Regulation of the chiral twist and supramolecular chirality in co-assemblies of amphiphilic L-glutamic acid with bipyridines

Article abstract of DOI:10.1002/chem.201002595

A series of amphiphilic L-glutamic acid derivatives with various saturated alkyl chains has been designed and their co-assembly with 4,4′-bipyridine in aqueous media has been investigated. While the individual amphiphiles formed hydrogels with water and self-assembled into fine fiber networks, the addition of 4,4′-bipyridine caused significant changes in the co-assembled nanostructures such that twisted chiral ribbons were formed. In these supramolecular systems, either fine structural changes or adjustment of the stoichiometric ratio of the two components had crucial effects on the formation of the chiral twists. Based on detailed investigations by SEM and XRD analyses, FTIR, CD, and UV/Vis spectroscopies, and molecular simulation, it is considered that a delicate synergistic balance between π-π stacking, hydrophobic, and chiral interactions is responsible for the formation of the chiral twists. An interesting sandwich structure, in which an excess of 4,4′-bipyridine is inserted into the space of primary cages constructed from the amphiphile and 4,4′-bipyridine, is proposed. Remarkably, the handedness of these chiral twists is related not only to the chiral center of the glutamic unit, but also the chain length of the alkyl tails. This work provides a deeper understanding of the formation mechanism of chiral twists, and exemplifies a feasible shortcut to the rational design of chiral structures from basic molecular structures to supramolecular systems. Twisted nanostructures: A chiral twist has been obtained through the co-assembly of single-chain L-glutamic acids with bipyridines (see picture). The sandwiching of additional 4,4′-bipyridine within the cage formed by the amphiphile and the bipyridine caused the chiral twist. The dimensions and chirality of the twist could be regulated by adjusting the alkyl chain length of the amphiphile. Copyright

Full text of DOI:10.1002/chem.201002595

FULL PAPER
DOI: 10.1002/chem.201002595
Regulation of the Chiral Twist and Supramolecular Chirality in
Co-Assemblies of Amphiphilic l-Glutamic Acid with Bipyridines
Xuefeng Zhu, Pengfei Duan, Li Zhang, and Minghua Liu*[a]
Abstract: A series of amphiphilic l-glu-
tamic acid derivatives with various sa-
turated alkyl chains has been designed
and their co-assembly with 4,4’-bipyri-
dine in aqueous media has been inves-
tigated. While the individual amphi-
philes formed hydrogels with water
and self-assembled into fine fiber net-
works, the addition of 4,4’-bipyridine
caused significant changes in the co-as-
sembled nanostructures such that twist-
ed chiral ribbons were formed. In these
supramolecular systems, either fine
structural changes or adjustment of the
stoichiometric ratio of the two compo-
nents had crucial effects on the forma-
tion of the chiral twists. Based on de-
tailed investigations by SEM and XRD
analyses, FTIR, CD, and UV/Vis spec-
troscopies, and molecular simulation, it
is considered that a delicate synergistic
balance between p–p stacking, hydro-
phobic, and chiral interactions is re-
sponsible for the formation of the
chiral twists. An interesting sandwich
structure, in which an excess of 4,4’-bi-
pyridine is inserted into the space of
primary cages constructed from the
amphiphile and 4,4’-bipyridine, is pro-
posed. Remarkably, the handedness of
these chiral twists is related not only to
the chiral center of the glutamic unit,
but also the chain length of the alkyl
tails. This work provides a deeper un-
derstanding of the formation mecha-
nism of chiral twists, and exemplifies a
feasible shortcut to the rational design
of chiral structures from basic molecu-
lar structures to supramolecular sys-
tems.
Keywords: amphiphiles · chirality ·
hydrogels · nanostructures · supra-
molecular chemistry
Introduction
salts of 12-hydroxystearic acid,[5a,b] glutamates,[5c,d] phospho-
lipids,[5e,f] sugar-based lipids,[5g,h] chiral geminal amphi-
Chiral structures such as helices and twists are among the
most fascinating morphologies with geometrical asymmetry,
which could store genetic information and be used as poten-
tial materials for chirotechnology.[1] It is an important issue
in supramolecular chemistry to mimic these chiral structures
and to obtain a comprehensive understanding of the mecha-
nism of their formation.[2] The creation of artificial helical
structures in relation to molecular design and controlled
self-assembly have been extensively investigated.[3] For ex-
ample, Lehn et al. have designed inorganic double-stranded
helicates through the self-organization of oligobipyridine li-
gands with Cu+ ions.[3a,b] Meijer et al. have created a helical
polymer using the self-recognition of hydrogen bonds in a
bifunctional monomer.[3c,d] Yashima et al. have synthesized
complementary double helices utilizing interstrand amidini-
um carboxylate salt bridges between two homopolymers.[3e,f]
Chiral twists, which have a similar structure to helices but
with different curvature, are also frequently found in supra-
molecular systems.[4] Several simple amphiphiles, such as
philes,[4a,b,5i] and amphiphilic peptides,[1g,5j] have been used to
ACHTUNGTRENNUNG
fabricate chiral twists. Some p-conjugated building blocks
have also proved effective for the construction of chiral
twists with a molecular motif of rod–coil or coil–rod–coil,
such as hexa-peri-hexabenzocoronene (HBC),[6a,b] oligophe-
nylenevinylenes
(OPVs),[6c–f]
oligophenyleneethynlenes
(OPEs),[6g] ALS (Aromatic, Linker, Steroidal group),[6h,i] C3-
symmetrical[6j,k] or X-shaped[6l] p-conjugated molecules, and
so on.[7] However, there remain many unknowns concerning
the formation of chiral twists. In particular, it is still a chal-
lenge to regulate chiral twists through either morphological
parameters or nanoscopic and macroscopic chiralities.[8] In
this paper, we report the formation and regulation of a
chiral twist using a two-component system[9] composed of a
series of l-glutamic acid derivatives and bipyridines
(Scheme 1).
