RESEARCH HIGHLIGHTS
National Science Review
2: 2–6, 2015
CHEMISTRY
The debut of all-boron fullerene
Su-Yuan Xie
Boron is the fifth element and next to carbon in the periodic table of the elements,
but boron has been researched less than
its neighbor carbon.
Borax sourced from a natural boronrich mine in Yamdrok Lake, Tibet,
China, has been used as a fusion agent
to make glass from ancient times. Elemental boron, however, was elusive
until 1808 when it was prepared from
electrolysis of melted boron trioxide
or by reducing boric acid with potassium. Structures of elemental boron
include an icosahedral (Ih ) B12 motif connected in an ordered manner,
as exemplified by α-rhombohedral
boron (Fig. 1a). The dangling bonds
in each Ih -B12 unit can be saturated by
hydrogen to produce borane species.
In stark contrast to the ubiquitous
Ih -B12 unit in elemental boron allotropes
and boranes, a different B12 all-boron
cluster with a C3v -symmetric planar structure that resembles a solid disk with three
boron atoms in the center surrounded
by a peripheral ring of boron atoms has
also been observed (Fig. 1b) [1]. With
six delocalized π electrons that exhibit
‘disk delocalization’, the planar B12 cluster is analogous to benzene with its wellknown cyclic delocalization. Bn clusters
adopt 2D planar or quasi-planar structures even up to n = 36 [2]. Because of
their diverse set of structural and bonding characteristics, scientists believe that
boron and carbon form a set of complementary chemical systems and have
long been fascinated with whether or
not structures such as cage-like all-boron
clusters are as prevalent as all-carbon
fullerenes. A series of all-boron cage
configurations stabilized by three-center
Figure 1. The structures of all-boron clusters (B12 and B40 ). (a) Icosahedral B12 motifs bound by threecenter two-electron σ bonds (indicated by dashed lines) to form a B12 layer in α-rhombohedral boron;
(b) Top and side views of a C3v -symmetric B12 cluster; (c) Top and side views of Cs -symmetric B40 with
two adjacent hexagonal holes; (d) Top and side views of a neutral B40 cluster with two hexagonal
and four heptagonal holes. This figure was redrawn according to the coordinates of a B40 cluster
provided in the supplementary information of [5].
bonds as well as polygonal holes on
the cage surfaces have been predicted
by Gang Su at the University of Chinese Academy of Sciences [3], Zhongfang Chen at the University of Puerto
Rico [4] and Jijun Zhao at Dalian University of Technology [4].
However, experimental observation
and characterization of cage-like allboron clusters was unsuccessful until a
joint team of Hua-Jin Zhai and Si-Dian
Li from Shanxi University, Jun Li from
Tsinghua University, Lai-Sheng Wang
from Brown University and Zhi-Pan Liu
from Fudan University reported a B40
cluster produced in a laser-vaporization
supersonic source and detected by photoelectron spectroscopy in the gas phase
[5]. Combined effort by experimental
and theoretical chemists has elucidated
that the B40 cluster adopts structures
including a Cs -symmetric planar anion
(Fig. 1c) as well as a D2d -symmetric neutral cage (Fig. 1d).
The authors named the all-boron cage
‘borospherene’, which satisfies Euler’s
C The Author(s) 2015. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For Permissions, please email: journals.
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RESEARCH HIGHLIGHTS
Theorem. In contrast to Ih -C60 fullerene,
the D2d -B40 cage is not perfectly smooth
but has eight quasi-planar B6 triangles,
two hexagonal holes and four heptagonal
holes resembling a Chinese red lantern.
The curved heptagons, similar to those in
all-carbon heptafullerenes [6], have been
previously predicted to exist in all-boron
fullerenes to release strain [4].
Discovery of this B40 fullerene is extraordinary because its signals in photoelectron spectra overlapped with those of
another isomer (2D flat B40 ; see Fig. 1c).
