type lectin with 15 domains is
retinalpresented.
pigment Capacity
epithelialforcells
and
tracheal
smoothto
muscle
cells)
and the
related C-type
lectin Endo180
sugar
binding
is confined
only few
domains
as depicted.
The occurrence
of
with eight
domains
as
well
as
in
the
cation-independent
P-type
lectin
with
15
domains
is presented.
lectin activity for GalNAc-4-SO4-bearing pituitary glycoprotein hormones in the cysteine-rich
Capacity
for sugar
binding
is confined
to only
fewwith
domains
as depicted.
occurrence
of section
lectin activity
3 domainThe
in the
N-terminal
of
domain,
a member
of the
β-trefoil fold
family
one (QxW)
for GalNAc-4-SO
pituitary
in the
cysteine-rich
domain, a member
the macrophage
mannose
receptorglycoprotein
(amino acids hormones
8–128), which
is linked
via a fibronectin-type-II4 -bearing
modulewith
to the
tandem-repeat
section,inisthe
alsoN-terminal
included into
the schematic
drawing
of therepeat-containing
β-trefoil fold family
one
(QxW)3 domain
section
of the macrophage
for these
lectins(amino
with more
than8–128),
one type
of CRD
protein
Moving further to the right side;
mannose
receptor
acids
which
is per
linked
viachain.
a fibronectin-type-II-repeat-containing
(g) to
thethe
association
of a distal
CRDisinalso
selectins
(attached
to an
epidermal-growth-factor
(EGF)-like
module
tandem-repeat
section,
included
into the
schematic
drawing for these
lectins with
domain and two to nine complement-binding consensus repeats) or in the siglec subfamily of I-type
more than one type of CRD per protein chain. Moving further to the right side; (g) the association
lectins using 1–16 C2-set immunoglobulin-like units as spacer equivalents to let the CRD reach out to
of a distal CRD in selectins (attached to an epidermal-growth-factor (EGF)-like domain and two to
contact ligands and to modulate capacity to serve in cis- or trans-interactions on the cell surface is
nine complement-binding consensus repeats) or in the siglec subfamily of I-type lectins using 1–16
shown. The force-dependent alterations of the topological arrangement of the two distal domains in
C2-setselectins
immunoglobulin-like
units as spacer equivalents to let the CRD reach out to contact ligands
accounts for catch bonds of selectins, a canonical immunoreceptor tyrosine-based inhibitory
and to
modulate
capacity
to
in cisor trans-interactions
on thepresent
cell surface
is shown. The
motif (ITIM) together with aserve
putative
tyrosine-based
motif is frequently
in the intracellular
force-dependent
alterations
the topological
arrangement of the
twoestablish
distal domains
in selectins
portion of siglecs.
C2-set of
domains
linked to fibronectin-type-III
repeats
the extracellular
accounts
forofcatch
bondslectins
of selectins,
a canonical
immunoreceptor
tyrosine-based
section
the I-type
L1 and neural
cell adhesion
molecule (NCAM)
(h). In theinhibitory
matrix, themotif
modular
proteoglycans
(i) (hyalectans/lecticans:
aggrecan,
brevican,present
neurocan
interact
(ITIM)
together
with a putative
tyrosine-based motif
is frequently
in and
the versican)
intracellular
portion
(i) withC2-set
hyaluronan
(andlinked
also link
via the link-protein-type
modulesthe
of the
N-terminal G1
of siglecs.
domains
toprotein)
fibronectin-type-III
repeats establish
extracellular
section
an L1
Ig-like
(ii) with
receptors
binding(NCAM)
to the glycosaminoglycan
chains
the
of thedomain
I-type (and
lectins
andmodule);
neural cell
adhesion
molecule
(h). In the matrix,
theinmodular
central region
and (iii) with carbohydrates
or proteins
(fibulins-1
andand
-2 and
tenascin-R)
via the
proteoglycans
(i) (hyalectans/lecticans:
aggrecan,
brevican,
neurocan
versican)
interact
(i) with
C-type lectin-like domain flanked by EGF-like and complement-binding consensus repeat modules
hyaluronan (and also link protein) via the link-protein-type modules of the N-terminal G1 domain (and
(kindly provided by H. Kaltner) (from [38], with permission).
an Ig-like module); (ii) with receptors binding to the glycosaminoglycan chains in the central region
and (iii) with carbohydrates or proteins (fibulins-1 and -2 and tenascin-R) via the C-type lectin-like
domain flanked by EGF-like and complement-binding consensus repeat modules (kindly provided by
H. Kaltner) (from [38], with permission).
Molecules 2016, 21, 629
Molecules 2016, 21, 629
6 of 36
6 of 35
Figure
Figure 3.
3. The
The three
three basic
basic types
types of
of modular
modular design
design of galectins, as given in the context of other lectins in
Figure 2.
2. Proto-type
Proto-type proteins
proteins(e.g.,
(e.g.,galectins-1,
galectins-1,-2-2and
and
non-covalently
associated
homodimers,
-7)-7)
areare
non-covalently
associated
homodimers,
the
the
vertebrate
tandem-repeat-type
galectins
are formed
bydifferent
two different
CRDs connected
by a(length
linker
vertebrate
tandem-repeat-type
galectins
are formed
by two
CRDs connected
by a linker
can varycan
by vary
alternative
splicing and
between
the chimera-type
galectin-3 has
an N-terminal
(length
by alternative
splicing
and species)
betweenand
species)
and the chimera-type
galectin-3
has an
tail composed
a peptide
site(s)with
for site(s)
Ser phosphorylation
and Gly/Pro-rich
sequence
repeats
N-terminal
tail of
composed
ofwith
a peptide
for Ser phosphorylation
and Gly/Pro-rich
sequence
enablingenabling
self-association
in the presence
of multivalent
ligands.ligands.
repeats
self-association
in the presence
of multivalent
3. Galectins:
Galectins:Definition
Definitionand
andStructures
Structures
To qualify
qualifytotobecome
become
a canonical
member
of this
family
a protein
must satisfy
the following
a canonical
member
of this
family
a protein
must satisfy
the following
criteria:
criteria:
(i) to have a CRD with a β-sandwich fold, first described in this field for the leguminous
(i) to have a CRD with a β-sandwich fold, first described in this field for the leguminous lectin
lectin concanavalin A and illustrated in the Gallery of Lectins [27], is a compact structural
concanavalin A and illustrated in the Gallery of Lectins [27], is a compact structural platform
platform consisting—in the case of human galectin-1—of two antiparallel β-sheets of five to six
consisting—in the case of human galectin-1—of two antiparallel β-sheets of five to six strands,
strands, respectively,
respectively,
(ii) to bind compounds from the group of β-galactosides and
(ii) to bind compounds from the group of β-galactosides and
(iii) to share a sequence signature (mostly the amino acids in contact with the ligand, especially
(iii) to share a sequence signature (mostly the amino acids in contact with the ligand, especially a
a central Trp for C-H/π-interaction with the B-face of galactose) [26,43,45–47].
central Trp for C-H/π-interaction with the B-face of galactose) [26,43,45–47].
the CRD
CRD is commonly
presented in
in three
three topological
topological modes
modes given
given in
in Figure
Figure 3,
3,
In vertebrates
vertebrates the
commonly presented
associated
with
a
tail
consisting
mostly
of
non-triple
helical
collagen-like
repeats
suited
for
aggregation
associated
homodimer (proto
(proto type)
type) and as a covalently linked
(chimera type), as a non-covalently associated homodimer
heterodimer (tandem-repeat
(tandem-repeat type).
type). Before
Before synthetic
synthetic work
work to
to prepare
prepare binders
binders for
for galectins
galectins is
is outlined,
outlined,
heterodimer
an overview
overview of
of diversity
diversityof
ofgalectin
galectinrepresentation
representationamong
amongspecies
speciesis isgiven.
given.
Computational
mining
Computational mining
of
of databases,
together
with
experimental
validation
of active
genes,
accounts
for phylogenetic
the phylogenetic
databases,
together
with
experimental
validation
of active
genes,
accounts
for the
tree
tree that
summarizes
the number
of galectins
in selected
species
(Figure
Reflecting
dynamics
that
summarizes
the number
of galectins
in selected
species
(Figure
4). 4).
Reflecting
thethe
dynamics
of
of
the
evolutionary
process,
the
galectin
display
is
not
constant
between
species,
an
important
fact
the evolutionary process, the galectin display is not constant between species, an important
precluding simple
and
calling
for thorough
analysis
of eachofindividual
protein. protein.
Equally
precluding
simpleextrapolations
extrapolations
and
calling
for thorough
analysis
each individual
important,
galectins
likely
form
a
network
of
members
of
each
group
(to
give
a
clinically
relevant
Equally important, galectins likely form a network of members of each group (to give a clinically
example,example,
colon cancer
cells
express
tested
galectins,
responsiveresponsive
to differentiation
and providing
relevant
colon
cancer
cellsmost
express
most
tested galectins,
to differentiation
and
prognostic prognostic
informationinformation
[48,49], posing
the challenge
define properties
the individual
proteins
and
providing
[48,49],
posing thetochallenge
to defineofproperties
of the
individual
cooperation/antagonism
in
situ.
proteins and cooperation/antagonism in situ.
is is
invaluable
due
to to
itsits
ability
to
To start
start characterizing
characterizingthe
theligand
ligandspecificity,
specificity,synthetic
syntheticchemistry
chemistry
invaluable
due
ability
make
lead
compounds
available
forfor
affinity
testing
(for(for
a compilation
of experimental
approaches
to
to
make
lead
compounds
available
affinity
testing
a compilation
of experimental
approaches
characterize
lectin
activity,
please
to
characterize
lectin
activity,
pleasesee
seeTable
Table1 1inin[26]).
[26]).Due
Duetotothe
theadvantage
advantageof
of requiring
requiring only small
microarrays have become instrumental for
amounts of lectin and ligand on a multi-assay scale, glycan microarrays
comparative analysis,
setting
the
stage
for
the
structural
follow-up
studies
(for a(for
flowascheme,
please
analysis, setting the stage for the structural follow-up
studies
flow scheme,
see Figure
[50–54].
and This
other and
glycan-based
techniques delineated
grading of
please
see 5)
Figure
5) This
[50–54].
other glycan-based
techniques the
delineated
thebioactivity
grading of
diverse glycans,
teaching
the lesson
thatthe
various
changes,
e.g., changes,
introduction
sialylation of
or
bioactivity
of diverse
glycans,
teaching
lessonstructural
that various
structural
e.g., of
introduction
sulfation
(as
mentioned
above)
and
extension
to
oligosaccharides
with
branching
and
sequence
repeats,
sialylation or sulfation (as mentioned above) and extension to oligosaccharides with branching and
can matterrepeats,
markedly
distinct
members
the family
(for examples,
please(for
see [55–65]).
lactose
sequence
canfor
matter
markedly
forofdistinct
members
of the family
examples,With
please
see
as reference,
enhancements
were found
for the saccharides
given
in Figure
5, for
[55–65]).
Withsignificant
lactose as affinity
reference,
significant affinity
enhancements
were found
for the
saccharides
example
with the
synthetic
p-nitrophenyl
lactoside,
a classical lactoside,
test case studied
up test
to now,
given
in Figure
5, for
example
with the synthetic
p-nitrophenyl
a classical
caserecently
studied
15
by NMR
spectroscopy
N-labeled galectin-1
[66]. Thesegalectin-1
structures,
at the
same
time, establish
up
to now,
recently byusing
NMR spectroscopy
using 15N-labeled
[66].
These
structures,
at the
a
platform
to
proceed
to
address
the
issue
of
selection.
One
preferred
galectin
ligand,
as
depicted
same time, establish a platform to proceed to address the issue of selection. One preferred galectin
in Figure
(bottom),
a repeat
of the disaccharide
Questions to[67].
be answered
and
ligand,
as 6depicted
in is
Figure
6 (bottom),
is a repeat of[67].
the disaccharide
Questionsare
to whether
be answered
are whether and to what extent homologous proteins have different binding properties and
whether a target-specific blocking might be accomplished.
Molecules 2016, 21, 629
7 of 36
to what extent homologous proteins have different binding properties and whether a target-specific
blocking might be accomplished.
Molecules 2016, 21, 629
Molecules 2016, 21, 629
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7 of 35
Figure 4. Canonical galectins (according to the classification given in Figure 2) in model organisms
Figure
4. Canonical
galectins (accordingtotothe
the classification given
given in Figure
2)2)ininmodel
organisms
Figure
Canonical
galectins
Figure(numerical
model
organisms on
on4.the
level of gene
(Roman(according
number), transcriptclassification
(Arabic number) andinprotein
information).
on theoflevel
of (Roman
gene (Roman
number),
transcript(Arabic
(Arabic number)
and
protein
(numerical
information).
the level
gene
number),
transcript
number)
and
protein
(numerical
information).
For details, please see [42,43].
For details,
please
[42,43].
For details,
please
seesee
[42,43].
Figure 5. Sequence of steps to identify high-affinity carbohydrate ligands for galectin binding. Galectin
Figure
5. Sequence
of
steps
toidentify
identifyhigh-affinity
high-affinity carbohydrate ligands
for galectin
binding.
Galectin
Figure
5. Sequence
steps
topotent
binding.
interaction
withofthe
most
binders based oncarbohydrate
this screening ligands
can thenfor
be galectin
studied further
onGalectin
the
interaction
with
the
most
potent
binders
based
on
this
screening
can
then
be
studied
further
on theon the
interaction
with
the
most
potent
binders
based
on
this
screening
can
then
be
studied
further
structural level.
structural level.
structural
level.