[a] X. Zhu, P. Duan, Dr. L. Zhang, Prof. Dr. M. Liu
Beijing National Laboratory for Molecular Science
CAS Key Laboratory of Colloid, Interface and
Chemical Thermodynamics
Institute of Chemistry
Chinese Academy of Sciences, Beijing, 100190 (P. R. China)
E-mail: liumh@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002595.
Scheme 1. Molecular structures of l-CnGAc (Cn) and bipyridines (xPy).
Chem. Eur. J. 2011, 17, 3429 – 3437
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We have designed a series of amphiphiles based on l-glu-
tamic acid with saturated fatty acid chains of different
lengths (Scheme 1). These compounds can self-assemble
into nanofibers in water, thus leading to hydrogels. Interest-
ingly, upon adding bipyridine to the system, the nanostruc-
tures underwent exciting changes due to strong hydrogen
bonding between the carboxylic acid and the pyridyl nitro-
gen atoms.[10] In particular, the addition of 4,4’-bipyridine
caused the formation of a chiral twist. Moreover, by chang-
ing the ratio of 4,4’-bipyridine to the amphiphile as well as
the chain length of the amphiphile, both the pitch and the
chirality of the twist could be regulated. Based on detailed
investigations by SEM and XRD analyses, as well as FTIR,
CD, and UV/Vis spectroscopic observations, the mechanism
of the twist formation has been clarified. It has emerged
that cooperativity between hydrophobic interactions of the
amphiphiles, p–p stacking of the aromatic bipyridine, and
hydrogen bonds between the bipyridine and l-glutamic acid
is essential for the formation of chiral twists. In addition, the
alkyl chain plays a crucial role in regulating the chirality of
the twists. That is to say, amphiphiles with a longer alkyl
chain transferred the molecular chirality of the l-glutamic
acid to the macroscopic chiral twist, whereas those with a
shorter alkyl chain did not. Our results provide a compre-
hensive understanding of the formation of the chiral twist.
formed. Interestingly, SEM and TEM observations of the
nanostructures revealed beautiful twists with exclusively
right-handedness rather than fine fibers, as shown in
Figure 2. The twists were uniform with a pitch of around
2.80 mm and a width of 24–30 nm. Surprisingly, when either
4ePy or 2Py was mixed with C18, only precipitates were ob-
tained. In these cases, the mixtures of C18 and bipyridines
also formed an organized nanostructure, which appeared as
nanotapes and nanoplates in the SEM images. This strongly
suggested that the C18 molecules could interact with all
three xPy molecules, giving rise to distinct co-assembly be-
haviors according to the structural differences of xPy.
Effect of C18/xPy ratio on morphological evolution: In
order to further probe the nature of the assembly of C18/
xPy, we varied the mixing ratio. At all investigated stoichio-
metries of the C18/4ePy or C18/2Py mixtures, precipitates
were invariably formed with the same nanoplates or nano-
tapes in their nanostructures (Figure 2d, e and Figure S1 in
the Supporting Information). However, hydrogels were
formed over a wide range of mixing ratios for C18/4Py. In-
terestingly, the self-assembled nanostructures were quite dif-
ferent. Figure 3 shows SEM images of various C18/4Py com-
binations. At C18/4Py=1:1, flat ribbons were predominant-
ly formed (Figure 3a). When a little more 4Py was added,
the ribbons started to twist (Figure 3b). On further increas-
ing the amount of 4Py, the twisted ribbons became more nu-
merous and their pitch gradually became smaller (Figure 3c,
d). Essentially uniform perfect twists were obtained at C18/
4Py=1:2 and remained unchanged up to a ratio of 1:3 (Fig-
ure 3e, f). In addition, the C18/4Py twists had exclusively
Results
Self-assembly of Cn and hydrogel formation: All of the Cn
molecules were difficult to dissolve in water at room tem-
perature. However, upon heat-
ing to the boiling point of
water, coupled with intense
ultrasonication, clear aqueous
solutions of Cn could be ob-
tained. After allowing each so-
lution to cool naturally to room
temperature, it became opales-
cent and sticky. When the test
vial was inverted the mixture
no longer flowed, indicating the
formation of
a
hydrogel.
