Carbohydrate Research 358 (2012) 96–105
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Conformational study of the open-chain and furanose structures of D-erythrose
and D-threose
Luis Miguel Azofra a, Ibon Alkorta a,⇑, José Elguero a, Paul L. A. Popelier b,c
a
Instituto de Química Médica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain
Manchester Interdisciplinary Biocentre (MIB), 131 Princess Street, Manchester M1 7DN, United Kingdom
c
School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
b
a r t i c l e
i n f o
Article history:
Received 3 May 2012
Received in revised form 18 June 2012
Accepted 19 June 2012
Available online 27 June 2012
Keywords:
D-Erythrose
D-Threose
DFT
NBO
AIM
a b s t r a c t
The potential energy surfaces for the different configurations of the D-erythrose and D-threose (openchain, a- and b-furanoses) have been studied in order to find the most stable structures in the gas phase.
For that purpose, a large number of initial structures were explored at B3LYP/6-31G(d) level. All the minima obtained at this level were compared and duplicates removed. A further reoptimization of the
remaining structures was carried out at B3LYP/6-311++G(d,p) level. We characterized 174 and 170 minima for the open-chain structures of D-erythrose and D-threose, respectively, with relative energies that
range over an interval of just over 50 kJ/mol. In the case of the furanose configurations, the number of
minima is smaller by approximately one to two dozen. G3B3 calculations on the most stable minima indicate that the a-furanose configuration is the most stable for both D-erythrose and D-threose. The intramolecular interactions of the minima have been analyzed with the Atoms in Molecules (AIM) and Natural
Bond Orbital (NBO) methodologies. Hydrogen bonds were classified as 1-2, 1-3 or 1-4, based on the number of C–C bonds (1, 2 and 3, respectively) that separate the two moieties participating in the hydrogen
bond. In general, the AIM and NBO methodologies agree in the designation of the moieties involved in
hydrogen bond interactions, except in a few cases associated to 1-2 contact which have small OH O
angles.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Carbohydrates are the most abundant organic compounds on
Earth, in terms of their total mass found in living organisms. They
show a large number of functions, ranging from energy storage, over
structural material, to bacterial and viral recognition targets. The
structural properties of monosaccharides are mainly determined
by the presence of a carbonyl group or a hemiacetal moiety and a
variable number of hydroxyl groups. Numerous DFT and ab initio
studies have shown the considerable conformational flexibility of
carbohydrates. Thus, it does not come as a surprise that the complexity of the conformational space of numerous carbohydrates
has been described in the literature: glucose,1–9 allopyranose,10
galactopyranose,11 mannopyranose,12 idopyranose,13 fructofuranose14 as well as the open-chain configurations of erythrose and
threose.15 In some cases, the effect of the inclusion of explicit
solvent molecules on a monosaccharide’s conformation has been
examined.6,16–19 In spite of the considerable interest in carbohydrates, very few studies have focused on the conformational
⇑ Corresponding author. Fax: +34 91 564 48 53.
E-mail address: [email protected] (I. Alkorta).
URL: http://www.iqm.csic.es/are.
0008-6215/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2012.06.011
preference of the smaller carbohydrates such as tetroses and pentoses. D-Erythrose and D-threose are the two naturally occurring
members of the aldotetroses family (aldoses with a total of four carbon atoms in their skeleton). The only difference between these two
molecules is the configuration of the hydroxyl group attached to C2
(Fig. 1). Experiment20,21 has shown that, in aqueous solution, openchain conformations of D-erythrose and D-threose are in equilibrium
with a mixture of the corresponding a- and b-furanoses, which arise
by internal hemiacetal cyclization.
To the best of our knowledge, only the open-chain conformations
of the D-erythrose and D-threose have been studied in the literature
using DFT and ab initio methods.15 In the present article, the conformations of the two anomeric forms of the furanoses, a- and b-, as
well as the open-chain structures have been examined.