Molecules 2016, 21, 629
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Molecules 2016, 21, 629
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Figure 6. Representative structures of galectin ligands.
Figure 6. Representative structures of galectin ligands.
Table 1. Amino acids of the CRDs of six galectins directly interacting with lactose residues are shown
Table 1. Amino acids of the CRDs of six galectins directly interacting with lactose residues are shown in
in red, and amino acids that interact with lactose via water-mediated hydrogen bonds are shown in
red, and amino acids that interact with lactose via water-mediated hydrogen bonds are shown in blue.
blue. Comparison with galectin-1 (PDB-1GZW), galectin-2 (PDB-1HLC), galectin-3 (PDB-1KJL),
Comparison with galectin-1 (PDB-1GZW), galectin-2 (PDB-1HLC), galectin-3 (PDB-1KJL), galectin-4
galectin-4 (PDB-1X50), galectin-7 (PDB-4GAL), and galectin-8 (PDB 3AP9). Amino acids letter codes
(PDB-1X50), galectin-7 (PDB-4GAL), and galectin-8 (PDB 3AP9). Amino acids letter codes are indicated
are indicated in parentheses for Gal-3 [26].
in parentheses for Gal-3 [26].
CRD-Subsite
CRD-Subsite
A
B
A
C
B
C
D
D
E
EF
F
G
G
H
H
I
I
JJ
K
K
L
L
M
N
O
Galectin-1
Galectin-1
Ser29
Val31
Ser29
Asn33
Val31
Asn33
Gly35
Gly35
His44
His44
Asn46
Asn46
Arg48
Arg48
His52
His52
Asp54
Asp54
Asn61
Asn61
Trp68
Trp68
Glu71
Glu71
Arg73
Arg73
--
Galectin-2
Galectin-2
Gly30
Val32
Gly30
Asn34
Val32
Asn34
Gly36
Gly36
His45
His45
Asn47
Asn47
Arg49
Arg49
Gln52
Gln52
Asn58
Asn58
Trp65
Trp65
Glu68
Glu68
Arg70
Arg70
Arg120
Arg120
Galectin-3
Galectin-3
Arg144
(R)
Ala146
Arg144 (A)
(R)
Asp148
(D)
Ala146 (A)
Asp148
(D)
Gln150 (Q)
Gln150
- (Q)
- (H)
His158
His158
Asn160 (H)
(N)
Asn160 (N)
Arg162 (R)
Arg162 (R)
Glu165
Glu165 (E)
(E)
Asn174
Asn174 (N)
(N)
Trp181
(W)
Trp181 (W)
Glu184 (E)
Arg186 (R)
Ser237 (S)
Galectin-4C
Galectin-4C
Ser220
Ala222
Ser220
Asn224
Ala222
Asn224
Lys226
Lys226
His236
His236
Asn238
Asn238
Arg240
Arg240
-Asn249
Asn249
Trp256
Trp256
Glu259
Glu259
Lys261
Lys261
Gln313
Gln313
Galectin-7
Galectin-7
Arg31
His33
Arg31
Asn35
His33
Asn35
Leu37
Leu37
His49
His49
Asn51
Asn51
Arg53
Arg53
-Asn62
Asn62
Trp69
Trp69
Glu72
Glu72
Arg74
Arg74
Gly124
Gly124
Galectin-8N
Galectin-8N
Arg45
Gln47
Arg45
Asp49
Gln47
Asp49
Gln51
Gln51
Arg59
Arg59
His65
His65
Asn67
Asn67
Arg69
Arg69
-Asn79
Asn79
Trp86
Trp86
Glu89
Glu89
Ile91
Ile91
Tyr141
Tyr141
In
In that
that case,
case, aa high-affinity
high-affinity and
and specific
specific carbohydrate
carbohydrate derivative
derivative would
would need
need to
to be
be identified
identified for
for
each
galectin.
Because
the
CRDs
of
the
galectins
are
rather
conserved,
this
represents
a formidable
formidable
each galectin. Because the CRDs of the galectins are rather conserved, this represents a
synthetic
example, distinct
synthetic challenge.
challenge. However,
However, if
if one
one takes
takes the
the N-terminal
N-terminal CRD
CRD of
of galectin-8
galectin-8 as
as an
an example,
distinct
1 -sulfated
preferences
have
indeed
developed
[68–74].
Loaded
with
either
LNF-III
(Figure
7
left)
or 33′-sulfated
preferences have indeed developed [68–74]. Loaded with either LNF-III (Figure 7 left) or
lactose
lactose (Figure
(Figure 77 right),
right), the
the structures
structures of
of the
the complexes
complexes and
and the
the intimate
intimate interactions
interactions(shown
(shownin
inFigure
Figure 8)
8)
reveal
that
it
becomes
conceivable
that
achieving
selectivity
gains
is
not
an
insurmountable
task.
reveal that it becomes conceivable that achieving selectivity gains is not an insurmountable task. This
This
example
clearly
demonstrates
suitably
functionalized
carbohydratederivatives
derivativescan
can even
even take
take
example
clearly
demonstrates
thatthat
suitably
functionalized
carbohydrate
the
can
offer
a
the place
place of
of rather
rather complex
complex glycans.
glycans.Indeed,
Indeed,already
alreadyaarationally
rationallyderivatized
derivatizeddisaccharide
disaccharide
can
offer
substantial
selectivity
gain,
despite
the
overall
rather
conserved
contact
pattern
of
galectins
for
a substantial selectivity gain, despite the overall rather conserved contact pattern of galectins for
lactose
1).
lactose (Table
(Table 1).
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Figure 7.
7. Comparison
Comparison of
of structures
structures of
of LNF-III
LNF-III (left)
(left) and
and 33′-sulfated
lactose derivatives
derivatives (right)
(right) that
that
1 -sulfated lactose
Figure
Figure
7. equally
Comparison
oftostructures
of LNF-III (left)
and 3′-sulfated
lactosemodifications
derivatives (right)
thatsugar
bind
almost
well
human
galectin-8N.
Suggested
additional
of
the
bind almost equally well to human galectin-8N. Suggested additional modifications of the sugar head
almost
equallywith
well their
to human
galectin-8N.
Suggested
modifications
ofglycans
the sugar
headbind
groups
together
substituents,
which
might additional
increase
the
toward
groups
together
with their
substituents,
which might
increase
the affinity
ofaffinity
glycansoftoward
galectins,
head
groups
together
with
their
substituents,
which
might
increase
the
affinity
of
glycans
toward
galectins,
while
limiting
synthetic
efforts
towards
this
achievement,
are
highlighted.
whilegalectins,
limitingwhile
synthetic
efforts towards this achievement, are highlighted.
limiting synthetic efforts towards this achievement, are highlighted.
Figure 8. Human galectin-8N (Connolly surfaces and binding sites) loaded with lacto-N-fucopentaose-III
Figure
8. Human galectin-8N (Connolly surfaces and binding sites) loaded with lacto-N-fucopentaose-III
(top) and 3′-sulfated lactose (bottom) (PDB 3AP9).
(top) and 31 -sulfated lactose (bottom) (PDB 3AP9).
In view of Figure 3, the structural design of the galectins will then also be an incentive for
Figure 8. Human galectin-8N (Connolly surfaces and binding sites) loaded with lacto-N-fucopentaose-III
tailoring
aspects
of binding
partners,
to optimize
selectivity will
and to
unravel
In
view spatial
of Figure
3, the
structural
design
of the galectins
then
alsostructure-activity
be an incentive for
(top) and 3′-sulfated lactose (bottom) (PDB 3AP9).
relationships,
as
the
protein,
too,
can
be
tailored,
for
example
engineering
new
homodimers
(Figure 9).
tailoring spatial aspects of binding partners, to optimize selectivity and to unravel structure-activity
relationships,
protein,
too,structural
can be tailored,
engineering
newalso
homodimers
(Figurefor
9).
In view as
of the
Figure
3, the
designfor
of example
the galectins
will then
be an incentive
tailoring spatial aspects of binding partners, to optimize selectivity and to unravel structure-activity
relationships, as the protein, too, can be tailored, for example engineering new homodimers (Figure 9).
Molecules 2016, 21, 629
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Strategically,
the ligand
structure
can
bebemodified
fitatatdifferent
different
sites,
with
the aglycone,
Strategically,
the ligand
structure
can
modified for
for optimal
optimal fit
sites,
with
the aglycone,
as theascase
of
p-nitrophenyl,
as
starting
point,
a
strategy
defined
as
subsite-assisted
binding
the case of p-nitrophenyl, as starting point, a strategy defined as subsite-assisted binding [75].[75].
Figure 9. Relative spatial orientation of CRDs of galectins, which can be differently responsible for
Figure 9. Relative spatial orientation of CRDs of galectins, which can be differently responsible for
intra/inter-cellular cross-linking (left: face view; right: lateral view). Blue arrows point the positioning
intra/inter-cellular cross-linking (left: face view; right: lateral view). Blue arrows point the positioning
of two CRDs.
of two CRDs.
4. Tailoring Monovalent Ligands for Selectivity
4. Tailoring Monovalent Ligands for Selectivity
In chemical terms, each structural part of the ligand can be examined for its contribution to
selectivity.
the aglycone
(in glycoclusters
the linker
the scaffold),
the atom
glycosidic to
In chemicalThus,
terms,
each structural
part of the
ligandto can
be examined
for atitsthe
contribution
linkage
and
the
sugar
(sequence
and
substitutions)
are
subject
to
changes
on
the
drawing
selectivity. Thus, the aglycone (in glycoclusters the linker to the scaffold), the atom at the board,
glycosidic
providing
compounds
to be
synthesized and
linkage
and theinteresting
sugar (sequence
and
substitutions)
aretested.
subject to changes on the drawing board,
providing interesting compounds to be synthesized and tested.
4.1. Modifications of the Aglycone
The classical
is the aromatic extension by a p-nitrophenyl group, reported for galectin-1
4.1. Modifications
of theexample
Aglycone
(formerly called galaptin) [57,58]. In detail, a 2.5-fold enhancement was seen by adding this group
The
classical
aromatic
extension
byobserved
a p-nitrophenyl
group,
for galectin-1
to lactose
[57],example
and evenisathe
19-fold
enhancement
was
when it was
usedreported
as an extension
to
(formerly
called
galaptin)
[57,58]. in
Inthe
detail,
a 2.5-fold
enhancement
seen bycontributions
adding this to
group
galactose
[58].
That alterations
aglycone
part have
the capacitywas
to generate
to lactose
[57],
and
even
a
19-fold
enhancement
was
observed
when
it
was
used
as
an
extension
selectivity has been delineated by testing a series of β-lactosides with aromatic substituents [76].
to galactose
[58].and
That
alterations inproved
the aglycone
part
have
capacity
to to
generate
contributions
β-Naphthyl
2-benzothiazolyl
especially
active,
in the
terms
of affinity
galectin-3
relative
to galectin-1
in isothermal
titration
calorimetry
andofinβ-lactosides
comparative with
inhibition
assays
[76–78]. As [76].
to selectivity
has been
delineated
by testing
a series
aromatic
substituents
illustrated
in2-benzothiazolyl
Scheme 1, the synthesis
of especially
the lactosides
started
available
peracetylated
β-Naphthyl
and
proved
active,
in from
termsreadily
of affinity
to galectin-3
relative
lactosyl
bromide
1,
which
was
obtained
in
quantitative
yield
by
treatment
of
peracetylated
lactose
to galectin-1 in isothermal titration calorimetry and in comparative inhibition assays [76–78].
with HBr in AcOH. As expected, the phase transfer-catalyzed (PTC) nucleophilic displacements of
As illustrated in Scheme 1, the synthesis of the lactosides started from readily available peracetylated
lactosyl bromides by the aryl alcohols or aryl thiols occurred with complete anomeric inversion to
lactosyl bromide 1, which was obtained in quantitative yield by treatment of peracetylated lactose with
afford exclusively the corresponding β-glycosides in good to excellent yields (78% for 2a and 75%
HBr in
expected,
the phase
transfer-catalyzed
(PTC)
nucleophilic
displacements
of lactosyl
forAcOH.
3) [76].AsThey
were ready
to be
evaluated for ligand
activity
with galectins
after Zemplén
bromides
by
the
aryl
alcohols
or
aryl
thiols
occurred
with
complete
anomeric
inversion
to afford
de-O-acetylation (NaOMe, MeOH). With β-napthyl thioglycoside such as the peracetylated derivative
exclusively
the corresponding
β-glycosides
in good
to excellent
yields
(78%using
for 2a
and 75%
for 3) [76].
2a in hand,
synthesis of anomeric
sulfones by
oxidation
of the sulfide
groups
mCPBA
(2.1 equiv,
They CH
were
to besulfones
evaluated
for ligand
activity with galectins
after
Zemplén
2Cl2ready
) generated
2b (92%)
after de-O-acetylation.
Most O-aryl
lactosides
werede-O-acetylation
estimated as
beingMeOH).
3-fold better
thanβ-napthyl
lactose in activity
assays, thus
underscoring
a preference for
aromatic aglycons.
(NaOMe,
With
thioglycoside
such
as the peracetylated
derivative
2a in hand,
What’s
more,
the
data
reveal
this
region
to
be
of
conspicuous
relevance
for
selectivity
[76–78].
synthesis of anomeric sulfones by oxidation of the sulfide groups using mCPBA (2.1 equiv, CH2 Cl2 )
generated sulfones 2b (92%) after de-O-acetylation. Most O-aryl lactosides were estimated as being
3-fold better than lactose in activity assays, thus underscoring a preference for aromatic aglycons.