Figure 1 shows SEM images of
the xerogels obtained from air-
dried Cn hydrogels. Worm-like
nanofibers of several microme-
ters in length were formed and
built up into three-dimensional
(3D) networks, in which water
molecules were trapped to form
hydrogels (Figure 1a, inset).[11]
Co-assembly behaviors of C18/
xPy: When C18 was mixed
with 4Py in a molar ratio of 1:2,
Figure 1. SEM images of Cn xerogels, C18 (a), C16 (b), C14 (c), and C12 (d). The inset is a photograph of the
a white opaque hydrogel was C18 hydrogel. Scale bar: 500 nm.
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Chiral Twists in l-Glutamic Acid–Bipyridine Systems
FULL PAPER
which showed an interesting
change depending on the chain
length of the amphiphile Cn.
Figure 4 shows SEM images of
Cn/4Py xerogels, which clearly
show the different features.
Firstly, C16/4Py showed simi-
lar self-assembly behavior to
that of C18/4Py (Figure 4a–c).
That is to say, mainly nanorib-
bons were formed at C16/4Py=
1:1 while chiral twists were pre-
dominantly observed at 1:2,
which remained unchanged up
to 1:3. Moreover, the chirality
of all of the twists retained the
same right-handedness. Again,
this suggested that the molecu-
lar chiral information of the
amphiphile C16 was transferred
to the supramolecular assem-
blies of C16/4Py and amplified
to the nanometer scale.[9e,10i]
Secondly, analogous twists
were observed for all of the in-
vestigated C14/4Py combina-
tions, including C14/4Py=1:1.
As denoted by the arrows in
Figure 4d–f, both left- and
right-handed twists were simul-
taneously observed in all C14/
4Py mixtures, which is different
from the exclusively right-
handed twists of C18/4Py and
C16/4Py. These surprising re-
sults were consistently obtained
in several repeated experi-
ments, which clearly indicated
that the chiral nature of the
molecules was not the only
factor responsible for the chir-
ality of the twists. Nanoscale
twists of opposite handedness
could be produced from the
same chiral molecule.[8a–d]
Figure 2. Morphologies of co-assembled C18/4Py (a, b, c), C18/4ePy (d), and C18/2Py (e) at molar ratios of
1:2. The insets are photographs of the samples. Scale bar: 1 mm.
Figure 3. SEM images of co-assembled C18/4Py at different molar ratios, 1:1 (a), 1:1.2 (b), 1:1.5 (c), 1:1.8 (d),
1:2 (e), and 1:3 (f). Scale bar: 500 nm (a), 2 mm (b–f).
Thirdly, in all of the C12/4Py
right-handedness, suggesting that this stemmed from the
nature of the original molecular chirality.
mixtures, nanotapes composed of fine nanofibers were pre-
dominantly formed, with only a small amount of twists and
helical fibers (Figure 4g–i). This further indicated that the
alkyl chain length of amphiphiles Cn had a dramatic influ-
ence on the co-assembled nanostructures.
It should be noted that the pH of the aqueous solutions
had a significant effect on the gel formation as well as the
gel structures. It was found that neutral pH was favorable
for twist formation. This might be due to the strong H-bond-
ing between the carboxylic acid and the pyridine at neutral
pH.[18c]
Effect of the length of the Cn alkyl chain on the twists: Sim-
ilar investigations were carried out for the other three am-
phiphiles Cn (C16, C14, and C12) with different bipyridines
xPy. When these Cn were mixed with 4ePy or 2Py, almost
the same nanoplates or nanosheets were observed as found
for C18/4ePy or C18/2Py, regardless of the chain length of
Cn or the Cn/4ePy or Cn/2Py ratio (Figures S2 and S3). In
contrast, Cn/4Py formed hydrogels, the morphologies of
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M. Liu et al.
