Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co-Assembly with Achiral Molecules

Article abstract of DOI:10.1002/anie.201510140

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecu

Full text of DOI:10.1002/anie.201510140

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International Edition: DOI: 10.1002/anie.201510140
German Edition: DOI: 10.1002/ange.201510140
Helical Structures
Inversion of the Supramolecular Chirality of Nanofibrous Structures
through Co-Assembly with Achiral Molecules
Guo-Feng Liu+, Ling-Yun Zhu+, Wei Ji, Chuan-Liang Feng,* and Zhi-Xiang Wei
Abstract: To understand the behavior of chiral nanostructures,
it is of critical importance to study how achiral molecules
regulate the chirality of such nanostructures and what the main
driving forces for the regulation processes are. In this work, the
supramolecular chirality of helical nanofibers consisting of
phenylalanine-based enantiomers is inverted by achiral bis-
(pyridinyl) derivatives through co-assembly. This inversion is
mainly mediated by intermolecular hydrogen bonding inter-
actions between the achiral additives and the chiral molecules,
which may induce stereoselective interactions and different
reorientations for the assembled molecules, as confirmed by
calculations. This work not only exemplifies a feasible method
to invert the helicity of chiral nanostructures by the addition of
achiral molecules, but also provides a method to explore their
functions in environments where chiral and achiral molecules
are in close proximity.
actions.[9] The chirality of supramolecular nanostructures is
usually determined by the molecular chirality.[10] Because of
the non-covalent interactions, some chiral nanostructures are
successfully controlled by external stimuli, such as light,[11]
temperature,[12] solvent,[13] pH,[14] rotary stirring,[15] and chiral
additives,[16] leading to a so-called helix-to-helix transition
between P and M. However, in these cases, chiral guests or
asymmetric factors are always present. In fact, chiral supra-
molecular structures usually do not exist alone in biological or
self-assembled aggregates, but are closely dependent on the
guests around them, which can be chiral, asymmetric, or
achiral. However, little is known about the helical inversion of
supramolecular nanostructures triggered by achiral mole-
cules, which is very important for understanding the origins of
chirality in supramolecular systems.
Inspired by the above situation, we selected phenyl-
alanine-based enantiomers (right-handed DPF and left-
handed LPF, Scheme 1) as basic units for constructing
supramolecular chiral nanostructures. Two achiral bis(pyri-
dinyl) derivatives (BPy1 and BPy2) with different chemical
structures were employed to regulate the chirality of the
nanostructures. The handedness of the nanostructures could
be inverted simply by exchanging the achiral molecule
(Scheme 1). This process was shown to be mainly mediated
by intermolecular hydrogen bonding interactions between the
achiral molecules and the enantiomers during co-assembly,
H
elical and twisted nanostructures with controllable chir-
ality are attracting attention owing to their crucial roles in the
fields of catalysis,[1] sensing or recognition,[2] chiroptical
switches,[3] medicine,[4] biomineralization,[5] and biology.[6] To
tune the chirality of nanostructures and to influence the
conformation of chiral assemblies, the participation of chiral
guests or asymmetric factors is usually required.[7] On the
other hand, it has remained challenging to tune the chiral
conformation (right-handed, P; left-handed, M) of nano-
structures by achiral molecules, but it is very important to
have an overall understanding of the assembly mechanisms of
chiral nanostructures in materials science and biology.[8] Such
studies should provide complementary information on regu-
lating the chirality of nanostructures and methods for
exploring their functions when in close proximity with achiral
molecules.
To obtain chiral nanostructures, supramolecular assem-
blies are often employed as they may self-assemble in
a controllable way through non-covalent interactions, such
as hydrogen bonding, p–p stacking, and electrostatic inter-
[*] G. F. Liu,[+] W. Ji, Prof. C. L. Feng
State Key Lab of Metal Matrix Composites
School of Materials Science and Engineering
Shanghai Jiao Tong University
Shanghai 200240 (China)
E-mail: clfeng@sjtu.edu.cn
Dr. L. Y. Zhu,[+] Prof. Z. X. Wei
Key Laboratory of Nanosystem and Hierarchical Fabrication
National Center for Nanoscience and Technology
Beijing 100190 (China)
Scheme 1. Helical inversion of the nanofibers. M and P denote left-
and right-handed helical or twisted nanofibers, respectively. LPF and
DPF are enantiomeric gelators. BPy1 and BPy2 are achiral bis(pyridinyl)
derivatives.