What’s more, the data reveal this region to be of conspicuous relevance for selectivity [76–78].
Molecules 2016, 21, 629
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Scheme 1. Synthetic strategy leading to O- or S-lactosides under phase transfer-catalyzed (PTC) conditions.
Scheme 1. Synthetic strategy leading to O- or S-lactosides under phase transfer-catalyzed (PTC) conditions.
On the structural level, the O-3 hydroxyl group of the glucose moiety of lactose is the main
On
theofstructural
level,
O-3
group
of the
glucose
moiety of
lactose
is the main
contact
part ofstrategy
lactosethe
with
galectins,
as first
detected
by
chemical
mapping
β-methyl
lactoside
Scheme
1.this
Synthetic
leading
tohydroxyl
O- or S-lactosides
under
phase
transfer-catalyzed
(PTC)
conditions.
contact
of this part
of lactose
with [79].
galectins,
as first detected
by chemical
mapping
β-methyl
lactoside
derivatives
(deoxy
and fluoro)
Semi-empirical
calculations
correlated
well with
the chemical
modifications
done
the
anomeric
position
withgroup
the effect
onglucose
charge
density
O-3
ofwith
the
On(deoxy
the structural
level,
the
O-3
hydroxyl
of the
moiety at
ofwell
lactose
is glucoside
the
derivatives
andatfluoro)
[79].
Semi-empirical
calculations
correlated
themain
chemical
residue of
ofthis
lactose.
The
O-3 with
electron
density
calculations
made on
the lactoside
derivatives
contact
part
lactose
galectins,
as
firstthe
detected
by
mapping
lactoside
modifications
done
at of
the
anomeric
position
with
effectwere
onchemical
charge
density
atβ-methyl
O-3 of
the
glucoside
evaluated
on
galectin-1
in
the
study
[80–82].
Electron-withdrawing
aglycones,
for
example
the
derivatives
(deoxy
and
fluoro)
[79].
Semi-empirical
calculations
correlated
well
with
the
chemical
residue of lactose. The O-3 electron density calculations were made on the lactoside sulfone
derivatives
moiety in 2b, decreased
the
O-3 electronic
densities
favored
interactions
with
Glu71,
which was
modifications
done at the
anomeric
withElectron-withdrawing
theand
effect
on charge
density
at O-3
of the
evaluated
on galectin-1
in the
studyposition
[80–82].
aglycones,
forglucoside
example the
accounted
for substantially
by hydrogen
bonding
with were
the human
lectin
Hence,
a direct
residue
of lactose.
The O-3 electron
density
calculations
made on
the [83].
lactoside
derivatives
sulfone moiety in 2b, decreased the O-3 electronic densities and favored interactions with Glu71,
correlationonexists
between
inhibitory
and the charge
densityforatexample
O-3 calculated
on
evaluated
galectin-1
in thethe
study
[80–82].potencies
Electron-withdrawing
aglycones,
the sulfone
which
was in
accounted
for substantially
hydrogen
bonding
with
the human
lectinthat
[83].
Hence,
electrostatic
has aby
theoretical
density
of −0.346
compared
ofwas
its
moiety
2b,potential.
decreasedLac-O-pNP
the O-3 electronic
densitieselectron
and favored
interactions
with
Glu71,towhich
a direct
correlation
exists
between
the
inhibitory
potencies
and
the
charge
density
at
O-3
calculated
o-nitrophenyl
(−0.322).by
This
differencebonding
of electron
density
could explain
the relative
accounted
foranalog
substantially
hydrogen
with
the human
lectin [83].
Hence,inhibitory
a direct
on electrostatic
potential.
Lac-O-pNP
haspotencies
a2b,theoretical
electron
of
´0.346
compared
to
potency variation
on
galectin-1.
Compound
having
thethe
lowest
O-3density
electronat
density,
also
has the
correlation
exists
between
the inhibitory
and
charge
density
O-3
calculated
on
that electrostatic
of
its o-nitrophenyl
analog
This
difference
of electron
could
explain
highest
inhibitory
potency
[76].(´0.322).
Among
the
compounds
byof isothermal
calorimetry
(ITC),
potential.
Lac-O-pNP
has a theoretical
electron tested
density
−0.346density
compared
to that
of its the
−1)
benzothiazole
3potency
showed
the
best
thermodynamic
performance
of −6.7
kcal/mol;
Ka 65 inhibitory
×O-3
10−3 M
o-nitrophenyl
(−0.322).
This
difference
of electron
density(ΔG
could
explain
the
relative
inhibitoryanalog
variation
on galectin-1.
Compound
2b,
having
therelative
lowest
electron
for
galectin-3
[76].
potency
variation
on
galectin-1.
Compound
2b,
having
the
lowest
O-3
electron
density,
also
has
the
density, also has the highest inhibitory potency [76]. Among the compounds tested by isothermal
When
assaying
derivatives
harboring
a best
4-substituted
1,2,3-triazole
unit, derived
lactosyl
highest
inhibitory
potency
[76].
Amongthe
the
compounds
tested by
isothermal
calorimetry
(ITC),
calorimetry
(ITC),
benzothiazole
3 showed
thermodynamic
performance
(∆G from
of ´6.7
kcal/mol;
−3 M−1)
azide 4´using
copper-catalyzed
azide–alkyne
cycloaddition
(Scheme
2)
to
afford
heteroaryl
lactoside
benzothiazole
3
showed
the
best
thermodynamic
performance
(ΔG
of
−6.7
kcal/mol;
Ka
65
×
10
3
´
1
Ka 65 ˆ 10 M ) for galectin-3 [76].
6 ingalectin-3
essentially
quantitative yield after Zemplén deprotection, the potential of substitution at this
for
[76].
When assaying derivatives harboring a 4-substituted 1,2,3-triazole unit, derived from lactosyl
site was
reinforced
When assaying[66].
derivatives harboring a 4-substituted 1,2,3-triazole unit, derived from lactosyl
azideazide
4 using
copper-catalyzed azide–alkyne cycloaddition (Scheme 2) to afford heteroaryl lactoside 6
4 using copper-catalyzed azide–alkyne cycloaddition (Scheme 2) to afford heteroaryl lactoside
in essentially
quantitative
yieldyield
afterafter
Zemplén
deprotection,
the
atthis
this site
6 in essentially
quantitative
thepotential
potential of
of substitution
substitution at
CN deprotection,
O Zemplén
HO
OH
AcO
OAc
was reinforced
[66].
5
OH
OAc
site was reinforced
[66].
O
O
O
1. CuSO4, vit C, DMF
O
HO
microwave, 70 oC
HO
OH
30 min
OH
OAc
CN
O
HO
OH
AcO
OAc
5
OH
2. MeONa/MeOH
OAc
4
6
O
O
quant.
O
1. CuSO
O
4, vit C, DMF
O
HO
O
AcO
o
microwave, 70 C
HO
N3
AcO
OH
OAc2. Representative
Scheme
synthesis
of
regioselectively
4-substituted
1,2,3-triazole
30 min
OH
OAc
AcO
OAc
O
AcO
O
N3
N N
N
O
N N
N
O
CN
lactoside 6 using
CN
copper-catalyzed
azide–alkyne cycloaddition
2. MeONa/MeOH (CuAAC, click chemistry).
4
6
quant.
4.2. Modifications
at the Glycosidic
Linkage
Scheme 2. Representative
synthesis
of regioselectively 4-substituted 1,2,3-triazole lactoside 6 using
Scheme 2. Representative synthesis of regioselectively 4-substituted 1,2,3-triazole lactoside 6 using
copper-catalyzed
azide–alkyne
cycloaddition
(CuAAC,
click chemistry).
The
introduction
of
S,
Se
or
C
atoms
into(CuAAC,
the glycosidic
makes the glycoside resistant to
copper-catalyzed azide–alkyne cycloaddition
click linkage
chemistry).
glycosidases. Also, stereoelectronic aspects, bond angles and flexibility are altered [84–86]. Testing the
4.2. Modifications at the Glycosidic Linkage
S/Se-bridged digalactosides (TDG, SeDG), rather small discriminatory effects were noted whereas
4.2. Modifications at the Glycosidic Linkage
dithio/seleno
bridgingofwas
unfavorable
galectins
[87]. However,
S/Se-digalactoside
totoa
The introduction
S, Se
or C atomsfor
into
the glycosidic
linkage makes
the glycosideactivity
resistant
plantintroduction
toxin opens
avoiding
cross-reactivity,
if desiring
to
block
the
toxin
[87].
The
glycosidases.
Also,the
stereoelectronic
aspects,
bond
angles
and flexibility
are
altered
Testing
the
The
of possibility
S, Se or C for
atoms
into
the
glycosidic
linkage
makes
the[84–86].
glycoside
resistant
to
synthesis Also,
of the
SeDG is summarised
in Scheme
3.small
The peracetylated
bromide
treated
S/Se-bridged
digalactosides
(TDG,aspects,
SeDG),
ratherangles
discriminatory
effects
were
notediswhereas
glycosidases.
stereoelectronic
bond
and
flexibilitygalactosyl
are altered
[84–86].
Testing the
with selenourea
in acetone
to generate
a rather
salt.
Production
of the selenolate
anion
from
this to
salt
dithio/seleno
bridging
was (TDG,
unfavorable
for
galectins
[87].discriminatory
However,
S/Se-digalactoside
activity
a
S/Se-bridged
digalactosides
SeDG),
small
effects
were
noted
whereas
followed
byopens
reaction
with
7 gavefor
SeDG.
A related
approach, using
thiourea,
givesthe
TDG
via [87].
glycosyl
plant
toxin
the
possibility
avoiding
cross-reactivity,
if
desiring
to
block
toxin
The
dithio/seleno bridging was unfavorable for galectins [87]. However, S/Se-digalactoside activity to
thiol intermediates,
which
are generally
in 3.
S-glycoside
synthesis.galactosyl bromide is treated
synthesis
of the SeDG
is summarised
inuseful
Scheme
The peracetylated
a plant toxin opens the possibility for avoiding cross-reactivity, if desiring to block the toxin [87]. The
with selenourea in acetone to generate a salt. Production of the selenolate anion from this salt
synthesis
of the SeDG is summarised in Scheme 3. The peracetylated galactosyl bromide is treated
followed by reaction with 7 gave SeDG. A related approach, using thiourea, gives TDG via glycosyl
with thiol
selenourea
in acetone
to generate
a salt.
Production
of thesynthesis.
selenolate anion from this salt followed
intermediates,
which
are generally
useful
in S-glycoside
by reaction with 7 gave SeDG. A related approach, using thiourea, gives TDG via glycosyl thiol
intermediates, which are generally useful in S-glycoside synthesis.
Molecules 2016, 21, 629
Molecules 2016, 21, 629
12 of 36
12 of 35
Scheme 3. Structures of O-digalactoside (ODG) and its S- and Se- mimics (TDG and SeDG), with
Scheme 3. Structures of O-digalactoside (ODG) and its S- and Se- mimics (TDG and SeDG), with
synthesis of
of SeDG.
SeDG.
synthesis
Proceeding from the reducing end, the next part that can be altered is the sugar headgroup.
Proceeding from the reducing end, the next part that can be altered is the sugar headgroup. Here,
Here, a brief look at Figure 6 indicates that certain positions are preferred sites for increasing
a brief look at Figure 6 indicates that certain positions are preferred sites for increasing reactivity to
reactivity to galectins, inspiring exploitation.
galectins, inspiring exploitation.
4.3. Modification at Bioinspired Positions
4.3. Modification at Bioinspired Positions
The synthetic extensions of disaccharides to natural oligosaccharides has enabled to map contact
The synthetic extensions of disaccharides to natural oligosaccharides has enabled to map contact
sites in the CRDs beyond lactose. An example for the synthesis of relevant structures for galectins is
sites in the CRDs beyond lactose. An example for the synthesis of relevant structures for galectins
the addition of a fucose moiety in an α1,2-linkage to lactose, constituting the histo-blood group
is the addition of a fucose moiety in an α1,2-linkage to lactose, constituting the histo-blood group
H-trisaccharide, with general principles of oligosaccharide synthesis outlined in [88]. The synthesis
H-trisaccharide, with general principles of oligosaccharide synthesis outlined in [88]. The synthesis
of oligosaccharides enables generation of stereochemically pure and homogeneous substances for
of oligosaccharides enables generation of stereochemically pure and homogeneous substances for
evaluation of their binding to lectins. It also enables the incorporation of these substances onto
evaluation of their binding to lectins. It also enables the incorporation of these substances onto
microarrays for identification of ligands for lectins (Figure 5). Chemical synthesis of glycoside bonds
microarrays for identification of ligands for lectins (Figure 5). Chemical synthesis of glycoside bonds
relies on having both suitably protected donors and acceptors available. The synthesis of 2′-fucosyl
relies on having both suitably protected donors and acceptors available. The synthesis of 21 -fucosyl
lactose exemplifies an approach to generating a lactos derivative [89]. Matta and coworkers have
lactose exemplifies an approach to generating a lactos derivative [89]. Matta and coworkers have
shown that lactose can be converted to 9 via acetonide formation and benzoylation [90] and used this
shown that lactose can be converted to 9 via acetonide formation and benzoylation [90] and used this
in a synthesis of 2′-fucosyl lactose. This leaves the 2′-OH free to be a nucleophile in a glycosidation
in a synthesis of 21 -fucosyl lactose. This leaves the 21 -OH free to be a nucleophile in a glycosidation
reaction. In a variation of Matta’s synthesis, a benzylated thiofucopyranoside 10 was shown to act as
reaction. In a variation of Matta’s synthesis, a benzylated thiofucopyranoside 10 was shown to act
a donor using Crich and Smith’s activation protocol [91]. The use of benzyl protecting groups, which
as a donor using Crich and Smith’s activation protocol [91]. The use of benzyl protecting groups,
do not participate in the glycosidation reaction, ensure that the required α-stereoisomer 11 is formed
which do not participate in the glycosidation reaction, ensure that the required α-stereoisomer 11
after activation of the thioglycoside. Most likely, the undesired β-anomer would have resulted if
is formed after activation of the thioglycoside. Most likely, the undesired β-anomer would have
acylated protecting groups had been used, unless anomerisation could occur or be induced in situ [92].
resulted if acylated protecting groups had been used, unless anomerisation could occur or be induced
Subsequent removal of the benzoyl and acetonide protecting groups generates 13 and, finally,
in situ [92]. Subsequent removal of the benzoyl and acetonide protecting groups generates 13 and,
2′-fucosyl lactose is obtained by hydrogenolytic removal of the benzyl groups.
finally, 21 -fucosyl lactose is obtained by hydrogenolytic removal of the benzyl groups.