acid groups were also involved
in various hydrogen bonds.[12b]
Moreover, the appearance of
À
two peaks due to N H stretch-
ing vibrations at 3327 and
3310 cmÀ1 further evidenced
the complicated nature of the
hydrogen bonds. In addition,
bands attributable to asymmet-
ric and symmetric CH2 stretch-
ing vibrations at 2918 and
2850 cmÀ1, respectively, indicat-
ed that the alkyl chains were
closely packed in a highly or-
dered fashion (all-trans zigzag
conformations).[13]
The FTIR spectrum of the
C18/4Py=1:1 xerogel displayed
two new strong vibrational
bands at around 2500 and
1945 cmÀ1 (Table 1 and Fig-
ure S4a), which are typical of
carboxylic acid–pyridine hydro-
gen-bond
Such interaction is further evi-
denced by the shifts of the vi-
Figure 4. SEM images of co-assembled C16/4Py (a, b, c), C14/4Py (d, e, f), and C12/4Py (g, h, i) at different
molar ratios, 1:1 (a, d, g), 1:2 (b, e, h), and 1:3 (c, f, i). Scale bar: 500 nm. The white arrow marks designate the
chiral twists, and M and P denote the left- and right-handed twists, respectively.
interactions.[10b,c,f,g]
FTIR spectra of the supramolecular assemblies: FTIR spec-
tra were recorded to explore molecular interactions in the
supramolecular assemblies. For the C18 xerogel (Table 1
and Figure 5a), the occurrence of the amide I and amide II
bands at 1645 and 1545 cmÀ1, respectively, indicated that the
amide groups formed strong hydrogen bonds.[12] Bands at
1731, 1694, and 1676 cmÀ1 suggested that the two carboxylic
Table 1. Main vibrational bands [cmÀ1] in the FTIR spectra of C18 and
C18/xPy assemblies.
Assignment
C18
C18/4Py
C18/4ePy
C18/2Py
Figure 5. FTIR spectra of the xerogels, C18 (a), C18/4Py=1:1 (b), C18/
4Py=1:2 (c), C12/4Py=1:1 (d), and C12/4Py=1:2 (e).
1:1
1:2
1:1
1:2
1:1
1:2
nNH
3327,
3310
2918
3315
3315
3318
3317
3313
3312
nCH2
nOH[a]
nC=O
2917
2850
2500
1945
1729
2917
2850
2499
1944
1729
2917
2850
2506
1947
1733
2917
2850
2495
1947
1733
2917
2849
2547
1940
1745
1715
2917
2850
2525
1943
1745
1717
brational bands of C=N and C=C in the pyridyl rings from
1591 and 989 cmÀ1 to 1603 and 1005 cmÀ1, respectively.[10b,d]
This suggested that carboxylic acid–pyridine hydrogen
bonds were formed between C18 and 4Py. Meanwhile, three
vibrational bands at 1731, 1694, and 1676 cmÀ1 disappeared
and only one band remained at 1729 cmÀ1. This indicated
that the carboxylic acid groups of C18 that had previously
been involved in diverse H-bonds now interacted exclusively
with 4Py due to the stronger carboxylic acid–pyridine hydro-
gen-bond interactions.[10b] However, the amide I and ami-
de II bands at 1652 and 1546 cmÀ1 demonstrated that the hy-
drogen bonds between amide groups remained intact. The
asymmetric and symmetric CH2 stretching vibrations at 2917
and 2850 cmÀ1 indicated that the highly ordered structure of
the alkyl chains was preserved upon mixing.
2850
1731
1694,
1676
1645
amide I
1652
1603
1652
1603
1592
1543
1467
1648
1611
1648
1609
1597
1543
1472
1465
1019
992
1651
1605
1589
1552
1472
1463
1007
1652
1605
1589
1552
1472
1464
1007
nC=N[b]
amide II
dACHTUNGTRENNUNG
1545
1471
1455
1546
1470
1546
1472
1464
1019
1005
1005
990
[a] Stretching vibration of the hydroxy group in carboxylic acid–pyridyl
hydrogen bonds. [b] Stretching vibration of the pyridine ring. [c] Twisting
vibration of the pyridine ring.
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Chiral Twists in l-Glutamic Acid–Bipyridine Systems
FULL PAPER
The FTIR spectral changes seen for C18/4ePy=1:1 were
similar to those described above for C18/4Py=1:1 (Table 1
and Figure S4b). Two new bands at 2506 and 1947 cmÀ1 (the
stretching vibration of a pyridyl-bonded hydroxy group of
the carboxylic acid function) and shifts in the bands at 1591
and 989 cmÀ1 to 1609 and 1019 cmÀ1 (the vibrations of the
C=N and C=C of 4ePy, respectively), suggested the forma-
tion of a H-bonded C18/4ePy complex with a similar supra-
molecular structure to that of C18/4Py=1:1.
drogel compared with that of an aqueous solution of 4Py in-
dicated proton transfer from the carboxylic acid group of
C12 to the pyridyl N atom of 4Py. From these observations,
we could infer that shortening of the alkyl chain led to an
increase in the water-solubility of C12. As a consequence,
the weakened hydrophobic interaction and carboxylic acid–
pyridine hydrogen bonds might be responsible for the fact
that no sandwiched 4Py and no twist formation were ob-
served.
