Supramolecular helical fluid columns from self-assembly of homomeric dipeptides

Article abstract of DOI:10.1002/chem.200801607

Herein, we demonstrate that with the widespread theme of residue patterning and stereochemical restraints of self-complimenting proteinogenic amino acids, a new and rich class of homomeric dipeptides exhibiting two-dimensional fluid aggregates with hierarchical ordering can be obtained. In particular, a simple way of achieving a class of functional dipeptides, wherein the first and the second residues chosen are L-/D-alanines and L-/D-leucines, has been accomplished. The supramolecules synthesized can be regarded as intermediates between polycatenars and taper-shaped amphiphiles because they possess two lipophilic segments interlinked by a peptide unit (spacer). Two pairs of enantiomers and their respective diastereomers derived from these amino acids are evidenced to self-organize into a helical columnar phase through hydrogen bonding by means of FTIR, UV/Vis, and chiroptical circular dichroism (CD) spectral analyses as well as by optical, calorimetric, electrical switching, and X-ray studies. The CD and X-ray studies have revealed that the form chirality (handedness) and the magnitude of out-of-plane fluctuations of the lattice planes of the fluid supramolecular columnar structures are solely directed by the stereochemistry encoded in the spacer. Of special significance, the less frequently found oblique helical columnar phase formed by a pair of enantiomers derived from L-/D-alanines, unlike those derived from other amino acids, exhibit ferroelectric behavior; the measured spontaneous polarization is as high as 440 nCcm^-2. Besides, all these supramolecules form stable organogels in ethanol and the CD and SEM studies on a representative gel suggest the presence of helical structures.

Full text of DOI:10.1002/chem.200801607

DOI: 10.1002/chem.200801607
Supramolecular Helical Fluid Columns from Self-Assembly of Homomeric
Dipeptides
Channabasaveshwar V. Yelamaggad,*[a] Govindaswamy Shanker,[a] R. V. Ramana Rao,[b]
Doddamane S. Shankar Rao,[a] Subbarao Krishna Prasad,[a] and
Vommina V. Suresh Babu[b]
Abstract: Herein, we demonstrate that
with the widespread theme of residue
patterning and stereochemical re-
straints of self-complimenting proteino-
genic amino acids, a new and rich class
of homomeric dipeptides exhibiting
two-dimensional fluid aggregates with
hierarchical ordering can be obtained.
In particular, a simple way of achieving
a class of functional dipeptides, where-
in the first and the second residues
chosen are l-/d-alanines and l-/d-leu-
cines, has been accomplished. The su-
pramolecules synthesized can be re-
garded as intermediates between poly-
catenars and taper-shaped amphiphiles
because they possess two lipophilic seg-
ments interlinked by a peptide unit
(spacer). Two pairs of enantiomers and
their respective diastereomers derived
from these amino acids are evidenced
to self-organize into a helical columnar
phase through hydrogen bonding by
means of FTIR, UV/Vis, and chiropti-
cal circular dichroism (CD) spectral
analyses as well as by optical, calori-
metric, electrical switching, and X-ray
studies. The CD and X-ray studies
have revealed that the form chirality
(handedness) and the magnitude of
out-of-plane fluctuations of the lattice
planes of the fluid supramolecular col-
umnar structures are solely directed by
the stereochemistry encoded in the
spacer. Of special significance, the less
frequently found oblique helical colum-
nar phase formed by a pair of enantio-
mers derived from l-/d-alanines, unlike
those derived from other amino acids,
exhibit ferroelectric behavior; the mea-
sured spontaneous polarization is as
high as 440 nCcmꢀ2. Besides, all these
supramolecules form stable organogels
in ethanol and the CD and SEM stud-
ies on a representative gel suggest the
presence of helical structures.
Keywords: amino acids · gelation ·
hydrogen bonds · liquid crystals ·
peptides
Introduction
definite length into two-dimensional lattices with different
symmetries are examples of such novel structures. General-
ly, fluid columns result from the noncovalent interactions of
a variety of molecular architectures, such as discotics,[2,3]
bent-core molecules,[4] polycatenars,[5] and dendrimers,[6] as
shown in Figure 1. However, to achieve even greater struc-
tural order and complexity in these fluid 2D motifs with se-
lectivity, directionality, reversibility as well as the possibility
of controlling the strength of the interactions through self-
assembly and intermolecular hydrogen bonding, supramo-
lecules such as amphiphiles,[7] bolaamphphiles,[8] ami-
des,[9,10,11a–e,12,13] hydrazides,[11f] and peptides,[14–16] seem to be
the most promising and advanced approach.