2. Computational methods
The conformational searches were conducted in two steps. In
the first step, a large number of structures were generated for each
of the three possible configurations (open-chain, a-furanose, and
b-furanose). The initial structures of the open-chain configuration
were generated starting from the combination of three possible
values of each rotatable bond: gauche, gauche,0 and trans (g, g0
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
Open-chain
4
D-erythrose
HO
O
2
D-threose
HO
3
4
OH
1
2
3
4
OH
OH
HO
OH
2
0
O
1
OH
4
0
OH
3
β-furanose
α-furanose
1
4
HO
1
2 OH
HO
1
OH
0
O OH
3
2
3
0
O
OH
O
4
OH
O OH
1
3
2
HO
Figure 1. Open-chain and a- and b-furanose configurations of D-erythrose and D-threose. This numeric labeling will be used throughout the article.
density function locates critical points, points where the gradient
of the electron density is null. These critical points are classified
based on the sign of the local curvature of the electron density.
Thus, it is possible to find maxima [denoted (3, 3)] or minima
[(3, +3)], as well as two types of saddle points, (3, +1) and (3,
1). The (3, 3) critical points practically coincide with nuclear
positions, the (3, 1) points are known as bond critical points,
the (3, +1) corresponds to ring critical points and the (3, +3) minima
are called cage critical points. In general, the bond critical points
can provide elementary information on the covalent and weak
interactions present in molecules and molecular complexes. Here
they are used to characterize the intramolecular interactions of
the molecules considered.
The Natural Bond Orbitals (NBO) theory36 analyzes the orbital
interactions. Most of the donor–acceptor interactions, for example
hydrogen bonds, donate from a filled orbital of the electron donor
to an empty orbital of the electron acceptor. These calculations
were carried out with the NBO-3.1 program.37
D-erythrose
D-threose
Open-chain structures
60
50
40
Erel (kJ/mol)
and t). In the open-chain configuration of D-erythrose and D-threose, there are six rotatable bonds, and consequently the number
of initial structures is 729 (=36) for each molecule. In the case of
the a- and b-furanose configurations, 20 different conformations
of the ring were taken into account (10 envelope and 10 twist
conformations) for each case. In addition, three different possible
positions of each hydroxyl group were examined (g, g0 and t). Thus,
the total number of conformations initially considered for each
furanose configurations is 540 (=2033). All these structures were
optimized at B3LYP/6-31G(d) level.22,23 The optimized structures
were compared among them in order to remove duplicates. For
that purpose, an in-house program was written that systematically
compared all the structures obtained and removed those that show
root mean square values smaller than 0.05 Å when all atomic coordinates are compared. This cutoff value separates the structures
into two groups: those having similar geometries that only differ
due to the numerical optimization procedure (<0.05 Å) and all
the others (>0.05 Å) that are considered different. The unique
structures at B3LYP/6-31G(d) level were reoptimized at B3LYP/6311++G(d,p)24 level and compared again; this led to the elimination of some more structures. Vibrational frequencies were also
calculated at B3LYP/6-311++G(d,p) level to confirm that the final
structures indeed correspond to minima.
In some cases, G3B3 calculations25,26 were performed to obtain
more accurate energy values in vacuum for the most stable conformers. The G3B3 method, which is a modification of the original
G3 method, uses optimized geometries at B3LYP/6-31G(d) level,
and then carries out QCISD(T), MP4 and MP2 calculations with large
basis sets in order to improve the energy. Thus, it is computationally less expensive than the original G3 method with a similar quality. All calculations were performed using the GAUSSIAN09 package.27
In order to characterize the conformation of the furanose rings,
the pseudorotation parameter P and the amplitude Q were calculated using the Cremer–Pople method.28 The ring puckering analysis methodology developed by Cremer and Pople is based on the
search of structural parameters from the midplane of the ring.
The two most important ones are the P parameter, which classifies
the conformation of the ring (in the case of furanoses envelope or
twist), and the Q parameter, which describes the total puckering
amplitude, that is, how much the structure is distorted with respect to the planar case. The P and Q parameters were calculated
with the RING96 program28,29 and automatically assigned to the corresponding conformation of the Altona-Sundaralingam conformation ring30,31 with a program written in our group.
The electron density of the systems has been analyzed by the
Atoms In Molecules (AIM) methodology32,33 using the MORPHY98
and AIMAll programs.34,35 The topological analysis of the electron
30
20
10
0
0
50
100
150
Ranking
Figure 2. Ranking of all open-chain conformations of D-erythrose and D-threose
according to the energies (B3LYP/6-311++G(d,p) level) relative to their respective
global minima.