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510140.
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which can further induce different rearrangements and
assemblies.
The syntheses of DPF, LPF, and BPy2 are outlined in the
Supporting Information,[17] and BPy1 is commercially avail-
able. Both DPF and LPF self-assembled into transparent
nanofibrous hydrogels in aqueous solution (Supporting
Information, Figure S1 and S2). However, the formed hydro-
gels were unstable and decomposed within twelve hours. An
equimolar amount of a bis(pyridinyl) derivative (BPy1 or
BPy2) was then mixed with DPF or LPF. The co-assembled
hydrogels were obtained by a heating and cooling process
(Figure S1). Compared with the DPF and LPF hydrogels, the
co-assembled hydrogels, DPF–BPy1 and LPF–BPy1, were
much more stable for a longer period of time (Figure S1). This
was further confirmed by rheology measurements, as the
storage (G’) and loss moduli (G’’) of the co-assembled
hydrogels were higher than those of the DPF and LPF
hydrogels (Figures S3–S5). Typically, greater G’ and G’’
values were observed for the LPF–BPy1 and LPF–BPy2
hydrogels, suggesting that the achiral molecules formed more
stable assembled structures with LPF than with DPF.
Furthermore, higher G’ and G’’ values were also observed
with BPy2 compared with BPy1. This may be due to the fact
that BPy2 can interact through two additional amide–amide
hydrogen bonds, indicating that the amide groups of the
achiral molecule play an important role in rendering the co-
assembly of BPy2 and LPF or DPF more stable and in
determining the rheological properties of the hydrogels. All of
these results imply that the hydrogels formed by the co-
assembly of the LPF/DPF enantiomers and the achiral
compounds and their rheological properties depend on both
the molecular chirality and the structure of the achiral
molecule.
Figure 1. SEM images of a) right-handed helical fibers in the DPF–
BPy1 xerogel, b) left-handed helical fibers in the LPF–BPy1 xerogel,
c) left-handed helical fibers in the DPF–BPy2 xerogel, and d) right-
handed helical fibers in the LPF–BPy2 xerogel.
The nanofibrous structures of the co-assembled hydrogels
were imaged by scanning electron microscopy (SEM;
Figure 1). An equimolar mixture of DPF and BPy1 (DPF–
BPy1) formed right-handed (P) helical fibers (Figure 1a),
whereas an equimolar mixture of LPF and BPy1 (LPF–BPy1)
co-assembled into left-handed (M) helical nanofibers (Fig-
ure 1b). Furthermore, whereas LPF and BPy2 assembled into
right-handed twisted nanoribbons (Figure 1c), the DPF–
BPy2 nanofibers were formed as left-handed twisted nano-
ribbons (Figure 1d). These results suggest that nanoscale
twists of opposite handedness could be generated from the
same chiral molecule. This chiral inversion was obviously
triggered by achiral BPy1 and BPy2, which differs from
previous approaches.[11–16] These results were consistently
obtained in several experiments.
To gain further insight, the hydrogels were analyzed by
CD spectroscopy. For the DPF hydrogels, significant positive
Cotton effects were observed at 210 nm and 268 nm, whereas
the LPF hydrogels exhibited negative Cotton effects at
210 nm and 268 nm (Figure S6). Similarly, positive Cotton
effects (210 nm and 268 nm) were observed for the DPF–
BPy1 hydrogel and negative Cotton effects for the LPF–BPy1
hydrogel (Figure 2a). Interestingly, for the DPF–BPy2 hydro-
gel, the CD spectrum exhibited a positive Cotton effect at
210 nm and a negative Cotton peak at 268 nm (Figure 2b).