As noted above with respect to Scheme 4, potent binding partners for galectins present distinct
As noted above with respect to Scheme 4, potent binding partners for galectins present distinct
substitutions at the N-acetyllactosamine core, for example the α1,3-linked N-acetylgalactosamine
substitutions at the N-acetyllactosamine core, for example the α1,3-linked N-acetylgalactosamine
residue, which turns the histo-blood group H-type trisaccharide to the A-type tetrasaccharide. Such a
residue, which turns the histo-blood group H-type trisaccharide to the A-type tetrasaccharide.
site for a substitution is referred to as a rational design based on structural information (bioinspiration)
Such a site for a substitution is referred to as a rational design based on structural information
Scheme 5 documenting typical reaction sequences toward the production of compounds following
(bioinspiration) Scheme 5 documenting typical reaction sequences toward the production of
this bioinspiration.
compounds following this bioinspiration.
In general, irrespective of the aglycone residues on unprotected β-D-lactopyranosides, it is possible
In general, irrespective of the aglycone residues on unprotected β-D-lactopyranosides, it is
to use the well-known behavior of tin-ether or tin-acetal activation that greatly and regioselectively
possible to use the well-known behavior of tin-ether or tin-acetal activation that greatly and
potentiate the nucleophilicity of the O-3′ oxygen to modify this particular position. For example,
regioselectively potentiate the nucleophilicity of the O-31 oxygen to modify this particular position. For
lactoside 14 can be selectively propargylated to give compound 15 in very good yield after acetylation
example, lactoside 14 can be selectively propargylated to give compound 15 in very good yield after
[76]. The propargyl ether of 8 was then treated by a [1–3]-dipolar cycloaddition [93] between the
acetylation [76]. The propargyl ether of 8 was then treated by a [1,3]-dipolar cycloaddition [93] between
alkyne and nitrile oxide, generated in situ from benzaldehyde oxime and N-chlorosuccinimide
(through intermediate benzhydroximoyl chloride) to afford 16 in 91% yield, which after Zemplén
Molecules 2016, 21, 629
13 of 36
the alkyne and nitrile oxide, generated in situ from benzaldehyde oxime and N-chlorosuccinimide
Molecules
2016, 21, 629 benzhydroximoyl chloride) to afford 16 in 91% yield, which after13Zemplén
of 35
(through
intermediate
Molecules 2016,
21, 629 the desired ligand 17 almost quantitatively. An analogous strategy was
13 ofused
35
deprotection
provided
to
deprotection provided the desired ligand 17 almost quantitatively. An analogous strategy was used to
give the corresponding 31 -sulfated O- and S-naphthyl lactosides 21 and 22 from 19 and 20, respectively.
give
the corresponding
3′-sulfated Oand S-naphthyl
lactosides 21 and analogous
22 from 19 strategy
and 20, respectively.
deprotection
provided the
ligand
17 almost quantitatively.
was used to
It is also
worth noting
that thedesired
above chemical
sequences provided An
compounds reactive
with galectins-1
It
is the
alsocorresponding
worth noting3′-sulfated
that the Oabove
chemical sequences
provided
compounds
with
give
and S-naphthyl
lactosides 21
and 22 from
19 and 20,reactive
respectively.
and -3
in a much
faster
than theroute
one previously
described
for
the synthesis
18 showing
galectins-1
and
-3noting
inroute
a much
than the one
previously
described
for theof
synthesis
18 a KD
It is also worth
that faster
the above chemical
sequences
provided
compounds
reactive of
with
of 320showing
nM
against
galectin-3
[94].
a Kand
D of-3
320
against
galectin-3
[94]. the one previously described for the synthesis of 18
galectins-1
innM
a much
faster
route than
showing a KD of 320 nM against galectin-3 [94].
Scheme 4. Example of oligosaccharide synthesis to produce naturally occurring galectin ligands.
Scheme 4. Example of oligosaccharide synthesis to produce naturally occurring galectin ligands.
Scheme 4. Example of oligosaccharide synthesis to produce naturally occurring galectin ligands.
Scheme 5. Chemical modifications of lactosides at both the aglycones and at the 3′-position.
Scheme 5. Chemical modifications of lactosides at both the aglycones and at the 3′-position.
Scheme
5. Chemical
of lactosides
at both the aglycones
at the 31 -position.
To initiate
to explainmodifications
its marked reactivity
to galectins-3,
-4 and -8, and
the isoxazole
lactoside 17
was docked
into
the
active
site
of
human
galectin-3.
Possible
interactions
with
all
13
amino acid
To initiate to explain its marked reactivity to galectins-3, -4 and -8, the isoxazole lactoside
17
residues
listed
in
Table
1
can
thus
be
monitored.
It
can
be
clearly
seen
that
17
may
form
strong
wasinitiate
dockedtointo
the active
site of reactivity
human galectin-3.
Possible
all 13lactoside
amino
acid
To
explain
its marked
to galectins-3,
-4 interactions
and -8, the with
isoxazole
17 was
hydrogen
bonding
with
Asp148,
H158,
possibly
thecan
key
remote
Arg144
with
the
isoxazole
moiety
residues
listed
in Table
1 human
can thus
beand
monitored.
It
be
clearly
seen
that
form
strong
docked
into the
active
site of
galectin-3.
Possible
interactions
with
all17
13may
amino
acid
residues
10).bonding
studyand
of structure-activity
relationships
offers
remarkable
hydrogen
withthorough
Asp148,
H158,
the key remote
Arg144
with the
isoxazole
moiety
listed(Figure
in Table
1Evidently,
can thus
be
monitored.
It possibly
can be clearly
seen
that
17 (SARs)
may
form
strong
hydrogen
potential
to understand
selectivitystudy
and push
an enhancementrelationships
to its inherent(SARs)
limits.offers remarkable
(Figure 10).
Evidently, thorough
of structure-activity
bonding with Asp148, H158, and possibly the key remote Arg144 with the isoxazole moiety (Figure 10).
potential to understand selectivity and push an enhancement to its inherent limits.
Evidently, thorough study of structure-activity relationships (SARs) offers remarkable potential to
understand selectivity and push an enhancement to its inherent limits.
Molecules 2016, 21, 629
Molecules 2016, 21, 629
14 of 36
14 of 35
Figure 10. Schematic representation of the CRD site of human galectin-3 (PDB 3AYE) showing all
Figure 10.
Schematic
representation
theinteracting
CRD sitewith
of compound
human galectin-3
3AYE) showing all
amino
acids involved
as per Table 1ofand
17 (docked(PDB
in by superposition
amino acids
involved
as ligand);
per Table
and interacting
with compound
17surface
(docked
in byand
superposition
with natural
lactose
left 1Connolly
surface representation
with red
(anionic)
blue
surfacelactose
(cationic
amino acids).
with natural
ligand);
left Connolly surface representation with red surface (anionic) and blue
surface (cationic amino acids).
In summary, a binding partner of a (ga)lectin has been formally separated into distinct sections.
The advantage of this approach is that any structural change introduced by synthesis can be
In summary,
a binding
partner
of a (ga)lectin
hascan
been
separated
intoofdistinct
evaluated and
optimized.
Combinatorial
integration
thenformally
fully realize
the potential
tailoringsections.
the monovalent
testing should
include
the natural
of lectins
The advantage
of this structure.
approachOfisnote,
that activity
any structural
change
introduced
by panel
synthesis
can (of
be the
evaluated
same family Combinatorial
and of other families
sharing the same
nominal
glycan
specificity)
preclude premature
and optimized.
integration
can then
fully
realize
the to
potential
of tailoring the
conclusions on selectivity. Hence, keeping an eye on the size of the lectin network, as illustrated in
monovalent
structure. Of note, activity testing should include the natural panel of lectins (of the
Figure 4, and on cross-reactive activities is essential. With this part done, the next source of
same family and of other families sharing the same nominal glycan specificity) to preclude premature
inspirations for tailoring synthetic binders of lectins comes from the natural presentation of glycans
conclusions
on selectivity.
keeping an eye
on the
of theare
lectin
network,
illustrated in
as multivalent
clustersHence,
up to microdomains.
Affinity
and size
selectivity
increased
whenas
properly
Figure 4,taking
and on
cross-reactive
activities
is essential.
Withthethis
done,
theglycoside
next source
ofeffect
inspirations
spatial
factors of the
presentation
into account,
finepart
work
on the
cluster
with pioneering
experiments
on thecomes
hepaticfrom
asialoglycoprotein
receptor, a C-type
lectinas
shown
for tailoring
synthetic binders
of lectins
the natural presentation
of glycans
multivalent
schematically
in
Figure
2,
and
synthetic
glycoclusters
serving
as
excellent
precedent
[95,96].
clusters up to microdomains. Affinity and selectivity are increased when properly taking spatial
factors of
the presentation into account, the fine work on the glycoside cluster effect with pioneering
5. Tailoring Spatial Aspects for Selectivity
experiments on the hepatic asialoglycoprotein receptor, a C-type lectin shown schematically in Figure 2,
A wide variety of scaffolds can be used to present glycans as bioactive ligands, generating
and synthetic glycoclusters serving as excellent precedent [95,96].
glycoclusters or glycodendrimers [97–101]. Of course, any favorable structural feature found on the
level of testing monovalent compounds can now be implemented. The following schemes can thus
5. Tailoring Spatial Aspects for Selectivity
be considered as examples tested with galectins, starting with a selection of bivalent compounds.
derived
glucuronic
acid and
phenylene-1,4-diamine
(glycophanes;
A wideMacrocyclic
variety ofscaffolds
scaffolds
can from
be used
to present
glycans
as bioactive ligands,
generating
[102,103]) had been used as fragments in building bivalent mannosides 23 and 24 (Scheme 6) [104].
glycoclusters or glycodendrimers [97–101]. Of course, any favorable structural feature found on the
This inspired the use of such chiral scaffolds based on p-xylylenediamine to prepare structurally
level of testing monovalent compounds can now be implemented. The following schemes can thus be
defined bivalent lactoside 26 [105]. Compound 26 is considered rigid when compared with 25. Molecular
considered
as examples
tested
with galectins,
with a galactose
selectionheadgroups
of bivalent
modeling
can be used
to investigate
distance starting
profiles between
in compounds.
such bivalent
Macrocyclic
scaffolds
derived forfrom
and phenylene-1,4-diamine
compounds. The
use of macromodel
example,glucuronic
indicated that acid
the intergalactoside
distance in 26
was 24.5 [102,103])
Å. While both
and 26used
showed
affinity
galectin 3bivalent
in a solidmannosides
phase binding 23 and
(glycophanes;
had25 been
as similar
fragments
inforbuilding
assay,
the
glycophane
26
was
>10-fold
more
potent
than
25
towards
the
truncated
galectin
3.
24 (Scheme 6) [104]. This inspired the use of such chiral scaffolds based on p-xylylenediamine
to
Various different frameworks (scaffolds) can be incorporated in bivalent glycocluster design
prepare structurally defined bivalent lactoside 26 [105]. Compound 26 is considered rigid when
and synthesis and a selection are shown in Scheme 7 [105–107]. While topology within this series can
compared
with 25. Molecular modeling can be used to investigate distance profiles between galactose
be considered the same (i.e., they are all bivalent), the spatial relationships are different, allowing
headgroups
in suchtobivalent
compounds.
The use of
macromodel
example,
indicated
this property
be investigated
through bioassays.
Interactions
of the for
scaffold
with the
lectin thatthat the
intergalactoside
distance
26 was
24.5 cannot
Å. While
both 25 and
26for
showed
similar
for
may contribute
to, or in
hinder,
binding
be discounted
given,
example,
activityaffinity
observed
forgalectin
compounds
such
as
6
(Scheme
2).