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
3. Results and discussion
3.1. Energy and conformation of the minima
A total of 174 and 170 minima were found for the open-chain
configuration of D-erythrose and D-threose, respectively. These
numbers are much higher than those reported by Aviles-Moreno
and Huet15 where only 14 and 15 conformers where described.
The number of minima obtained in the furanose configurations is
much smaller due to the restrictions imposed by the ring. Thus,
the total number of minima is 14, 16, 22 and 19 for a-D-erythrofuranose, b-D-erythrofuranose, a-D-threofuranose and b-D-threo-
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
Figure 3. Molecular graphs of the most stable conformers of the open-chain configuration of the D-erythrose and D-threose calculated at B3LYP/6-311++G(d,p) level. The
relative energy with respect to the most stable conformer corresponds to B3LYP/6-311++G(d,p) and G3B3 (in parenthesis) computational levels. The values for the CCCC
dihedral bond angles are given. The position of the bond and ring critical points calculated within the AIM methodology is indicated with green and red dots, respectively.
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
ϕ
ϕ
ϕ
ϕ
ϕ
ϕ
Fig. 3 (continued)
furanose, respectively. The energies of the minima for the
open-chain configurations stretch over a range of 50 kJ/mol.
Fig. 2 ranks all open-chain conformations of D-erythrose and Dthreose according to the energies relative to their respective global
minima. In the case of D-erythrose, Fig. 3 shows 13 structures with
an Erel smaller than 10 kJ/mol and only 5 structures below this energy threshold in the case of D-threose.
The most stable minima in the open-chain configuration show
an intramolecular hydrogen bond between the hydroxyl group
attached to C-2 and the carbonyl moiety (C1@O). Although intramolecular hydrogen bonding is not a unique feature of the most
stable structures, it occurs in the set with high frequency. The
frequency of this hydrogen bond occurring decreases in the deciles
of energy ranked open-chain D-erythroses: 1-10: 9 times, 11-20: 6
times, 21-30: 3 times, 31-40: 3 times, 41-50: 2 times, 51-60: 4
times, 61-100: 2 times, 101-174: 1 time. In addition, the most
stable conformer of D-erythrose shows a C1H O4 contact while
in D-threose the O4H interacts with O2, generating a hydrogen
bond (HB) chain with the O2H O1 HB. In some cases, stabilizing
interactions between oxygen atoms and the carbonyl carbon atom
similar to the ones recently described in the conformational study
of the salicylic acid can be found.38 The conformations in which
these interactions appear can be considered as the precursor to
furanose formation.
The G3MB3 calculations return similar relative energies compared to those obtained at B3LYP/6-311++G(d,p) level shown in
Figure 3. The two levels predict the same conformation as the most
stable for both D-erythrose and D-threose. The most stable minima
found for the open-chain configuration of D-erythrose are exactly
the same as those described by Aviles-Moreno and Huet.15 However, in the case of D-threose, the most stable minimum found by
these authors corresponds to the third minima in our search, with
a relative energy of 5.8 and 6.3 kJ/mol at B3LYP/6-311++G(d,p) and
G3B3 levels, respectively.
The comparison of the B3LYP/6-311++G(d,p) and G3B3 energies
shows that in the cases of a-D-erythrofuranose and b-D-threofuranose, both levels predict the same conformation as the most stable.
In the case of b-D-erythrofuranose and a-D-threofuranose, the most
stable conformation changes from one level to the other since
several minimum structures are found with very small relative
energies (three minima less than 1 kJ/mol in a-D-threofuranose
and four less than 2 kJ/mol in b-D-erythrofuranose). For either case,
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
alpha-erythrose
beta-erythrose
alpha-threose
beta-threose
30
lations carried out within the G3B3 composite method agree with
the relative energies obtained at the G3B3 level.
The populations derived from the energy calculations for all the
conformers agree with those reported in solution for threose indicating a population of 51% and 38% for the a- and b-furanose configurations, respectively,21 while the remaining 11% is present as
the aldehyde hydrate, which has not been considered in this work.