In contrast, the FTIR spectrum of C18/2Py=1:1 featured
two vibrational bands at 1745 and 1715 cmÀ1 in the relevant
region for carboxylic acid functions, although two new
bands at 2547 and 1940 cmÀ1 indicated the occurrence of the
carboxylic acid–pyridine H-bond interaction (Table 1 and
Figure S4c). This suggested a distinct interaction mode of
C18 with 2Py compared to that with the former two pyri-
dines. That is to say, one carboxylic acid group of C18
formed an H-bond with 2Py as in the case of 4Py or 4ePy,
while the other was seemingly free in the C18/2Py com-
plex.[12b]
X-ray diffraction measurements on the supramolecular as-
semblies: X-ray diffraction measurement was also used to
evaluate the nanostructures of the supramolecular assem-
blies. Figure 6 displays the XRD patterns of the xerogels of
As expected, the other Cn xerogels displayed FTIR spec-
tra almost the same as that of C18, except that the asym-
metric and symmetric CH2 stretching vibrations gradually
shifted from 2918 and 2850 cmÀ1 (C18) to 2922 and
2852 cmÀ1 (C12), respectively, implying looser packing with
shortening of the alkyl chains of Cn (Figure S4d and
Table S1).[13]
Figure 6. XRD spectra of the xerogels, C18 (a), C18/4Py=1:1 (b), C18/
4Py=1:2 (c), and C18/4Py=1:3 (d).
For all of the Cn/4Py=1:1 xerogels, the same vibrational
bands at 2500, 1945, and 1729 cmÀ1 confirmed the formation
of carboxylic acid–pyridine hydrogen bonds (Table S1).[10b]
Significantly, these three bands remained unchanged, despite
the dramatic morphological evolution upon changing the
C18/4Py ratio from 1:1 to 1:2. However, remarkable differ-
ences were observed at lower vibrational frequencies. That
is to say, the FTIR spectrum of the C18/4Py=1:2 xerogel
featured two minimally shifted bands (1592 and 990 cmÀ1)
from 1591 and 989 cmÀ1 for the C=N and C=C vibrations of
the pyridine ring besides the two more significantly shifted
bands at 1603 and 1005 cmÀ1 found for the C18/4Py=1:1
xerogel[10b,d] (Figure 5b, c). Apparently, the two less shifted
bands could be attributed to the vibrations of weakly bound
4Py molecules. From these observations, we inferred that in
the C18/4Py=1:2 hydrogel, the redundant 4Py is probably
sandwiched into the space between the two 4Py units of the
primary C18/4Py=1:1 complex. Such FTIR spectral changes
were also obtained for the C16/4Py and C14/4Py xerogels
when the ratio was changed from 1:1 to 1:2. However, the
FTIR spectrum of C12/4Py=1:2 was identical to that of the
1:1 xerogel (Figure 5d, e), possibly indicating that no sand-
wich structure akin to those formed with the three longer-
chain amphiphiles was adopted in this case. Moreover, the
shifts of the asymmetric and symmetric CH2 stretching vi-
brations to 2922 and 2852 cmÀ1 for C12/4Py, as compared to
2918 and 2850 cmÀ1 in the other three cases, implied that the
alkyl tails of C12/4Py were less tightly packed.[13] Compar-
ing the UV/Vis spectra (Figure 7d, solid line), a 4 nm red
shift of the maximum absorbance band of the C12/4Py hy-
pure C18 and the composites C18/4Py. According to
Braggꢁs equation, the d-spacing of the C18 xerogel was esti-
mated to be 3.29 nm, suggesting a bilayer structure formed
by C18 molecules.[12b] Well-defined patterns were also ob-
served for the C18/4Py xerogels and the interlayer distance
became larger (ca. 3.65 nm). This indicated that a composite
bilayer was formed in which the 4Py molecules were
packed in an orderly manner between the C18 molecules
through carboxylic acid–pyridine interactions. Similar XRD
results were obtained for all of the other combinations (Fig-
ure S5), suggesting that the basic unit of each of the nano-
structures was a certain bilayer structure, regardless of
whether nanotwists or nanosheets were formed.
Supramolecular chirality of the twists: SEM observation re-
vealed the microscopic chirality of the twists. To further
characterize their nanoscale chirality, circular dichroism
(CD) spectra were measured. Even though the pure Cn
molecules could aggregate into ordered nanofibers and sub-
sequently form hydrogels, neither UV/Vis spectral activity
nor CD activity was detected in the range 200–400 nm due
to the lack of a chromophore (Figure S6d). Aqueous solu-
tions of the three individual bipyridines xPy were UV/Vis
spectrally active (4Py: 240, ca. 270 nm; 2Py: 234, 281 nm;
4ePy: ca. 250, 256, 263 nm) but CD silent due to their achi-
ral nature (Figure 7, dotted line). For co-assembled C18/
xPy, no remarkable changes in the UV/Vis spectra were ob-
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M. Liu et al.
acid–pyridyl H-bonds play
a
pivotal role in chirality tran-
scription.