In
the
series
shown
the
distances
between
the
galactose
residues
can
3 in a solid phase binding assay, the glycophane 26 was >10-fold more potent than 25 towards the
vary
from 15.5
truncated
galectin
3. Å to 35 Å, as estimated by molecular modeling. A variety of scaffold types have been
investigated including both tertiary (27–29) and secondary (30, 31) diamides, diglycosyldiamides
Various different frameworks (scaffolds) can be incorporated in bivalent glycocluster design and
synthesis and a selection are shown in Scheme 7 [105–107]. While topology within this series can
be considered the same (i.e., they are all bivalent), the spatial relationships are different, allowing
this property to be investigated through bioassays. Interactions of the scaffold with the lectin that
may contribute to, or hinder, binding cannot be discounted given, for example, activity observed for
compounds such as 6 (Scheme 2). In the series shown the distances between the galactose residues
can vary from 15.5 Å to 35 Å, as estimated by molecular modeling. A variety of scaffold types have
Molecules 2016, 21, 629
15 of 36
been investigated including both tertiary (27–29) and secondary (30, 31) diamides, diglycosyldiamides
(33, 34), ditriazoles (32) and the previously mentioned glycophane 26 together with a C-glycosyl
version 35 and a more flexible variant of the latter 36. Highlights from biological study of such a series
include observations that the tertiary diamide 27 was 5-fold more selective for proteolytically truncated
Molecules 2016, 21, 629
15 of 35
galectin-3 (complete removal of the non-CRD sections, shown in Figure 3) than 26 [104] and that the
(33, 34),
ditriazoles (32)
the previously
26 together
with a compared
C-glycosyl with
more flexible
glycocluster
36 and
showed
100-fold mentioned
selectivityglycophane
for the chicken
galectin-3
version 35 and
a more flexible
chicken proto-type
galectin-1A
[106].variant of the latter 36. Highlights from biological study of such a
include observations that the tertiary diamide 27 was 5-fold more selective for proteolytically
Theseries
synthesis
of 25 and 26 is summarised in Scheme 8 [104]. The glycosidation reaction of
truncated galectin-3 (complete
removal of the non-CRD sections, shown in Figure 3) than 26 [104]
1,6-anhydro
donor
37
with 1 C4 conformation,
which is more reactive than a related donor with 4 C1
and that the more flexible
glycocluster 36 showed 100-fold selectivity for the chicken galectin-3
conformation,
occurs
the presence
triethylsilyloxyprop-1-ene
to initially give a β-glycoside but
compared
with in
chicken
proto-typeofgalectin-1A
[106].
this species undergoes
rapid
situ26anomerisation
presence
of The
SnCl
α-glucuronide
The synthesis
of 25inand
is summarised in
in the
Scheme
8 [104].
glycosidation
reaction
of
4 to give the
4C1
1,6-anhydro
donorconverted
37 with 1C4 to
conformation,
which of
is more
reactive than
a relatedgroups
donor with
38 [108,109].
This was
39 by removal
the acetate
protecting
followed
by
conformation,
occurs
in gave
the presence
of triethylsilyloxyprop-1-ene
initially give areaction
β-glycoside
an acetylation
reaction
that
a 6,3-lactone
intermediate andtosubsequent
of but
this with
this species undergoes rapid in situ anomerisation in the presence of SnCl4 to give the α-glucuronide
allyl alcohol gave 39. The glycosidation reaction between benzoylated trichloroacetimidate 40 and
38 [108,109]. This was converted to 39 by removal of the acetate protecting groups followed by an
39 and subsequent
removal
the aallyl
ester using
Pd(0) gave
41. Next reaction
the coupling
acid
41 with
acetylation reaction
thatofgave
6,3-lactone
intermediate
and subsequent
of this of
with
allyl
p-xylylenediamine
Ring closure
metathesis
the Grubbs-I
catalyst followed
by alkene
alcohol gave gave
39. The42.
glycosidation
reaction
between using
benzoylated
trichloroacetimidate
40 and 39 and
subsequent
of the allyl
ester
using Pd(0) 26,
gave
41. Nextalkene
the coupling
of acid
with
reduction
and acylremoval
group removal
gave
glycophane
whereas
reduction
and41deacylation
p-xylylenediamine
gave 42.
using
the Grubbs-I
catalyst
followed
by alkene as the
under similar
conditions gave
25.Ring
Theclosure
use of metathesis
the benzoyl
protecting
groups
in 40
was important
reduction
and
acyl
group
removal
gave
glycophane
26,
whereas
alkene
reduction
and
deacylation
analogous per-O-acetylated donor gave none of the desired glycoside but only an orthoester, which
under similar conditions gave 25. The use of the benzoyl protecting groups in 40 was important as
can be observed in the course of glycosidation.
the analogous per-O-acetylated donor gave none of the desired glycoside but only an orthoester,
Synthesis
ofbe35observed
with the
which can
in thep-xylylenediamine
course of glycosidation.based scaffold is shown in Scheme 9 [106].
The propargyl
derivative
was
prepared frombased
benzylidene
via ainpartial
reductive
cleavage
Synthesis
of 3544
with
thefirst
p-xylylenediamine
scaffold is43
shown
Scheme
9 [106]. The
propargyl derivative
44 by
was4-O-propargylation.
first prepared from benzylidene
43 via a partial
cleavage
of
of the benzylidene
followed
Next formation
of the reductive
C-glycosyl
derivative
45
the
benzylidene
followed
by
4-O-propargylation.
Next
formation
of
the
C-glycosyl
derivative
45
was
was carried by anomeric allylation [110], followed by selective removal of the 6-O-benzyl group and
carried by anomeric allylation [110], followed by selective removal of the 6-O-benzyl group and
subsequent
oxidation. The copper (I)-promoted triazole formation with azide 4 gave 46. Subsequent
subsequent oxidation. The copper (I)-promoted triazole formation with azide 4 gave 46. Subsequent
couplingcoupling
to p-xyxylenediamine
followed by deacetylation gave 36, whereas ring closure metathesis
to p-xyxylenediamine followed by deacetylation gave 36, whereas ring closure metathesis
using Grubbs-II
catalyst
followed
by de-O-acetylation
35.
using Grubbs-II
catalyst
followed
by de-O-acetylationgave
gave 35.
Scheme 6. Structures of bivalent glycoclusters based on glycophane and an acyclic variant. Space-filling
Scheme 6.
Structures
of bivalent
glycoclusters
on galactose
glycophane
and an
acyclic
variant.
Space-filling
model
of 26 is shown
on right
with distancebased
between
anomeric
carbon
atoms
indicated.
model of 26 is shown on right with distance between galactose anomeric carbon atoms indicated.
Molecules 2016, 21, 629
16 of 36
Molecules 2016, 21, 629
16 of 35
Scheme 7. A panel of synthetic lactose-presenting bivalent compounds with inter-galactoside distances
Scheme 7. A panel of synthetic lactose-presenting bivalent compounds with inter-galactoside distances
are shown. Various types of scaffolds have been used including secondary and tertiary diamides,
are shown. Various types of scaffolds have been used including secondary and tertiary diamides,
diglycosyldiamides, ditriazoles and glycophanes.
diglycosyldiamides, ditriazoles and glycophanes.
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17 of 36
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17 of 35
Scheme 8. Synthesis of glycophane 26 and the acyclic analogue 25.
Scheme 8. Synthesis of glycophane 26 and the acyclic analogue 25.
Scheme 8. Synthesis of glycophane 26 and the acyclic analogue 25.
Scheme 9. Synthesis of glycophane 36 and the acyclic analogue 35.
Scheme 9. Synthesis of glycophane 36 and the acyclic analogue 35.
Scheme 9. Synthesis of glycophane 36 and the acyclic analogue 35.
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The chemistry shown in Scheme 10 exemplifies an approach to the synthesis of diamides where
21 -fucosyl
lactose headgroups
are incorporated
[107].an
Diamides
can
corresponding
The chemistry
shown in Scheme
10 exemplifies
approach
to be
theprepared
synthesisfrom
of diamides
where
glycosylamines
such
as
49.
The
Ugi
reaction
is
a
four
component
reaction
involving
amine,
acid,
2′-fucosyl lactose headgroups are incorporated [107]. Diamides can be prepared from corresponding
aldehyde
and
isocyanide
that
is
suitable
for
tertiary
amide
generation.
When
a
diacid
such
as
glycosylamines such as 49. The Ugi reaction is a four component reaction involving amine, acid,
terephthalic
acid
is
used
with
49,
it
efficiently
gives
the
divalent
glycocluster
50
after
protecting
group
aldehyde and isocyanide that is suitable for tertiary amide generation. When a diacid such as
removal. Alternatively,
reduction
of it
theefficiently
azide 51 ingives
the presence
of Pd-C
and hydrogen
gaveprotecting
a glycosyl
terephthalic
acid is used
with 49,
the divalent
glycocluster
50 after
amine
and
subsequent
coupling
of
this
with
terephthaloyl
chloride
and
then
deacetylation
gave
group removal. Alternatively, reduction of the azide 51 in the presence of Pd-C and hydrogen gave
a
diamide 52.
Theand
secondary
amides
in 52 prefer
a trans-arrangement
in 51 deacetylation
prefer cis and
glycosyl
amine
subsequent
coupling
of this
with terephthaloylwhereas
chloridethose
and then
this difference
gives
to different
spatial
between the carbohydrate
headgroups
and
gave
diamide 52.
Therise
secondary
amides
in arrangements
52 prefer a trans-arrangement
whereas those
in 51 prefer
can
account
for
differences
in
preferences
for
lectins.
The
direct
copper-promoted
triazole
formation
cis and this difference gives rise to different spatial arrangements between the carbohydrate
from 51, prepared
from
21 -fucosyl
azide (Scheme
11), and subsequent
protecting
group
removal gave
headgroups
and can
account
for differences
in preferences
for lectins.
The direct
copper-promoted
ditriazole
53.
This
latter
compound
showed
high
affinity
(IC
=
8
µM)
for
galectin-4
and
~6-fold
triazole formation from 51, prepared from 2′-fucosyl azide 50
(Scheme 11), and subsequentwas
protecting
more
selective
to
this
lectin
than
52
[107].
group removal gave ditriazole 53. This latter compound showed high affinity (IC50 = 8 μM) for
Valency
to new
glycocluster
topologies
and can be taken to the
galectin-4
and increases
was ~6-foldgive
morerise
selective
to this
lectin than 52
[107].
levelValency
of tri- increases
and tetravalent
glycoclusters,
as
shown
next
with
notable
enhancements
for and
the
give rise to new glycocluster topologies and can be taken
to the level of trichimera-type
galectin-3 [111].
Synthetically,
2-propynyl
lactosides werefor
conjugated
under Sonogashira
tetravalent
glycoclusters,
as shown
next with
notable enhancements
the chimera-type
galectin-3
palladium-catalyzed
cross-coupling
conditions
with
triiodobenzene
and
suitably
derivatized
[111]. Synthetically, 2-propynyl lactosides were conjugated under Sonogashira palladium-catalyzed
pentaerythritol conditions
cores (Schemes
and 13). Starting
2-propynyl
lactoside
55, a series of
tricross-coupling
with12triiodobenzene
and with
suitably
derivatized
pentaerythritol
cores
and tetra-valent
derivatives
and 60 were
thus prepared
common
general
protocol
(Schemes
12 and 13).
Starting 56,
with59,
2-propynyl
lactoside
55, a seriesusing
of tri-aand
tetra-valent
derivatives
((Ph
P)
PdCl
,
DMF:Et
N
(1:1,
v/v),
CuI,
rt,
3–5
h).
The
simultaneous
Sonogashira
cross-coupling
of
3 and
2
3 prepared using a common general protocol ((Ph3P)2PdCl2, DMF:Et3N (1:1,
56, 59,
602 were thus
55 to CuI,
1,3,5-triiodobenzene
54 (Scheme 12)Sonogashira
and to pentaerythritol
tetrakis(mand
p-iodobenzyl)ether 54
57
v/v),
rt, 3–5 h). The simultaneous
cross-coupling
of 55 to
1,3,5-triiodobenzene
or
58
(Scheme
13)
afforded
an
efficacious
entry
into
this
family
of
carbohydrate
clusters
56
(trimer
80%),
(Scheme 12) and to pentaerythritol tetrakis(m- and p-iodobenzyl)ether 57 or 58 (Scheme 13) afforded
andefficacious
tetramers entry
59 (meta)
(para)ofincarbohydrate
78 and 75% yields,
respectively.
Complete
of
an
intoand
this60
family
clusters
56 (trimer 80%),
and de-O-acetylation
tetramers 59 (meta)
these60clusters
was
resulting
in freely water-soluble
clusters 56, 59,ofand
60 in
more than
and
(para) in
78uneventful,
and 75% yields,
respectively.
Complete de-O-acetylation
these
clusters
was
90%
yields.
uneventful, resulting in freely water-soluble clusters 56, 59, and 60 in more than 90% yields.
Scheme 10. Synthesis of diamides with the histo-blood H trisaccharide headgroup.
Scheme 10. Synthesis of diamides with the histo-blood H trisaccharide headgroup.
Molecules 2016, 21, 629
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19 of 36
19 of 35
19 of 35
Scheme 11. Synthesis of the bivalent compound 53.
Scheme
11.11.Synthesis
53.53.
Scheme
Synthesisofofthe
thebivalent
bivalentcompound
compound
Scheme 12. Trivalent lactose-presenting cluster 56 with equivalent inter-galactoside distances
Scheme12.12.
Trivalent
lactose-presenting
cluster
56 with
equivalent
inter-galactoside
distances
Scheme
Trivalent
lactose-presenting
cluster
56
with
equivalent
inter-galactoside
distances
was
was obtained
using the
core 54 and 2-propynyl
lactoside
55. It showed
remarkable reactivity
with
was
obtained
using
the
core
54
and
2-propynyl
lactoside
55.
It
showed
remarkable
reactivity
with
obtained
galectin-3 using
[111]. the core 54 and 2-propynyl lactoside 55. It showed remarkable reactivity with
galectin-3
[111].
galectin-3 [111].
Molecules 2016, 21, 629
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20 of 36
20 of 35
Scheme 13. Tetrameric lactoside clusters 59 and 60 with different inter-galactoside distances
Scheme 13. Tetrameric lactoside clusters 59 and 60 with different inter-galactoside distances prepared
prepared using palladium-catalyzed cross coupling Sonogashira reactions.
using palladium-catalyzed cross coupling Sonogashira reactions.