25
3.2. Analysis of the intramolecular interactions: AIM and NBO
20
In order to analyze the intramolecular hydrogen bond interactions, two methodologies were used: the topological analysis of
the electron density within the AIM method and the Natural Bond
Orbital (NBO). In the AIM approach, the presence/absence of an
interatomic BCP determines the existence/non-existence of a HB
interaction. In contrast, in the NBO method, a numerical value is
obtained for the interaction of the lone pair of the oxygen atom
with the r⁄ H–O orbital. The AIM and NBO analyses will be discussed separately first and then compared.
Erel (kJ/mol)
35
15
10
5
0
1
6
11
Ranking
16
21
Figure 4. Ranking of the conformers of a- and b-D-erythrose, and a- and b-Dthreose according to the energies (B3LYP/6-311++G(d,p) level) relative to their
respective global minima.
the difference in relative energy is never larger than 2 kJ/mol for
the most stable conformers. The minima obtained for the furanose
configurations range between 25 and 35 kJ/mol with respect to the
most stable conformation in each configuration (Fig. 4, the structures have been numbered in increasing order of relative energies).
The number of structures with relative energy smaller than 10 kJ/
mol is, respectively, 5, 9, 7, and 4 for a-D-erythrofuranose, a-Dthreofuranose, b-D-erythrofuranose, and b-D-threofuranose (Fig. 5).
The presence of stabilizing intramolecular HBs is a constant for
all the low energy minima. The interacting moieties depend on the
relative orientation of the hydroxyl groups. Thus, in the a-Derythrofuranose, which presents the three hydroxyl groups on
the same side of the furanose ring, two HBs are found in several
conformations. In the rest of the configurations that show two
hydroxyl groups on one side of the furanose ring, only one HB is
observed. In addition, some Oxygen–Oxygen interactions are found
with bond paths that do not connect the expected hydrogen atom
with the oxygen atom.
At B3LYP level, the most stable structures correspond to an
envelope conformation of the ring, except in a-D-erythrofuranose,
where a 2-exo 3-endo twist conformation (2T3) is the most stable.
The most frequent configurations found in the most stable minima
are 2E and E2. Thus, 2E is present in two of the lowest energy conformations of a-D-erythrofuranose and in another two of a-Dthreofuranose, while E2 configuration is present in the b-form
(twice in b-D-erythrofuranose and thrice in b-threofuranose). A
graphical representation of the puckering parameters (P and Q)
of the most stable conformers (see Supplementary data, Table S1
and Fig. S1) shows that most of the conformers are in ‘southern’
forms.
Table 1 compares the energies of the most stable minima in
each configuration for erythrose and threose. In the case of erythrose, the most stable configuration corresponds to the a-furanose at the two computational levels. In the case of threose,
B3LYP/6-311++G(d,p) predicts that the open-chain is the most
stable while the G3B3 method favors a-furanose. Discrepancies between these two computational levels have previously been reported when comparing open and cyclic structures.39,40 The
analysis of our results shows that the single point QCISD(T) calcu-
3.3. Aim
The hydrogen bonds (HB) are characterized in the AIM methodology by the presence of a BCP associated with a bond path between an HB acceptor, here an oxygen atom, and the hydrogen
atom of the HB donor moiety, here a hydroxyl group. In the
open-chain configuration, the presence of CH O HB interactions
where the CH group corresponds to the terminal acetal group
(C-1) was observed in nine conformations. In general, and depending on the conformation, all oxygen atoms can be involved in HB
interactions, the only exception being the oxygen atom of the furanose ring, which is never involved in any BCP for all the examples
we have studied. However, the possibility that the ring oxygen
atom can be involved in HB interaction has been described for
the sucrose disaccharide.41
Based on the AIM criteria, the HBs have distances that range between 1.87 and 2.44 Å in the open configurations, and between
1.92 and 2.33 Å in the furanose configurations. The values of the
electron density at the BCP range between 0.030 and 0.010 au
and the corresponding Laplacian between 0.107 and 0.033 au,
which are within the ranges proposed almost two decades ago42
to characterize HBs based on electron density descriptors.