Molecular self-assembly, a prevailing means of generating
novel networks that combine several properties and respond
to external stimuli, has great significance in biological and
material sciences.[1] Columnar (Col) liquid-crystal (LC)
phases[2] built up by the self-organization of columns of in-
[a] Dr. C. V. Yelamaggad, G. Shanker, Dr. D. S. S. Rao, Dr. S. K. Prasad
Centre for Liquid Crystal Research
Jalahalli, Bangalore 560013 (India)
Fax : (+91)80-2838-2044
E-mail: Yelamaggad@gmail.com
Of these supramolecules, biocompatible polypeptides[14]
and low molar mass peptides[15,16] have been attracting a
great deal of attention owing to their ability to form thermo-
tropic or lyotropic Col phases with a variety of supramolec-
ular structures. For example, poly(g-n-alkyl-l-glutamate),
which has an intrinsic rigid a-helical chain resulting from
[b] Dr. R. V. R. Rao, Prof. V. V. S. Babu
Department of Studies in Chemistry
Central College Campus
Bangalore University, Bangalore 560001 (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801607.
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FULL PAPER
Results and Discussion
Synthesis: The proposed enan-
tiopure oligomers were synthe-
sized in six simple and high
yielding steps using the key in-
termediates shown in Scheme 1.
The coupling of appropriate
Na-Fmoc amino acids (Fmoc=
9-fluorenylmethoxycarbonyl;
Figure 1. Models of fluid columns formed by different molecular architectures, such as a) discogens, b) bent-
core systems (the bow/polar axes, indicated by an arrow, are along the column axis), c) polycatenars, d) den-
drimers or amides, e) bolaamphiphiles, and f) polypeptides. Colh =hexagonal columnar phase, Colr =rectangu-
lar columnar phase.
27a,b and 28a,b) with various
n-alkoxyanilines (21–23) in the
the intramolecular hydrogen-bonding network of constituent
amino acid (monomers) residues, aggregates into a 2D hex-
agonal lattice.[12] Several low molar mass, but polymerizable,
Brønsted acidic (amides) amphiphiles preferentially form
the inverted hexagonal (HII) lyotropic LC phase.[13] Semi-
fluorinated twin dendritic benzamides are shown to self-or-
ganize into bilayered pyramidal supramolecular columns.[14]
Recently, a large number of dendritic (depsi) dipeptides
have been realized and are shown to self-assemble into
porous and supramolecular Col structures.[15] Notably, Col
LCs formed by depsipeptides[16] derived from folic acid and/
or glutamic acid residues are regarded as the biocompatible
fluids for use in membranes and sensors. Herein, we demon-
strate for the first time that, with the widespread theme of
residue patterning and stereochemical restraints of self-com-
plementing proteinogenic amino acids,[17] a new and rich
class of homomeric oligopetides exhibiting two-dimensional
fluid aggregates with hierarchical ordering can be obtained.
Thereby, anisotropic functional materials can be accom-
plished in which many important features, such as chirality,
ferroelectric behavior, gelating capabilities, and biocompati-
bility, can be incorporated that are attractive from the view
points of both life science and technologically important ma-
terials. For this purpose, a number of aspects were consid-
ered to give such systems a synthetic and structural versatili-
ty, whilst maximizing the probability of mesophase stabiliza-
tion. In particular, supramolecules consisting of two lipophil-
ic (n-trialkoxybenzene) segments interlinked through a pep-
tide unit (spacer), and thus, representing the features of
both hexacatenars and taper-shaped amphiphiles were de-
signed. Such a design should also help in understanding the
structure–property relationship, which can be extrapolated
to know the behavior of macromolecules. Thus, as a first
step in this direction, a simple way of achieving a new class
of materials was conceived for the synthesis of dipeptides in
which the first and the second residues may be any one of
the amino acids available, yielding a large number of distinct
dipeptides. In this study, l-/d-alanines and l-/d-leucines are
chosen to form dipeptide sequences to achieve the targets.
The analogous but representative dipeptides substituted
with mono- and dialkoxy tails on each of the benzene cores
were also prepared to support the point that the space-filling
concept is essential to realize fluid columnar organization.
presence of N-methylmorpholine (NMM) and isobutoxycar-
bonyl chloride (IBC-Cl) gave Fmoc amino acid amides 17–
20; these amides, upon treatment with a 1:1 mixture of di-
ethylamine/dichloromethane, gave amines 13–16. The
amines obtained were coupled with Na-Fmoc amino acids
27a,b and 28a,b by using NMM and IBC-Cl to obtain pro-
tected dipeptides that were deprotected under the same re-
action conditions mentioned previously to obtain key
amines 5–8. Finally, target molecules 1–4 were formed in ex-
cellent yields when n-alkoxybenzoic acids (24–26) were cou-
pled with amines 5–8 in the presence of 2-(1H-benzotriazol-
1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate
(HBTU). It is appropriate to mention here that key amines
5–8 and 13–16, unlike other intermediates (9–12 and 17–20),
were isolated in quantitative yields and used directly in the
next step. The molecular structure of the peptides were as-
1
sessed by UV, FTIR, H, and 13C NMR spectroscopies; mass
spectrometry; and elemental analyses (see the Supporting
1
Information). Notably, the H–1H COSY NMR spectroscopy
profile illustrated coupling between the NH and methine
proton of the peptide chain (see the Supporting Information
for details), which supported the structural assignment.