The click-chemistry reaction can also be used to give trivalent glycoclusters [89]. Hence, the reaction
The click-chemistry
reaction
can also
be used toderivative
give trivalent
glycoclusters
Hence,
the
of 2′-fucosyl
azide 51 (Scheme
14) with
phloroglucinol
61, and
subsequent [89].
protecting
group
1 -fucosyl azide 51 (Scheme 14) with phloroglucinol derivative 61, and subsequent
reaction
of
2
removal gave tritriazole 62. The distances between galactose residues in 62 was 23.5 Å. This latter
protecting
removal
gaveselectivity
tritriazole for
62. the
The chicken
distances
between relative
galactosetoresidues
in 62 was
compoundgroup
showed
>100-fold
galectin-3
the homodimeric
23.5
Å.
This
latter
compound
showed
>100-fold
selectivity
for
the
chicken
galectin-3
relative
to
galectin-1A, and was 10-fold more potent for the chimera-type protein than the corresponding
the
homodimeric
galectin-1A,
and
was
10-fold
more
potent
for
the
chimera-type
protein
than
the
trilactoside, which was in turn more potent than a related dilactoside. As scaffold with similar
corresponding
trilactoside,
which
wasbeen
in turn
moree.g.,
potent
than a coordinated
related dilactoside.
As[112,113]
scaffold
degree of rigidity,
other choices
have
realized,
metal-ion
complexes
with
similar
degree
of
rigidity,
other
choices
have
been
realized,
e.g.,
metal-ion
coordinated
or cyclopeptides [114,115], the latter especially suitable to block binding of galectin-4.
complexes
[112,113]
or cyclopeptides
[114,115],
theoflatter
especially
block
binding
Turning
to applications,
relative affinity
values
glycoclusters
forsuitable
galectinstocan
discriminate
of
galectin-4.
between
family members, as is the case between galectins-1 and -3. When combining the oligovalent
Turning
applications,
relative affinity
of glycoclusters
for galectins
can-4discriminate
display
with to
a bioinspired
substitution
at thevalues
3′-position,
affinity to galectins-3
and
was further
between
family
members,
as
is
the
case
between
galectins-1
and
-3.
When
combining
the
oligovalenta
enhanced, increasing selectivity against galectin-1 [116,117]. As a different type of application,
1 -position, affinity to galectins-3 and -4 was further
display
with
a
bioinspired
substitution
at
the
3
synthetic glycocluster has proven as potent tool to determine the stoichiometry of ligand-dependent
enhanced,
increasing selectivity
[116,117].
a different tail
typeonofthe
application,
galectin-3 oligomerization,
which against
dependsgalectin-1
on a critical
length of As
the N-terminal
side of the
aprotein
synthetic
glycocluster
has
proven
as
potent
tool
to
determine
the
stoichiometry
of
ligand-dependent
[118,119]. The feasibility to increase the level of branching made it tempting to further raise
galectin-3
oligomerization,
which
on a critical
length of the N-terminal
tail
on the side
of the
the valency,
entering the realm
ofdepends
glycodendrimers.
Poly(amidoamine)
starburst
(PAMAM)
scaffold
protein
[118,119].
The
feasibility
to
increase
the
level
of
branching
made
it
tempting
to
further
raise
(Scheme 15) serves as an impressive example. Such a lactosylated glycodendrimer (65) up to generation
the
entering
the realm
of glycodendrimers.
Poly(amidoamine)
starburst (PAMAM)
scaffold
fivevalency,
(G5) (128
lactosides),
anchored
through an aromatic
p-isothiocyanatophenyl
β-D-lactoside
63 as
(Scheme
15)
serves
as
an
impressive
example.
Such
a
lactosylated
glycodendrimer
(65)
up
to
generation
ligand (Scheme 15), too, reacted differentially with a galactoside-specific plant toxin and galectins-1
five
(128Of
lactosides),
anchoredofthrough
an aromatic
β-D-lactoside
63 as
and (G5)
-3 [120].
note, the numbers
generation
in designp-isothiocyanatophenyl
progress had a strong impact
on reactivity.
ligand
(Scheme
15),
too,
reacted
differentially
with
a
galactoside-specific
plant
toxin
and
galectins-1
In spite of the numerous strategies leading to the syntheses of glycodendrimers, it became
and
-3 [120].
the numbers
of generation
design
progress
a strong
impact
on molecules
reactivity.
obvious
that Of
thenote,
chemical
procedures
leading to in
them
would
greatlyhad
benefit
by using
core
In spite
of the numerous
syntheses
of glycodendrimers,
became
already
possessing
the higheststrategies
possible leading
number to
of the
exposed
surface
functionalities. Initaddition,
obvious
that
the
chemical
procedures
leading
to
them
would
greatly
benefit
by
using
core
molecules
in order to better control the physicochemical properties of dendrimers, while accelerating the
already
the highest
number
of exposed
surface
functionalities.
In addition,
increase possessing
of surface groups,
it waspossible
suggested
that different
building
blocks
be used in between
each
in
order to This
better
control led
the to
physicochemical
properties
of peel”
dendrimers,
accelerating
the
generation.
approach
the general concept
of “onion
strategywhile
[121–125]
For instance,
a glycodendrimer having 18-exposed lactoside residue was readily build-up in a few steps by using
Molecules 2016, 21, 629
21 of 36
increase of surface groups, it was suggested that different building blocks be used in between each
generation.
approach led to the general concept of “onion peel” strategy [121–125] For instance,
Molecules 2016,This
21, 629
21 of 35
a glycodendrimer having 18-exposed lactoside residue was readily build-up in a few steps by using
a hexaphenylbenzene
hexaphenylbenzene core such as 69
69 and
and aa tris(hydroxymethyl)aminomethane
tris(hydroxymethyl)aminomethane (TRIS) precursor
suitably substituted (Scheme 7). The
The synthetic
synthetic sequence
sequence leading
leading to this type of glycodendrimer
glycodendrimer
was initiated
initiated by
by aadouble
doubleStille
Stille
coupling
reaction
involving
bis(tributylstannyl)acetylene
(66)
coupling
reaction
involving
bis(tributylstannyl)acetylene
(66) and
and
para-iodoanisole
in the
presenceofoftetrakis(triphenylphosphine)palladium(0)
tetrakis(triphenylphosphine)palladium(0)toto give
give the
para-iodoanisole
(67) (67)
in the
presence
corresponding symmetrical diarylalkyne (68) in
in 71%
71%yield.
yield. A
A[2+2+2]
[2+2+2] cyclotrimerization of 68 catalyzed
catalyzed
by dicobalt
dicobalt octacarbonyl
octacarbonyl[Co
[Co
subsequent
methyl
ether
deprotection
2(CO)
8] and
subsequent
methyl
ether
deprotection
withwith
BBr3 BBr
provided
rapid
2 (CO)
8 ] and
3 provided
rapid
to perhydoxylated
69 in
87%Propargylation
yield. Propargylation
ofpropargyl
69 with propargyl
bromide
access access
to perhydoxylated
69 in 87%
yield.
of 69 with
bromide and
K2COand
3 in
˝
K
DMF
65 C hexapropargylated
afforded hexapropargylated
ether
in 82%
yield.
the orthogonal
AB3
DMF
afforded
ether 70 in
82%70yield.
Using
theUsing
orthogonal
AB3 synthon
2 CO65
3 in°C
synthon
2-bromoacetamido-tris[(propargyloxy)methyl]-aminomethane
(71)targeted
gave the18-mer
targeted
18-mer
2-bromoacetamido-tris[(propargyloxy)methyl]-aminomethane
(71) gave the
precursor
precursor
(79%).
The introduction
of the modified
lactopyranoside
was efficiently
achieved
72 (79%). 72
The
introduction
of the modified
lactopyranoside
terminitermini
was efficiently
achieved
using
using
Cu(I)-catalyzed
azide-alkyne
[1,3]-dipolar
cycloaddition
reaction
(Scheme
thisend,
end, lactosyl
Cu(I)-catalyzed
azide-alkyne
[1,3]-dipolar
cycloaddition
reaction
(Scheme
8).8).ToTothis
azide 4 was “clicked” on either 70 or 72 to afford hexa-lactosylated
hexa-lactosylated dendrons (not shown) and 18-mer
dendrimer 73, respectively,
respectively,in
ingood
goodoverall
overallyields
yieldsafter
afterusual
usualtransesterification
transesterification(MeONa
(MeONain
inMeOH).
MeOH).
1 -fucosyl lactose-presenting cluster 62 prepared using click-chemistry reaction
Scheme
Scheme 14.
14. Trivalent
Trivalent 22′-fucosyl
lactose-presenting cluster 62 prepared using click-chemistry reaction
from
from 51
51 and
and phloroglucinol
phloroglucinol derivative.
derivative. In
In this
this case,
case, inter-galactoside
inter-galactoside distances
distances are
are shorter
shorter than
than in
in56.
56.
Molecules 2016, 21, 629
Molecules 2016, 21, 629
22 of 36
22 of 35
Scheme 15. Poly(amidoamine) dendrimer of generation G2 (65) incorporating 16 lactoside residues
Scheme 15. Poly(amidoamine) dendrimer of generation G2 (65) incorporating 16 lactoside residues
linked through thiourea linkages.
linked through thiourea linkages.
Another type of glycodendrimer with ligand radiating from propargylated hexaphenylbenzene
Another
type of glycodendrimer
with ligand
frombeen
propargylated
hexaphenylbenzene
cores
and containing
up to 54 peripheral
sugarradiating
ligands has
synthesized
by the powerful
cores
and containing
up to 54cycloadditions
peripheral sugar
has been synthesized
by the
powerful
Cu(I)-catalyzed
[1,3]-dipolar
usingligands
both convergent
and divergent
approaches.
Cu(I)-catalyzed
cycloadditions
using
convergent
and divergent
approaches.
An expeditive [1,3]-dipolar
and systematic
route toward
the both
synthesis
of innovative
glycoclusters
and
An
expeditive
and
systematic
route
toward
the
synthesis
of
innovative
glycoclusters
and
G(1)-glycodendrimers built from circular and aromatic polypropargylated scaffolds have thus been
G(1)-glycodendrimers
from circular
and aromatic were
polypropargylated
scaffolds
thushigh
been
developed (Scheme 16).built
The multivalent
glycoconjugates
obtained with simple
andhave
generally
developed
(Scheme
16). Thenucleophilic
multivalentsubstitutions
glycoconjugates
were synthons
obtainedand
with
simple and generally
yielding reactions
including
to generate
copper-catalyzed
azide
high
yielding
reactionsreaction
including
nucleophilic
substitutions
to generate
synthons
and
copper-catalyzed
alkyne
cycloaddition
to covalently
attach
the minimally
modified
galectin
ligand
as lactoside
azide
alkyne
cycloaddition
reaction to covalently
attach
the minimally
modified
galectin ligand
moieties.
Interestingly,
the hydrophobic
character of
the inner
aromatic core
was counterbalanced
asbylactoside
moieties.
the hydrophobic
character derivatives.
of the inner aromatic core was
the hydrophilic
sugarInterestingly,
derivatives, affording
fully water-soluble
Based on hexachlorocyclotriphosphazene
(N3P3Claffording
6) as a central
(Scheme
17), which is
counterbalanced
by the hydrophilic sugar derivatives,
fullymolecule
water-soluble
derivatives.
known
to be
planar having three up and (N
three
lactosylated
dendrons
a
Based
onnearly
hexachlorocyclotriphosphazene
Cl6 ) assubstituents
a central molecule
(Scheme
17), and
which
3 P3down
architecture
obtained
(Scheme
18) [126].
Insubstituents
detail, a functionalized
ABdendrons
3 trimer,
isdumbbell-shape
known to be nearly
planarwere
having
three up
and three
down
lactosylated
derived
from TRIS (71,
Scheme 16),
together
with
an AB518)
building
block
(76),
derived from AB
the3
and
a dumbbell-shape
architecture
were
obtained
(Scheme
[126]. In
detail,
a functionalized
3P3Cl
6, were 16),
used.
Lactosylation
using (76),
clickderived
chemistry
with
commercially
trimer,
derived available
from TRISN(71,
Scheme
together
with an was
AB5 achieved
building block
from
the
lactosyl azideavailable
75 possessing
glycol
spacer.
As
also
used
in
glycodendrimer
synthesis
commercially
N3 P3aCltetraethylene
,
were
used.
Lactosylation
was
achieved
using
click
chemistry
with
6
for glycodendrimersome
surface
programming
[127],
to give
trimer
77 and
pentamer 78 insynthesis
good to
lactosyl
azide 75 possessing
a tetraethylene
glycol
spacer.
As also
used
in glycodendrimer
excellent
yields. Transformation
the bromides at
bothtodendron’s
focal
was readily
for
glycodendrimersome
surfaceofprogramming
[127],
give trimer
77point
and pentamer
78performed
in good to
by
an
S
N
2
substitution
using
sodium
azide
to
afford
compounds
79
and
80,
respectively.
Pentavalent
excellent yields. Transformation of the bromides at both dendron’s focal point was readily performed
hypermonomer
80 wasusing
then sodium
used toward
decavalent
dumbbell-shape
lactosylated
compound
82
by
an SN 2 substitution
azidethe
to afford
compounds
79 and 80,
respectively.
Pentavalent
by the above double
with dipropargylated
tetraethylene glycol
81. Standard
Zemplén
hypermonomer
80 wasclick
thenreaction
used toward
the decavalent dumbbell-shape
lactosylated
compound
82 by
de-O-acetylation
(MeONa,
MeOH)
generated
79
(3-mer),
80
(5-mer)
and
83
(10-mer)
in
almost
the above double click reaction with dipropargylated tetraethylene glycol 81. Standard Zemplén
quantitative yields in each case.