Compared with DPF–BPy1, a chiral transition into the
Figure 2. CD and VCD spectra of the DPF–BPy1 (a), LPF–BPy1 (c),
DPF–BPy2 (b), and LPF–BPy2 (d) hydrogels were obtained by using
a 0.1 mm quartz cuvette with a total gelator concentration of
2.0 mgmLÀ1
.
opposite optically active hydrogel was achieved only by
exchanging the achiral additive (BPy1 and BPy2), which is in
good agreement with the SEM results. Thus a relationship
between the handedness of the helical fibers and the CD signs
could be established. The right-handed (P) DPF–BPy1 nano-
fibers exhibited a positive Cotton effect at 268 nm, whereas
left-handed (M) LPF–BPy1 fibers showed a negative Cotton
effect at 268 nm (Figure 2a). In contrast, P-twisted LPF–
BPy2 nanoribbons exhibited a positive Cotton effect at
268 nm (Figure 2a), and M-twisted DPF–Bpy2 nanoribbons
showed a negative Cotton effect at the same wavelength
(Figure 2b). It must be pointed out that the left-handed
nanofibers and right-handed nanoribbons are not exactly
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enantiomers, and their CD spectra should not be exact mirror
images. In both cases, the chirality of the supramolecular
nanostructures was successfully inverted by exchanging the
achiral additive (BPy1 vs. BPy2).
The optical activity and the chirality of these hydrogels
were investigated by vibrational circular dichroism (VCD)
=
spectroscopy. For the LPF hydrogels, the C O stretching
band exhibited a negative VCD signal at 1800–1700 cmÀ1 and
a negative VCD signal at 1700–1620 cmÀ1 (Figure S7). On the
other hand, the signs of both of these VCD bands were
positive for the DPF hydrogels (Figure S7). The VCD spectra
of DPF and LPF were almost mirror images of each other.
Similarly, the VCD spectrum of LPF–BPy1 showed a negative
VCD signal at 1800–1700 cmÀ1 and a positive one at 1700–
1600 cmÀ1 (Figure 2c). The DPF–BPy1 hydrogel resulted in
a positive VCD signal at 1800–1700 cmÀ1 and a negative one
at 1700–1600 cmÀ1 (Figure 2c). Surprisingly, the C O stretch-
=
ing band of the LPF–BPy2 hydrogel showed a significant
positive VCD band at 1800–1700 cmÀ1 and a negative VCD
signal at 1700–1600 cmÀ1. For DPF–BPy2, however, the signs
of the VCD bands at 1800–1700 cmÀ1 and 1700–1600 cmÀ1
were negative and positive, respectively (Figure 2d). As
discussed above, the observed chiral and morphological
transformations of the hydrogel nanostructures should be
attributed to the presence of achiral additives, which leads to
supramolecular chirality inversion and chiroptical inversion.
To understand the arrangement of molecular aggregates
in the co-assembled hydrogels, colorless cocrystals of LPF–
BPy1 were crystallized from tetrahydrofuran/para-xylene
(3:5, v/v) solution. Detailed crystallographic data can be
found in Table S2. The crystal structure and packing diagram
of LPF–BPy1 suggested that the main driving force for the co-
assembly was the formation of two types of intermolecular
Figure 3. The packing structure of LPF–BPy1 stabilized by hydrogen
bonds (pink dashed lines) in the cocrystal state.
Based on the experimental crystal structure and the
optimized geometry, time-dependent density functional
theory (TDDFT) calculations with the long-range-corrected
(LRC) functional wB97x and the 6-31G(d,p) basis set were
conducted to calculate the CD spectra of the co-assembly. For
the dimer (M)-LPF, two negative peaks A and B were found
between 190 and 400 nm (Figure 4). Peak A is located at
approximately 243 nm, corresponding to the S8 and S10
À
À
À
=
hydrogen bonds (C N···H O and N H···O C; Figure 3).
First, one-dimensional (1D) self-assembly in a head-to-tail
fashion occurred through the formation of hydrogen bonds
(bond length: 1.76 Š) between a pyridyl nitrogen atom and
the HO group of a carboxylic acid (H···N, Table S2). Second,
three-dimensional (3D) crystal networks were obtained
through the formation of hydrogen bonds between an
À
=
amide N H group and a carboxylic acid C O moiety
(2.10 Š, H···O, Table S2). These two kinds of intermolecular
hydrogen bonds thus are the main driving force for stabilizing
the cocrystal framework (Figure 3).