Discussion
It was clear from the above re-
sults that the Cn amphiphiles
could form hydrogels with 4Py,
but formed precipitates with
4ePy or 2Py. The two compo-
nents self-assembled coopera-
tively to form various nano-
structures such as fibers, rib-
bons, sheets, and twists. More-
over, the morphologies of the
nanostructures could be tuned
by varying the chain length of
Cn,[4b,5i,14] the chemical struc-
ture of xPy, and their stoichio-
metric ratio.[15] It was clear that
multifarious noncovalent inter-
actions were involved, such as
carboxylic acid–pyridyl hydro-
gen bonds, amide–amide hydro-
gen bonds, aromatic p–p stack-
ing, and alkyl hydrophobicities.
Figure 7. CD and UV/Vis spectra of C18/4Py (a, solid line), C16/4Py (a, gray solid line), C18/4ePy (b), C18/
2Py (c), and C12/4Py (d) at molar ratios of 1:2, as well as those of aqueous solutions of pure xPy (dotted
line), 4Py (a), 2Py (b), and 4ePy (c).
served. However, significant CD signals were detected. Spe-
cifically, a strong positive Cotton effect was observed for the
C18/4Py hydrogel with a crossover at 277 nm, which corre-
sponded to the electronic transition of the 4Py rings (Fig-
ure 7a, solid line). Also, a positive Cotton effect with a
crossover at 285 nm was detected for C18/4ePy, whereas a
negative Cotton effect with a crossover at 266 nm was found
for C18/2Py (Figure 7b, c, solid lines). The strong CD activi-
ties observed for C18/xPy demonstrate that upon coopera-
tive self-assembly, the molecular information of chiral C18
was successfully transcribed to the achiral xPy chromophor-
es.[9e,10i] Moreover, the opposite sign of the CD signals sug-
gested a distinct chirality transcription mode for C18/2Py
compared to that for C18/4Py or C18/4ePy.[1l,m,9g] This fur-
ther corroborated their different molecular interactions as
revealed by the FTIR results.
Furthermore, it was suggested that the formation of chiral
twists may be attributed to synergistic effects among these
noncovalent interactions.
Each l-glutamic acid-based amphiphile Cn has three hy-
drogen-bond sites, that is, two carboxylic acid groups and
one amide function, and as a result several types of carbox-
ylic acid–carboxylic acid, carboxylic acid–amide, and amide–
amide hydrogen bonds could be formed.[12b] Through hydro-
phobic interactions of their alkyl chains, the amphiphiles
themselves first aggregate into a bilayer structure. These bi-
layer subunits are further stacked in a layer-by-layer fashion
and then self-assemble into fibrils, which become inter-
twined to form nanofibers of diameter about 21 nm.[5g,16]
Subsequently, the nanofibers become woven into 3D net-
works, which can trap water molecules, thereby leading to
hydrogels (Figure S7).
Similarly, C16/4Py displayed a strong positive Cotton
effect and the crossover appeared at 260 nm, a much lower
wavelength compared to 277 nm for C18/4Py (Figure 7a,
gray solid line). This suggested that the alkyl chain length of
the amphiphiles could influence the aggregation behavior of
the chromophores 4Py. Even though C14/4Py displayed the
same UV/Vis spectrum as C18/4Py and C16/4Py, it was CD
silent owing to the coexistence of right- and left-handed
twists (Figure S6c). The CD inactivity of C12/4Py may be a
result of weaker carboxylic acid–pyridyl hydrogen-bonding
interactions because of proton transfer from C12 to 4Py, as
evidenced by a 4 nm red shift from 240 to 244 nm (Fig-
ure 7d, solid line). It would seem that strong carboxylic
When xPy was introduced, the self-recognizing carboxylic
acid–pyridyl hydrogen bonds took priority over the primary
carboxylic acid–carboxylic acid and carboxylic acid–amide
hydrogen bonds of the Cn molecules. Therefore, supra-
molecular Cn/xPy was produced as the basic building block
for further assembly (Figure 8 and Figure S8). FTIR spectra
verified that carboxylic acid–pyridyl hydrogen bonds were
predominantly formed instead of the diverse hydrogen
bonds between carboxylic acids. However, the amide–amide
hydrogen bonds were well preserved. Owing to the highly
directional hydrogen bonds and the strong hydrophobic in-
teractions between the alkyl chains, an ordered composite
bilayer structure was formed. Compared with the bilayers
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Chiral Twists in l-Glutamic Acid–Bipyridine Systems
FULL PAPER
structure.[10j] This was further demonstrated by PCMOD
simulations of C18/4ePy, which showed that the distance be-
tween the intramolecular pyridyl N atoms of 4ePy dramati-
cally decreased from 1.00 to 0.90 nm with the introduction
of one additional equivalent of 4ePy, whereas this distance
remained almost unchanged with 4Py (Figure S8). Such
structural flexibility was in complete contrast to the structur-
al rigidity with 4Py. This flexibility of 4ePy meant that there
was insufficient p-p stacking interaction strength to facilitate
a chiral twist.