Molecules 2016, 21, 629
23 of 36
de-O-acetylation (MeONa, MeOH) generated 79 (3-mer), 80 (5-mer) and 83 (10-mer) in almost
quantitative
yields
Molecules 2016, 21,
629 in each case.
23 of 35
Molecules 2016, 21, 629
23 of 35
Scheme
16. Synthetic
Synthetic procedures leading
to hexaphenylbenzene
hexaphenylbenzene core (69)
which was
either directly
Scheme
Scheme 16.
16. Synthetic procedures
procedures leading
leading to
to hexaphenylbenzene core
core (69)
(69) which
which was
was either
either directly
directly
propargylated
to
give
70
or
treated
with
a
propargylated
TRIS
intermediate
(71)
to
afford
a scaffold
propargylated
to
give
70
or
treated
with
a
propargylated
TRIS
intermediate
(71)
to
afford
propargylated to give 70 or treated with a propargylated TRIS intermediate (71) to afford aa scaffold
scaffold
harboring
18-propargyl residues
residues (72).
harboring
harboring 18-propargyl
18-propargyl residues(72).
(72).
Scheme 17. Accelerated synthesis of multivalent lactosides using click chemistry with scaffold 72
Scheme 17. Accelerated synthesis of multivalent lactosides using click chemistry with scaffold 72
and
a lactosyl
azide (4) tosynthesis
give a glycodendrimer
with 18-exposed
lactoside
residues.
Scheme
17. Accelerated
of multivalent lactosides
using click
chemistry
with scaffold 72 and
and a lactosyl azide (4) to give a glycodendrimer with 18-exposed lactoside residues.
a lactosyl azide (4) to give a glycodendrimer with 18-exposed lactoside residues.
Molecules 2016, 21, 629
Molecules 2016, 21, 629
24 of 36
24 of 35
Scheme
Lactose-presentingglycoclusters
glycoclusters with
with an
focal
point
were
used
Scheme
18.18.
Lactose-presenting
an azide
azidefunctionality
functionalityatatthe
the
focal
point
were
used
for the convergent synthesis of highly substituted glycodendrimers 79 (3-mer), 80 (5-mer), and 83
for the convergent synthesis of highly substituted glycodendrimers 79 (3-mer), 80 (5-mer), and 83
(10-mer) possessing an extended tetraethylene glycol spacer for accessibility to galectins.
(10-mer) possessing an extended tetraethylene glycol spacer for accessibility to galectins.
Alternatively, globular-shaped glycodendrimers having six (6-mer, 87), eighteen (18-mer, 88),
Alternatively,
globular-shaped
glycodendrimers
six (6-mer,
eighteen
(18-mer,
and
and
thirty six (36-mer,
90) were constructed
using thehaving
extended
lactoside87),
residues
(Scheme
19) 88),
[126].
thirty
six (36-mer,
were constructed
using the extendedderivative
lactoside residues
19) [126]. Thus,
Thus,
using a 90)
hexapropargylated
cyclotriphosphazene
(84) and (Scheme
the copper-catalyzed
using
a hexapropargylated
cyclotriphosphazene
(84) and
the copper-catalyzed
[1,3]-dipolar
[1–3]-dipolar
cycloaddition
together with azidederivative
75, the three
compounds
were prepared
in 82%,
65%,
and
77%
yields,
respectively.
After
Zemplén
trans
esterification,
the
unprotected
lactosylated
cycloaddition together with azide 75, the three compounds were prepared in 82%, 65%, and 77%
1H-NMR spectra clearly probed completion of
dendrimers
were obtained.
Interestingly,
yields,
respectively.
After Zemplén
trans high-field
esterification,
the unprotected lactosylated dendrimers
1
theobtained.
multiple click
processes,high-field
as judged by
the complete
disappearance
of the
critical alkyne
were
Interestingly,
H-NMR
spectra
clearly probed
completion
of theprotons
multiple
at
δ
2.64
ppm.
In
addition,
all
the
relative
integrations
of
each
triazole
protons
around
δ
8
ppm
click processes, as judged by the complete disappearance of the critical alkyne protons atwere
δ 2.64
in perfect
agreement
withrelative
those ofintegrations
the newly formed
internal
region
(18Haround
vs. 6H, respectively
for in
ppm.
In addition,
all the
of each
triazole
protons
δ 8 ppm were
18-mer 88). Moreover, MALDI-TOF experiments afforded typical isotopic patterns for 90 with the
perfect agreement with those of the newly formed internal region (18H vs. 6H, respectively for
expected molecular weight signal ([M + Na]+ adduct) at m/z 23,750 Da. Using an analogous strategy,
18-mer 88). Moreover, MALDI-TOF experiments afforded typical isotopic patterns for 90 with
lactodendrimers harboring 20-mer, 30-mer, 60-mer, and finally 90-mer were also constructed (Scheme 20)
the expected molecular weight signal ([M + Na]+ adduct) at m/z 23,750 Da. Using an analogous
starting from thiolated pentaerythritol and hexasubstituted benzene derivatives [126]. The above
strategy, lactodendrimers harboring 20-mer, 30-mer, 60-mer, and finally 90-mer were also constructed
Molecules 2016, 21, 629
25 of 36
Molecules 2016, 21, 629
25 of 35
(Scheme 20) starting from thiolated pentaerythritol and hexasubstituted benzene derivatives [126].
Thestructures
above structures
weretoshown
to have
low polydispersity
PDI (Mw/Mn)
(<1.08)
with molecular
were shown
have low
polydispersity
PDI (Mw/Mn)
(<1.08) with
molecular
weight
weight
ranging
to than
more75than
kDa [126].
ranging
from from
5 kDa5tokDa
more
kDa75
[126].
O
O
O
O
O
O
N
N
N
O
N
N
O
O
O
O
N
O P P O
N N
P
O O
O
N
N
N N
O
O
O
O
O
N N
N
O
O
O
O
O
N
N
N
O
O
O
O
N
O
O
O
O
O
87 R = H
N
N N
N
O
O
O
N
N
O
N
N N
O
O
MeONa , MeOH
r.t. , 18h
90%
O
O
86 R = Ac
88 R = H
O
O
O
O
O
75
O
O
N
O
84%
THF/H2O
(3h.) , r.t. (18h.)
CuSO4×5H2O , Na Asc
O
N
O
O
O
O
50oC
N
O
O
NH
O
O
O
O
O
O
O
O
O
O
N
N
O
O
85 R = Ac
N
N N
N
N N
O
O
N
N
N
O
O
O
N
NH
O
N
N
N
O
O
O
O
N
N N
O
O
O
O
O
N
N N
O
N
N
N
N
O
H
N
O
O
O
MeONa , MeOH
r.t. , 18h
82%
O
O
O
N
P P O
O
N N
P
O O
N
H
O
N
N
N N
N
O
O
O
N
N
N
N
O
N N
N
O
N
O
O
O
O
O
O
O
O
N
O
O
N
N
O
O
HN
N N
N
N
N
O
N
N
HN
O
N
O
N
N
N
O
O
O
O
O
O
N
O
O
O
N
N
N
N N
N
N N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
THF/H2O
50oC (3h.) , r.t. (18h.)
CuSO4×5H2O , Na Asc
79
65%
O
O
O
OR
O
RO
O
O
N
P P O
O
N N
P
O O
RO
O
89 R = Ac
O
O
84
OR
O
O
RO
RO
RO
NaOMe, MeOH
r.t., 18 h
93%
90 R = H
80 CuSO4×5H2O , Na Asc
THF/H2O
50oC, 3h, r.t. 18h
77%
O
O
O
O
O
O
N
O
N
N
N N
N
N
N
O
O
N N
N
O
N
O
O
N N
N
O
O
N N
N
O
O
O
O
O
N
N
N
O
O
O
O
O
N N
N
O
O
O
O
N
N
N
O
O
O
O
O
N
N
O
O
O
N
O
N
HN
O
N
O
O
O
N
N
N
N
O
N
N
N
N
N
O
O
N
H
O
N
N N
O
N
N N
N
N N
O
O
O N
P P O
O
N PN
O O
O
O
N
N
N
O
N
N
O
O
O
N
N N
O
O
O
N
O
O
N
N
N N
O
O
O
O
O
O
O
O
O
O
O
N
O
N
O
O
O
O
O
O
O
N NN
N
O
O
N
N N
O
O
O
O
O
O
O
O
O
O
N
N N
O
O
O
O
O
NN
N
N N
O
O
O P
N N O
O P NP
O
O
O
O
O
O
HN
O
O
O
O
O
O
O
N
O
O
O
N
O
N
N
O N O
P P O
ON N
P
O O
O
O
O
NH
N
O
O
N
P P O
O
N N
P
O O
N
N
N
O
N
O
H
N
O O
P
N NO
O P NP
O
O
O
N
N
N
N
O
O
O
O
NN
N
O
O
O
N N
N
O
O
O
O
NH
O
O
O
N
N
O
O O
P
N N O
O P P
N O
O
O
O
O
N N
N
O
O N O
P P O
N PN
O
O
O
O
O
N
N N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Scheme 19. Convergent synthesis of a globular glycodendrimer with six (6-mer, 87), eighteen
Scheme
19. Convergent synthesis of a globular glycodendrimer with six (6-mer, 87), eighteen
(18-mer, 88), and thirty six (30-mer, 90) sugar headgroups.
(18-mer, 88), and thirty six (30-mer, 90) sugar headgroups.
Molecules 2016, 21, 629
26 of 36
Molecules 2016, 21, 629
26 of 35
O
O
O
O
O
O
O
O
O
O
O
O
N
N N
O
O
N
N N
O
O
N
NN
O
O
O
O
O
O
O
O
O
O
NN
N
O
O
N
NN
O
O
NN
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
NN
N
O
N
H
O
NN
N
NN
N
O
O
O
O
O
O
O
O
O
O
O
NN
N
NN
N
NN
N
O
NN
NN
N
N
N
N
N N
N
NN
N NN
NN N
N
N
N
O O
N NN
O
O
O
O
O
O
O
N
O
O
O
HN
O
HN O
O
O NH
O
HN O
O
NN
N
NN
HN
NN
N
NN
N N
N
N
O
N
O
O
N
O
O
N
O
N HN
O
O
N
O
O
O
O
N
N
O
N
OP P O
O NP
O
N N
P
N
P
ONP O
O O
O
O
HN
O
NN
O
O
N
HN
O
O O
N
H
O
NN
N
O
NN
O
N
N
N
N
O
O
NH
O
NN
N
O
NN O
N
O
N
N
O
O
O
O
O
O
O
O
O
91 (20-mer)
O
O
NN
N
N
N N
O
O
O
O
O
N NN
O
O
O
O
O
N NN
O
O
O
O
O
O
O
O
N NN
O
O
NN
N
O
O
O
O
O
O NPO
P N
ONPO
O
O
O
O
H
N
S
N
NN
O
O
O
N
N NN
O
O
O
O
O
O
N
N
N
O
O
O
O
O
O
O
O
N
N
N
O
O O
NP
OP N
P O
O NO
N
N
N
N
N
N O
O
O
O
O
O
S
S
O
O
O
O
O
O
O
O
S
O
O
S
NH
O
N
NN
N
N
O
NH
O
HN
NH
O
N
NN
O
O O
P
N N
O PNP O
O
O
O P NO
N P
O
O P N
O
O
N
S
O
N
NN
O
O
H
N
O
O
NN
N
O
N
NN
N
NN
N
N
O
N
O
O
N
NN
N
NN
N
NN
O
HO
OH
O
O
HO
HO
O
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
NN
O
N
N
N
NN
N
N
N
N
O
O
N
N
N
O
O
N
H
O
O
O
O
O
O
O
O
N
NN
O
O N
NN
O
O
N
NN
O
O
S
O
O
O
O
O
O
O
N
NN
O
O
O
O
N NN
N NN
O
O
O
O
O
O
O
O
N
NN
O
HN
O
O
O
ONP O
N
P
O NP O
O
94 (90-mer)
HN
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
NN
O
O
N
N NN
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
NN
O
O
O
N
O
NN
O
O
N
N
O
N
N
N
O
N
O
O
O
O
O
O
92 (30-mer)
O
O
N
N NN
O O
N
N N
N
O
N
N
NN
O
O
NN
O
O
O
O
O
O
O
HN
O
O
O
O
O
N
NN
O
N HN
N
N
N
HN
O
O
O
O
N
N
N
N
O
O
N
N
O
O
O
NN
N
NN
N
NN
O
N
NN
O
O
O
O
O
O
O
O
HN
S
O
O
O
O
O
O
N
NN
N NN
O
O
O
O
O
O
O
O
N
N
N
O
NN
N
O
N
N
N
O
O
O
O
O
O NPO
P N
ONPO
O
O
O
O
H
N
N
NN
O
O
O
O
NH
O
OP N O
N P
OP N O
O
O
N NN
S HN
S O
O
O
O
O
N
N
N
N
N
O
N
O
O PON
O
N P
OP N O
O
O
O
O
O
N
N N
O
O
O
O
O
O
O
NN
N
O
O
S
O
NN
O
N
O
HN
O
S
NH S
O
O NPO
P N
O NP
O O
NN
N
O
O
O
O
O NPO
P N
O NPO
O
O
O
OP NO
N P
OP N O
O
O
O
O
O
NN O
N
O
O
O
N
O
N
N
N
N N NN N
N
N N
O
O
O
O
O
O
O
O
O
O
NN
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
O
HO
O
O
O
O
O
O
N
O
O
NN
O
O
NH N
O
O
O
O
N
N
O
N NN
O
N
O
O
NN
N
N
N
O
N
N
H
NN
N
N
NN
NN
O
NN
N
NN
NN
NH
NN
O
O
N
O
O
O
N
N
N
O
O
N
NH
HN
O
O
O NH
N
O O
O
O
N
NH
N
N
O
N
O
O
O
O
N
O
O
N
O
O
O
O
N N N
O
O
N
O O
N N
N N
N
O
N
N
N
N
N
O
N NN
NN
O
N N
NN N
NN NN N
O
NN
N
N
NN
NN
O
O
N
N
O
O
NN
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
N
O
O
O
O
N
O
N
N
N
O
N
O
N
O
NN
O
N
O
O
O
NN
O
N
O
93 (60-mer)
N
O
O
O
O
HN
S
O
O
O
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
NH
O
N
N
N
O
O
O
HN
O
N
O
N
N
O N
O
N
N
N
N
N
O
O
O
O
O O
O P NP
N NO
P
O O
HN
O
O
O
O PO
N
N PO
OP N O
O
O
O
O
O
NN
N
N
N N
O
N
O
N N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
6
Scheme 20. Illustration of the structures of variations of the theme to build glycodendrimers with the
Scheme 20. Illustration of the structures of variations of the theme to build glycodendrimers with the
same core as in Scheme 18 harboring 20 (91), 30 (92), 60 (93), and 90 (94) lactoside residues.
same core as in Scheme 18 harboring 20 (91), 30 (92), 60 (93), and 90 (94) lactoside residues.