Figs. 3 and 5 show the molecular graphs, which include the critical points and bond paths of the most stable conformations of
open chain and each furanose configuration. The hydrogen bonds
have been classified as 1-2, 1-3, and 1-4 based on the number of
C–C bonds (1, 2, and 3, respectively) that separate the two interacting moieties. The total number of HB contacts of each type characterized for all the conformers are gathered in Table 2.
The representation of the interatomic distance H O versus the
OH O angle (Fig. 6) shows clearly the three types of HBs. Thus, for
a given interatomic distance, the smaller angles are associated
with a 1-2 HB, the intermediate angles with a 1-3 HB, and the
largest angles with a 1-4 HB. This classification is a clear indication
of the geometrical restrictions on the interaction due to the size of
the pseudo-ring formed.
The representation of q and r2q versus the intermolecular
distance (Fig. 7) shows again a clear differentiation between the
three different types of HBs. Thus, the largest values of q and r2q
are associated with 1-2 interactions, being smaller for the 1-4 HBs.
These results follow the tendencies described previously for those
cases where the interaction forms a cyclic structure; thus, for a given
distance, the values of the electron density descriptor become smaller as the size of the ring increases.43 These exponential relationships
between q and r2q versus the interatomic distance are in agreement with previous reports on intermolecular interactions.44,45
L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
101
α
4
1
3
2
α
β
Figure 5. Molecular graph of the most stable conformers of a-D-erythrofuranose, a-D-threofuranose, b-D-erythrofuranose and b-D-threofuranose calculated at B3LYP/6311++G(d,p) level. The relative energy with respect to the most stable conformer corresponds to B3LYP/6-311++G(d,p) and G3B3 (in parenthesis) levels. The conformation
assigned is indicated. The position of the bond and ring critical points calculated within the AIM methodology is indicated with green dots and red dots, respectively.
3.4. NBO analysis
As an example of the orbitals involved in a HB interaction
based on the NBO methodology, those responsible of one of
the HBs in the most stable conformer of the open-chain of D-erythrose and b-D-erythrofuranose have been represented in Figure
8. We bring in the standard cutoff value of 2.1 kJ/mol for the
orbital interactions, and only considered interactions with an
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
β
Fig. 5 (continued)
energy exceeding this threshold. A total of 597 interactions that
can be associated to HB interactions were found. The largest energy value of the orbital interaction is 42.8 kJ/mol.
All the HBs predicted by the AIM method are confirmed by
the NBO analysis except for a few cases where the energy is below
the 2.1 kJ/mol cutoff, the smallest value being 1.34 kJ/mol. Of those
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L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
Erythrose
B3LYP
G3B3
B3LYP
G3B3
Threose
a
Open-chain
a-Furanose
b-Furanose
7.3
16.7 (0%)
0.0
8.6 (1%)
0.0
0.0 (91%)
4.8
0.0 (69%)
9.4
7.7 (9%)
1.6
1.3 (29%)
(a) 0.038
0.033
1-2 HB
1-3 HB
1-4 HB
0.028
ρBCP
Table 1
Relative energy of the most stable minima of each configuration (kJ/mol) and
predicted population for each configuration at the G3B3 computational levela
0.023
All minima calculated for each configuration have been considered.
0.018
0.013
Table 2
Total number of HB interactions based on the AIM methodology found in all the
characterized conformers
(open-chain)
D-Threose
(open-chain)
a-D-Erythrofuranose
b-D-Erythrofuranose
a-D-Threofuranose
b-D-Threofuranose
1-2 HB
1-3 HB
1-4 HB
36
62
17
52
62
19
18
10
—
8
4
—
10
—
—
—
—
—
2.0
2.1
2.2
2.3
2.4
2.5
(b) 0.14
0.12
1-2 HB
1-3 HB
1-4 HB
0.10
0.08
0.06
160
155
0.04
1-2 HB
1-3 HB
1-4 HB
150
OH···O Angle
1.9
H···O Distance
LAP BCP
D-Erythrose
0.008
1.8
145
0.02
1.8
140
135
2.0
2.2
H···O Distance
2.4
Figure 7. (a) Electron density and (b) Laplacian at the BCP (au) versus the
interatomic distance (Å). The exponential relationships have square correlation
coefficients, R2, of 0.92, 0.98 and 0.99 for the electron density of the 1-2, 1-3 and 1-4
HBs and of 0.81, 0.98, and 0.99 for the Laplacian, respectively.