Actually, not only the rigidity of the bipyridine, but also
the position of its pyridyl nitrogen atoms, as in 2Py, had an
influence on the formation of twists. FTIR data indicated
that different positions of the pyridyl nitrogen atoms had a
great influence on the interaction mode of the carboxylic
acid–pyridyl hydrogen bonds. PCMOD results further re-
vealed that each 2Py could only form a hydrogen bond with
one C18 in forming C18/2Py assemblies, whereas 4Py and
4ePy could each interact with two C18 in forming the re-
spective assemblies (Figure S8). These differences ruled out
the formation of a twist in the case of C18/2Py assemblies.
The different modes of the carboxylic acid–pyridyl H-bonds
were corroborated by CD spectroscopic analysis. C18/2Py
showed a strong negative Cotton effect on account of the
transcription of the chiral information from the S-configura-
tion glutamic acid function (l-glutamic acid) to 2Py. In
sharp contrast, each 4Py or 4ePy molecule interacted with
two C18 amphiphiles, and the combination of the same two
S-configuration glutamic acid functions led to a cooperative
interaction of the chiral centers and induced a positive
Cotton effect for the C18/4Py and C18/4ePy assemblies.[9g]
Finally, hydrophobic interactions were also essential for
the formation of the twists. In the case of these homologous
amphiphiles, hydrophobicity is directly related to the length
of their alkyl chains. FTIR investigation revealed that the
hydrophobicity of C16 is analogous to that of C18. There-
fore, strong CD signals with the same positive Cotton effect
and exclusively right-handed twists were obtained. With the
shorter alkyl chains of C14, imperfections in the packing of
the alkyl chains were evident from FTIR measurements,
which showed asymmetric and symmetric CH2 stretching vi-
brations at 2921 and 2852 cmÀ1, respectively. Consequently,
in contrast to the flat ribbons of C18/4Py=1:1 and C16/
4Py=1:1, twists started at C14/4Py=1:1. Moreover, both
right- and left-handed twists were always obtained, which
was another remarkable difference from the exclusively
right-handed twists of the longer-chain amphiphiles. Consid-
ering that C14/4Py=1:2 could also form sandwich struc-
tures, we inferred that the p–p stacking and hydrophobic in-
teractions might have competing parallel effects on the in-
teractions of the chiral centers of C14. Therefore, the kinet-
ic balance between them invariably produced opposing
right- and left-handed twists. As befits this interpretation,
they were consistently CD-silent at the nanoscale. The CH2
stretching vibrations (2922, 2852 cmÀ1) indicated that the
C12 alkyl chain was more loosely packed than that of C18.
In addition, the carboxylic acid–pyridyl hydrogen bond di-
Figure 8. Model for the formation mechanism of a co-assembled chiral
twist from C18/4Py (for details, see discussion text).
formed by the pure Cn molecules, the composite bilayers
were further stabilized by additional p-p stacking interac-
tions between the aromatic bipyridines xPy. This basic unit
imparted the resulting bilayers with extra rigidity. As a
result, flat ribbons were formed instead of worm-like fibers.
At a 1:1 ratio of C18/4Py, the carboxylic acid and bipyridine
combined to produce this structure and self-assembled into
flat strips or ribbons due to the increased rigidity of the
headgroups. When a slight excess of 4Py was added, the flat
ribbons started to twist because of the involvement of more
p–p stacking interactions. Apparently, the additional 4Py
was inserted into the cavity. We have carried out a PCMO-
DEL simulation on such a structure. It showed a remarkable
decrease in the distance between the two intermolecular car-
boxylic groups of the C18 molecule from 0.71 nm in C18/
4Py=1:1 to 0.67 nm in C18/4Py=1:1.5. This result supports
the view that when additional 4Py is introduced, a sandwich
supramolecular structure may be formed. Incorporation of
this bipyridine into the cavity increases the rigidity of the
headgroups and forces the two carboxylic groups of the C18
molecule closer together. Consequently, the chiral centers of
the C18 molecules are packed much more closely, similar to
those in linear p-conjugated systems, and this eventually
causes the nanoribbon to form a twist. Up to C18/4Py=1:2,
sandwiched 4Py continues to be incorporated and uniform
twists are essentially formed, which then remain unchanged
with the addition of further 4Py.