The size of the glycodendrimers containing both the acetylated and the deprotected functionalities,
The
sizeparticularly
of the glycodendrimers
containing
acetylated
the deprotected functionalities,
and more
their solvodynamic
radii,both
wasthe
estimated
by and
pulsed-field-gradient
stimulated
and
more
particularly
their
solvodynamic
was pulse
estimated
pulsed-field-gradient stimulated
echo
(PFG-STE)
NMR
experiments
usingradii,
bipolar
pairsbylongitudinal-eddy-current
delay
(BPP-LED)
in CDCl
3 and
D2O, respectively,
25 °C, pulse
as described
in detail [126]. Diffusion values
echo
(PFG-STE)
NMR
experiments
using at
bipolar
pairs longitudinal-eddy-current
delay
˝
were
determined
by
the
average
of
individual
values
corresponding
to
the
decay
of
the
signal
intensity
(BPP-LED) in CDCl3 and D2 O, respectively, at 25 C, as described in detail [126]. Diffusion values were
of differentbyprotons
located
at different values
level incorresponding
the molecule. to
The
were of
determined
the average
of individual
theresulting
decay ofmeasurements
the signal intensity
consistent
with located
spherical
and unimolecular
character
of the
The Stokes–Einstein
different
protons
at different
level in the
molecule.
Theglycodendrimers.
resulting measurements
were consistent
equation
directly
yielded
the
corresponding
solvodynamic
diameters
using
viscosities
determined
with spherical and unimolecular character of the glycodendrimers. The Stokes–Einstein
equation
for
pure
deuterated
solvents.
As
expected,
diffusion
data
revealed
the
increase
of
solvodynamic
directly yielded the corresponding solvodynamic diameters using viscosities determined for pure
diameters solvents.
as the number
of peripheral
epitopes
enhanced,
under similar
solvent conditions,
deuterated
As expected,
diffusion
data was
revealed
the increase
of solvodynamic
diameters
showing dendrimers with solvodynamic diameters ranging from 2.6 to 10.1 nm. Systematic monitoring
as the number of peripheral epitopes was enhanced, under similar solvent conditions, showing
of bioactivity with the CRD of galectin-3, physiologically a product of proteolytic processing that
dendrimers with solvodynamic diameters ranging from 2.6 to 10.1 nm. Systematic monitoring of
removes the N-terminal tail (please see Figure 3), revealed ligand property for all compounds. As
bioactivity with the CRD of galectin-3, physiologically a product of proteolytic processing that removes
noted above for starburst glycodendrimers, the relative potency per sugar unit was highest at low
the N-terminal tail (please see Figure 3), revealed ligand property for all compounds. As noted above
to medium level of valency, here for the dumbbell-shapes decavalent 83 (r.p./sugar = 53) of a diameter
forofstarburst
glycodendrimers, the relative potency per sugar unit was highest at low to medium level
5.3 nm [126]. These examples document the ingenuity of chemical design to reach the level of
of giant
valency,
here with
for the
dumbbell-shapes
decavalent
83 (r.p./sugar
= 53) of
diameter ofvesicle-like
5.3 nm [126].
clusters
a core.
To take design
even further
into the direction
of aengineering
These
examples
document
the
ingenuity
of
chemical
design
to
reach
the
level
of
giant
clusters
with
structures, a different strategy has been successfully implemented.
a core. To take design even further into the direction of engineering vesicle-like structures, a different
strategy has been successfully implemented.
Molecules 2016, 21, 629
27 of 36
The way (glyco)lipids self-organize into monolayers or liposomes has been equally efficiently
achieved with amphiphilic Janus glycodendrimers (for a comparison of a glycolipid and the synthetic
analogue,
please
Scheme 21) [127]. The products are glycodendrimersomes or aggregates
of
Molecules
2016,see
21, 629
27 of 35
other types of design such as cubosomes with fully programmable properties in size and surface
The way (glyco)lipids self-organize into monolayers or liposomes has been equally efficiently
properties [127,128].
For example, ligand density on micro- and macroscales can be changed in
achieved with amphiphilic Janus glycodendrimers (for a comparison of a glycolipid and the synthetic
a definedanalogue,
manner,please
as branching
in glycodendrimers is. These products are ideal for interaction studies
see Scheme 21) [127]. The products are glycodendrimersomes or aggregates of
with lectins.
Here,
the
influence
of
parameter
both
sides on aggregation
examined.
These
other types of design such as any
cubosomes
with on
fully
programmable
properties incan
sizebe
and
surface
properties
[127,128].
For have
example,
ligandanswered
density onfundamental
micro- and macroscales
canonbegalectin
changed functionality
in a
assays with
human
galectins
already
questions
defined manner,and
as branching
in glycodendrimers
is. Thesedensity
products and
are ideal
for interaction
studies
in trans-interactions
as a function
of carbohydrate
headgroup
display
[129–131].
with lectins. Here, the influence of any parameter on both sides on aggregation can be examined.
Galectin-1’s
design as non-covalently associated homodimer (please see Figure 3) appears more suited
These assays with human galectins have already answered fundamental questions on galectin
for cross-linking
counter
receptors onand
theassurface
than
act as bridge
between
glycodendrimersomes,
functionality
in trans-interactions
a function
of to
carbohydrate
density
and headgroup
display
as indicated
by
comparative
analysis
with
engineered
variants
that
have
covalent
connections
[129–131]. Galectin-1’s design as non-covalently associated homodimer (please see
Figure 3) between
suited
for cross-linking
on the surface
to act as bridge
between
the two appears
CRDs more
[129].
Fittingly,
atomiccounter
force receptors
measurements
havethan
revealed
a comparatively
low
glycodendrimersomes,
as
indicated
by
comparative
analysis
with
engineered
variants
that
have
rupture of 37 ˘ 3 pN at a loading rate of 3 nN/s, when galectin-1 interacted with N-glycans of
covalent connections between the two CRDs [129]. Fittingly, atomic force measurements have revealed a
the glycoprotein asialofetuin [132]. An emerging aspect for endogenous lectins is the occurrence of
comparatively low rupture of 37 ± 3 pN at a loading rate of 3 nN/s, when galectin-1 interacted with
genetic polymorphisms.
It is a pertinent
question
they
haveforpathophysiological
N-glycans of the glycoprotein
asialofetuin
[132]. whether
An emerging
aspect
endogenous lectins issignificance.
the
As biomedical
effector,
it ispolymorphisms.
noteworthy that
lectinquestion
can have
anti-they
andhave
pro-inflammatory/tumoral
occurrence
of genetic
It is athis
pertinent
whether
pathophysiological
biomedical
it is noteworthy
thisinvasion
lectin can
have anti- and
activitiessignificance.
depending As
on the
context,effector,
for example
stimulatingthat
tissue
(glioblastoma)
or cartilage
pro-inflammatory/tumoral
activities
depending
on
the
context,
for
example
stimulating
tissue
degeneration (osteoarthritis) or acting as negative regulator in chronic inflammation or carcinoma cell
invasion (glioblastoma) or cartilage degeneration (osteoarthritis) or acting as negative regulator in
cycle progression [133–139].
chronic inflammation or carcinoma cell cycle progression [133–139].
Scheme 21. Typical structure of a natural glycolipid (LacCer) compared to a member of the recently
Scheme synthesized
21. Typicalfamily
structure
of a naturalamphiphilic
glycolipid
(LacCer)
compared
to a member
the recently
of self-assembling
Janus
glycodendrimers
capable
of formingof
stable
synthesized
family
of self-assembling amphiphilic Janus glycodendrimers capable of forming stable
liposomes
(glycodendrimersomes).
liposomes (glycodendrimersomes).
For the first time, a functional correlation of a single nucleotide polymorphism in a CRD of a
galectin could be discerned with this assay system, here for galectin-8’s F19Y variant whose
For expression
the first time,
a functional
correlation
of ofa rheumatoid
single nucleotide
polymorphism
in a CRD of
is strongly
associated with
occurrence
arthritis and
early age at disease
onset
[130,131,140].
This approach
thus takes
testing
to the
of vesicle-like
structures,
a galectin
could
be discerned
with this
assayactivity
system,
here
forlevel
galectin-8’s
F19Y
variant whose
andis
thestrongly
recently accomplished
hybridoccurrence
formation with
membranes
[141]and
let the
perspective
expression
associated with
of bacterial
rheumatoid
arthritis
early
age at disease
become envisioned to combine natural glycoconjugates with these synthetic glycodendrimers to
onset [130,131,140].
This approach thus takes activity testing to the level of vesicle-like structures, and
tailor surface properties in a so far unsurpassed manner according to the biomedical question to be
the recently
accomplished
formation
with
bacterial
[141]
let the
addressed.
Design ofhybrid
monolayers
with the
same
toolboxmembranes
will flank the
analysis
of perspective
aggregation become
envisioned
to
combine
natural
glycoconjugates
with
these
synthetic
glycodendrimers
to tailor surface
(trans-interaction). In such a setting, cis-bridging can be monitored, as exemplified by monitoring
galectin-1/ganglioside
GM1
interplay
[142].
properties in a so far unsurpassed manner according to the biomedical question to be addressed.
Design of monolayers with the same toolbox will flank the analysis of aggregation (trans-interaction).
In such a setting, cis-bridging can be monitored, as exemplified by monitoring galectin-1/ganglioside
GM1 interplay [142].
Molecules 2016, 21, 629
28 of 36
6. Conclusions
The increasingly unraveled physiological significance of carbohydrate-lectin recognition poses
challenging questions on the nanometric characteristics underlying the specificity and selectivity of the
functional pairing. As the example of the adhesion/growth-regulatory galectins attests, delineating
structure-activity relationships with respect to the intrafamily divergence in CRD sequence and its
spatial presentation requires sophisticated tools and transsectional cooperation. Here, the individual
parts of the toolbox provided by synthetic chemistry are invaluable sensors, e.g., to determine the
capacity for multivalent binding and induction of cell signaling that may cause harmful effects.
Actually, glycoclusters had been helpful to ascertain a reduction in cross-linking, leading to undesired
T cell activation, within the process of engineering an antiviral lectin for further testing of its potential
for therapeutic application [143]. Along the route to obtain insights into the biofunctionality of the
nanometric features, bringing test systems for activity assessment of the synthetic products as close as
possible to the in vivo conditions, e.g., by testing their effect on lectin binding to tissue sections [144],
will team up with the design of vesicle-like structures with increasing degree of resemblance to cell
surfaces. By rational tailoring on both sides, i.e., the ligand and the lectin, as hypothesis-driven process,
these advances bring understanding how endogenous lectins ‘read’ sugar-encoded information
into reach.
The monovalent [145,146] as well as the multivalent glycomimetic lactoside derivatives presented
herein offer several avenues to identify high affinity galectin-selective antagonists for the treatment
of various diseases. Nanometric glycodendrimers in particular, with their size ranging from 2 to
10 nm and their high water-solubility, constitute a valuable alternative to other types of insoluble
nanoparticles [147]. Of great interest in this regard, is the new family of glycodendrimersomes
described in the last section which are also nanometric (100–150 nm, for liposomes) or 100–200 µm
when forming giant vesicles. These biomimetics of cell membranes have the fortuitous possibility to
adapt their flexible inter-carbohydrate distances in response to the galectin receptors they encounter,
thus adding an additional advantage over other nanoparticles.
Acknowledgments: R.R. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC)
for a Canadian Research Chair and the Canadian Glycomics Network for financial support. P.V.M. thanks
Science Foundation Ireland for financial support (08/SRC/B1393 & 12/IA/1398) which was co-funded under
the European Regional Development Fund under Grant No. 14/SP/2710. P.V.M. also thanks the European
Commission for Marie Curie IntraEuropean Fellowships to Sebastien G. Gouin, Trinidad Velasco-Torrijos and
Dilip V. Jarikote, which supported some of the research reviewed herein. H.-J.G. thanks the excellence program of
the Ludwig-Maximilians-University Munich and the EC (for ITN network funding) for generous support.
Conflicts of Interest: The authors declare no conflict of interest.
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