Fortunately, the intensity ratio of the first
bands from the 2D planar and 3D spherical structures can be varied depending
on the conditions of supersonic expansion. The remarkable stability of the 3D
B40 fullerene with a sizable gap between
its highest occupied and lowest unoccupied molecular orbital energy levels stem
from its ‘spherical delocalization’ with
all of the 120 valence electrons form-
Rees
ing delocalized bonds (48 σ and 12 π
bonds), as determined by photoelectron
spectroscopy.
As the first all-boron fullerene observed experimentally, the D2d -B40 cluster marks a milestone in boron chemistry.
This elegant work bridges the gap between boron and carbon fullerenes, and
is important for understanding the guiding principles behind the structural and
bonding characteristics of all-boron clusters. Macroscopic synthesis and isolation
of pristine all-boron fullerene could be a
future task for chemists. Despite the challenges of this avenue of research, the B40
cluster may initiate a new direction of
boron chemistry as a potential new inorganic ligand or building block in chemical manipulation. For example, it may be
possible to modify the cage to produce
exohedral derivatives resembling boranes
or encapsulate metal atoms to form metalloborospherenes.
3
Su-Yuan Xie
College of Chemistry and Chemical Engineering,
Xiamen University, China
E-mail: [email protected]
REFERENCES
1. Zhai, HJ, Kiran, B and Li, J et al. Nat Mater 2003;
2: 827–33.
2. Piazza, ZA, Hu, HS and Li, WL et al. Nat Commun
2014; 5: 3113.
3. Yan, QB, Sheng, XL, and Zheng, QR et al. Phys Rev
B 2008; 78: 201401(R).
4. Wang, L, Zhao, JJ and Li, FY et al. Chem Phys Lett
2010; 501: 16–9.
5. Zhai, HJ, Zhao, YF and Li, WL et al. Nat Chem
2014; 6: 727–31.
6. Tan, YZ, Chen, RT and Liao, ZJ et al. Nat Commun
2011; 2: 420.
doi: 10.1093/nsr/nwv004
Advance access publication 23 February 2015
BIOLOGY & BIOCHEMISTRY
Powering brain power: GLUT1 and the era of structure based human
transporter biology
Douglas C. Rees
Every student of biochemistry quickly appreciates the central role of glycolysis
in cellular metabolism. What is not usually addressed in an introductory course
is how glucose gets inside a cell in the
first place. Specialized integral membrane
proteins known as transporters are responsible for glucose uptake; in mammals, glucose is imported by members of
the GLUT family of which 14 different
varieties have been identified in humans
[1]. GLUT transporters are members of
the major facilitator superfamily of transporters and catalyze the facilitated uptake of glucose in the thermodynamically
favored direction. The most widely distributed version is GLUT1 that is responsible for getting glucose into red blood
cells and across the blood brain barrier,
among many other roles [2].
As a relatively abundant membrane
protein (comprising ∼15% of the
membrane proteins in red blood cells),
GLUT1 has been the subject of many
pioneering transport studies, including
the key contribution of Widdas [3]
that glucose transport is mediated by
a carrier that can alternately access the
two sides of the membrane. An essential
role for GLUT1 is keeping brain cells
fueled with glucose; as the brain operates
at ∼20 W [4], this metabolic engine
consumes over 1018 molecules of glucose
per second. To satisfy this demand, the
brain needs a minimum of 1015 GLUT1
transporters operating at their maximal speed (∼103 s−1 ). In view of the
consequences of perturbing the cellular
energy supply, it is not surprising that
mutations in GLUT1 and other GLUT
family members are associated with various diseases, or that cancer cells requiring more glucose have increased levels
of this transporter to fuel their malignant
metabolism.
Given the essential physiological
roles, the recent crystal structure determination of GLUT1 by Nieng Yan and
co-workers [5] represents a landmark
accomplishment, by providing an atomic
resolution foundation to understand the
function of this remarkable protein at the
molecular level. GLUT1 is the first structurally characterized human transporter
of known substrate, and together with
ABCB10 [6], one of only two structurally
characterized human transporters. From
the structure, a mechanistic model for
GLUT1 transport was developed that
provides a framework for understanding