The rigid planar structure of the bipyridine unit would
seem to be an essential prerequisite for formation of the
chiral twist. When 4ePy was added, although similar struc-
tures were formed, as suggested by similar FTIR and CD
spectra to those of the C18/4Py mixture, no twist was
formed. This was due to the ethylene bridge between the
two pyridyl rings, which imparts much flexibility to the
Chem. Eur. J. 2011, 17, 3429 – 3437
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M. Liu et al.
(XRD) measurements. Samples were first vacuum-dried and made into
plates with KBr for Fourier-transform infrared (FTIR) spectral measure-
ments.
minished due to proton transfer from the carboxylic acid to
the pyridyl N when C12 interacted with 4Py, as verified by
UV/Vis spectra. As a result, the CD spectra were silent and
no twist was obtained.
Instruments: SEM was performed on a Hitachi S-4300 FE-SEM and
TEM images were obtained on a JEM-1011 EM at accelerating voltages
of 15 kV and 100 kV, respectively. FTIR spectra were recorded on a
Bruker Tensor 27 FTIR spectrometer at room temperature at a resolu-
tion of 4 cmÀ1. X-ray diffraction (XRD) analysis was performed on a
Rigaku D/Max-2500 X-ray diffractometer (Japan) with CuKa radiation
(l=1.5406 ꢃ), which was operated at a voltage of 45 kV and a current of
100 mA. UV/Vis and circular dichroism (CD) spectra were obtained on
JASCO UV-550 and JASCO J-815 CD spectrophotometers, respectively.
Conclusion
In summary, a simple supramolecular approach has been
proposed to achieve chirality transcription and twisted nano-
structures in a two-component system consisting of l-gluta-
mic acid-based amphiphiles and bipyridines. The mechanism
of the evolution of the chiral twist has been systematically
investigated by SEM and XRD analyses, FTIR, CD, and
UV/Vis spectral measurements, and molecular simulation. It
is suggested that carboxylic acid–pyridine hydrogen-bonding
interactions and a novel kind of sandwich-like supramolec-
ular structure are responsible for the twist formation. The
stoichiometric ratio of glutamic acid/bipyridine and the alkyl
chain length also exert significant effects. In such a self-as-
sembling system, a delicate balance between p–p stacking,
hydrophobic, and chiral interactions leads to fine tuning of
the nanostructures and the supramolecular chirality. Our re-
sults permit a comprehensive understanding of the self-as-
sembly of such chiral nanostructures through the synergistic
effects of multifarious noncovalent interactions. Studies on
amphiphiles with unsaturated alkyl chains and/or enantio-
meric glutamic head groups are underway.[18a,b]
Acknowledgements
This work was supported by the National Basic Research Program (nos.
2007CB808005, 2006CB932101), the National Natural Science Founda-
tion of China (nos. 21021003, 20773141), and the Fund of the Chinese
Academy of Sciences. Grateful thanks to Dr. Guocheng Zhang for valua-
ble guidance and discussion, to Dr. Peng Gao and Ms. Qingxian Jin for
performing the syntheses, and to Dr. Ling Zhong for help with the TEM
observations.
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Experimental Section
Materials: Amphiphiles Cn (n=18, 16, 14, 12) derived from l-glutamic
acid with different alkyl tails were synthesized as reported previous-
ly[12b,17] and thoroughly characterized (see the Supporting Information).
Three analytically pure bipyridines (xPy), namely 4,4’-bipyridine dihy-
drate (4Py), 2,2’-bipyridine (2Py), and 1,2-bis(4-pyridyl)ethane (4ePy),
were used directly as received.
Self-assembly procedures: The respective Cn (1.5ꢂ10À5 mol), in the form
of a white solid, and deionized water (Milli-Q water, 18.2 MWcm, 1 mL)
were mixed in a seal-capped vial, and the aqueous suspension was heated
to boiling. The vial was subsequently transferred to an ultrasonic bath
(100 W, 908C) and sonicated for several minutes until all of the solid had
dissolved to give a clear solution. The hot solution was then allowed to
cool naturally to ambient temperature (258C) at a cooling rate of about
38C minÀ1, whereupon a translucent hydrogel was obtained after 30 min.
For experiments involving self-assembly of the amphiphiles with the bi-
pyridines, 0.045m aqueous solutions of xPy were initially prepared as
stock solutions. Then, the required aliquot was injected into a seal-
capped vial containing the Cn (1.5ꢂ10À5 mol) and an appropriate
amount of water was added to make a total volume of 1 mL. Thereafter,
the mixtures were treated as described above. Finally, a white opaque hy-
drogel was obtained for Cn/4Py, while precipitates were obtained for Cn/
2Py and Cn/4ePy.
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Chiral Twists in l-Glutamic Acid–Bipyridine Systems
FULL PAPER
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Received: September 8, 2010
Revised: November 17, 2010
Published online: February 21, 2011
Chem. Eur. J. 2011, 17, 3429 – 3437
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www.chemeurj.org
3437