130
125
120
115
110
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
H···O Distance
Figure 6. Distribution of the H O distance (Å) versus the OH O (°) angles for the
1-2, 1-3 and 1-4 HB interactions.
interactions predicted by NBO but without the BCP that AIM would
require, the largest orbital interaction energy obtained was 8.4 kJ/
mol. Most interactions present in the NBO analysis but absent in
Open Chain D-erythrose
Oxygen lone pair
σ* H-O orbital
the AIM analysis are associated with 1-2 interactions with OH O
angles close to 110° or less.
The second order perturbation energy analysis within the NBO
methodology identifies those orbital interactions that stabilize
the energy of the system. The value of the orbital interaction
energy, E(2), provides an estimate of the donor–acceptor interaction. Here this interaction corresponds to the interaction of a lone
pair of an oxygen atom and the r⁄ of the interacting OH bond.
The representation of the E(2) versus the corresponding H O
interatomic distance (Fig. 9) for the three types of HBs, shows
that their values are mixed, especially for the 1-3 and 1-4 HBs.
β-D-erythrofuranose
Oxygen lone pair
σ* H-O orbital
Figure 8. Orbitals associated to the HB interaction in the most stable conformation of the open-chain D-erythrose and b-D-erythrofuranose.
104
L. M. Azofra et al. / Carbohydrate Research 358 (2012) 96–105
The NBO analysis shows the presence of more potential HB
interactions than those found by the AIM method. However, all
interactions detected with the NBO method but not with AIM show
small orbital interaction energy values and, in general, are associated with 1-2 contacts that have small OH O angles.
45
40
1-2HB
1-3HB
1-4HB
35
E(2)
30
25
Acknowledgments
20
15
10
5
0
1.80
1.90
2.00
2.10
2.20
2.30
2.40
2.50
O···H Distance
Figure 9. Second order perturbation NBO energy, E(2), (kJ/mol) versus the O H
interatomic distance (Å).
Supplementary data
45
40
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.carres.2012.06.011.
1-2HB
1-3HB
1-4HB
35
30
E(2)
L.M.A. thanks the Ministerio de Ciencia e Innovación for a Ph.D.
grant (No. BES-2010-031225). I.A. thanks the Ministerio de Educación (PR2009-0171) and the Royal Society of Chemistry for a travel
grant that allowed him to stay at the University of Manchester. We
also thank the Ministerio de Ciencia e Innovación (Project No.
CTQ2009-13129-C02-02) and the Comunidad Autónoma de Madrid (Project MADRISOLAR2, ref. S2009/PPQ-1533) for continuing
support. Gratitude is also due to the CTI (CSIC) for an allocation
of computer time and to Dr. Eric Elguero (IRD, Montpellier, France)
for the statistical analysis.
References
25
20
15
10
5
0
0.008
0.013
0.018
0.023
ρBCP
0.028
0.033
Figure 10. Electron density at the BCP, qBCP (au) versus second order perturbation
NBO energy, E(2), (kJ/mol).
For a given intermolecular distance, the 1-2 HBs exhibit, in general, small values of E(2) when compared to those obtained in 13 and 1-4 HBs.
The analysis of E(2) versus the electron density (Fig. 10) indicates that the 1-2 HBs show larger electron density values at the
BCP, for a given value of the E(2), than found in 1-3 and 1-4 HBs.
These results are associated with the larger values of the electron
density present in the smaller pseudo-rings.
4. Conclusions
A computational study of the conformational profile of erythrose and threose in the gas phase has been carried out. Three
possible configurations for both carbohydrates were studied:
open-chain, a-furanose, and b-furanose. A large number of conformational minima were obtained, especially for the open-chain configurations. The furanose conformations have been characterized
using the Cremer–Pople puckering parameters, P and Q. The
G3B3 calculations predict that the a-furanose configuration is the
most stable for both erythrose and threose.
An analysis of the electron density of the conformers characterized the intramolecular H O hydrogen bonds. They were classified according to the relative position of the interacting moieties
as 1-2, 1-3, and 1-4. The properties of the HBs within each group
are characteristic for a given group and differ from those obtained
for the other groups.
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