To simulate the chiroptical inversion phenomena of the
molecular aggregates, ab initio calculations were performed
within the framework of density functional theory (DFT) with
the B3LYP functional, Grimmeꢀs dispersion correction
(B3LYP-D), and the 6-31G(d,p) basis set. Here, we focus
only on the properties of LPF as the experimental CD spectra
of LPF and DPF are almost mirror images of each other. The
experimental crystal structure of (M)-LPF showed that two
molecules were nearly perpendicular to each other, and the
two terminal phenyl groups were arranged in a trans fashion
(Figure 4a). However, in the B3LYP-D/6-31G (d,p) opti-
mized structure of the (P)-LPF dimer, one molecule was
nearly parallel to the other one, with the smallest intermo-
lecular distance being approximately 3.2 Š, and the two
terminal phenyl groups are cis to each other (Figure 4b).
Figure 4. a) Structure of the (M)-LPF dimer determined by crystal-
structure analysis. b) B3LYP-D/6-31G (d,p) optimized geometry of the
(P)-LPF dimer. c,d) TDDFT calculated CD spectra of the two dimers at
the wB97x/6-31G (d,p) level of theory.
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excited states (see Table S3), which are associated with
intramolecular transitions from the amide linkage to the
central aryl group (see Figure S8a). Peak B is located at
200 nm and due to contributions from the S34 and S35 excited
states, which are associated with intramolecular p!p*
transitions in the terminal phenyl group of one molecule
and intermolecular transitions from the amide linkage of one
molecule to the central aryl group of another molecule (see
Figure S8a).
2012CB933803), and the SJTU-UM Collaborative Research
Program for Professors of Special Appointment (Eastern
Scholar) at the Shanghai Institutions of Higher Learning.
Keywords: chirality · co-assembly · helical structures ·
hydrogels · supramolecular chemistry
How to cite: Angew. Chem. Int. Ed. 2016, 55, 2411–2415
Angew. Chem. 2016, 128, 2457–2461
However, for the dimer (P)-LPF, in contrast to (M)-LPF,
one positive peak, A’, at approximately 256 nm and one
negative peak, B’, at about 206 nm were found between 190–
400 nm. Natural transition orbital (NTO) analysis showed
that peak A’ is mainly due to intermolecular p!p* tran-
sitions of the central aryl group, and peak B’ is associated with
transitions at the central aryl group and transitions from the
amide linkage or a terminal phenyl group to the central aryl
group (see Figure S8b). These two peaks are mainly due to
the exciton coupling of two molecules. The experimental CD
spectra, including the signs and shapes of the bands, were well
reproduced by our calculations. Our calculations thus fully
support the experimentally observed chiroptical inversion
phenomena.
Whereas BPy1 has only one kind of hydrogen-bonding
site (pyridine), BPy2 has two kinds of binding sites (pyridine
and amide), which leads to different interaction modes with
DPF and LPF. Typically, there is competition between the
COOH/pyridine and amide/amide groups of the achiral and
chiral compounds. BPy1 should interact with both DPF and
LPF via the COOH sites, whereas BPy2 interacts with them
through both the COOH and amide groups. These two
interactions lead to a parallel arrangement, which could
possibly cause the chirality inversion. Therefore, different
chirality phenomena can be expected for further achiral
additives, which could be obtained by varying the amide
group, for example, and more useful information on chirality
regulation by achiral molecules should be obtainable. Such
studies are currently in progress in our group.
In summary, the supramolecular chirality of nanofibrous
structures has been inverted by the addition of different
achiral molecules and the formation of intermolecular hydro-
gen bonds between these additives and the supramolecules,
which induce stereoselective interactions and different reor-
ientations. This system is one of very few examples where
achiral molecules have been shown to trigger chirality
inversions of nanostructures through co-assembly with
chiral compounds. It exemplifies a feasible shortcut to achieve
helical inversions by rationally designing basic molecular
structures. This method can be applied in complementary
studies on regulating the chirality of nanostructures and for
exploring their role in environments where chiral and achiral
molecules are in close proximity, for example, in biological or
self-assembled aggregates.[18]
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We thank the NSFC (51573092, 51273111, 51173105), the
National Basic Research Program of China (973 Program,
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Received: October 31, 2015
Published online: December 9, 2015
Angew. Chem. Int. Ed. 2016, 55, 2411 –2415
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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