Infrared, UV/Vis, and circular dichroism (CD) spectroscopic
studies: The FTIR spectra of the dipeptides indicated the
existence of strong intermolecular hydrogen bonding in both
ꢀ
the solid (see the N H and C=O stretching vibrations given
in the Experimental Section and also the Supporting Infor-
mation for more information) and LC states. For example, a
thin film of 3a sandwiched between the KBr plates was
cooled from the isotropic phase to room temperature, and
the FTIR spectrum was recorded at regular temperature in-
tervals. As expected, as the temperature decreased two
broad bands at around 3281 and 1633 cmꢀ1 due to N H and
ꢀ
C=O stretching vibrations, respectively, became stronger (in-
creased in intensity) and were pronounced near room tem-
perature (see the Supporting Information). These results
clearly indicate the hydrogen-bonding interaction between
the NH and CO groups, which become progressively stron-
ger on going from the isotropic phase to the mesophase, and
further to the glassy state.[11b,17c] In addition, disordering of
the n-alkoxy tails present on either side of the dipeptide was
indicated by the observation of asymmetric and symmetric
ꢀ1
ꢀ
C H stretching bands at 2937 and 2858 cm , respective-
ly.[11f]
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C. V. Yelamaggad et al.
Scheme 1. i) NMM, isobutoxycarbonyl chloride, THF, 08C!RT, 10 h, 70–74%; ii) diethylamine/dichloromethane (1:1), RT, 2 h, quantitative; iii) 27a,
27b, 28a, or 28b, NMM, isobutoxycarbonyl chloride, THF, 08C!RT, 12 h, 68–70%; iv) 5, 6, 7a–c, or 8a–c, HBTU, THF, N-ethyldiisopropylamine, 70–
72%.
Owing to the presence of multiple chromophores and hy-
drogen bonds, these dipeptides are expected to exhibit inter-
esting photophysical properties. Thus, UV/Vis absorption
and CD spectroscopic studies were carried out. The solution
UV/Vis spectra (in CH2Cl2) of all of the compounds showed
an absorption maxima centered at around 260 nm (see the
Supporting Information); as representative examples, the so-
lution UV/Vis spectra for 4a and 4b are shown in Figure 2a.
CD measurements carried out at 238C for dilute solutions
of all the dipeptides (7.3ꢁ10ꢀ5 m in CH2Cl2; cell length=
1 mm) showed a significant Cotton effect, establishing the
supramolecular chirality of the aggregates, wherein mole-
cules self-assemble into helical arrays through the hydrogen
bonding of successive peptide chains. For example, as shown
in Figure 2b, enantiomeric peptides 4a and 4b exhibit, as
anticipated, mirror-image Cotton effects with the absorption
bands at l=242 (molar ellipticity=ꢀ12.1), 273 (molar ellip-
ticity=+11.4), 239 (molar ellipticity=+10.6), and 275 nm
(molar ellipticity=ꢀ13.4). The zero crossing in both cases is
close to the absorption maximum of the chromophores (l
ꢁ260 nm; Figure 2a), which indicates exciton coupling due
to dipeptides aggregated through hydrogen bonds to gener-
ate a helical (chiral) superstructure. To investigate whether
molecular chirality coupled with hydrogen bonding has fa-
cilitated the self-assembly of dipeptides into fluid helical col-
umns, qualitative CD measurements were carried out for
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Self-Assembly of Homomeric Dipeptides
FULL PAPER
koxy tails, display Col LC be-
havior, which supports our
design view that space filling is
essential.
These compounds with three
distinct, incompatible regions
(paraffinic tails, aromatic cores,
and a peptide spacer) can be
termed amphiphilic hexacate-
nars. Most importantly, the
nature
(stereochemical
re-
strains) of the central peptide
segment in the molecules dic-
tates their packing, which in
turn reflects on the magnitude
of out-of-plane fluctuation of
the lattice plane. Enantiomers
3a and 3b exhibit an identical
and enantiotropic mesophase.
When placed between clean
glass plates and cooled from
the
isotropic
phase
(3a:
3b:
Figure 2. UV/Vis (a) and CD (b) spectra obtained for the dilute solutions of dipeptides 4a and 4b. The CD
spectra obtained in the mesophases as a function of temperature for dipeptide 4a (c) and 4b (d) are also
shown. Note that the enantiomeric pairs exhibited mirror image CD curves in both solution and mesophase
states.
175.18C, DH=5.9 Jgꢀ1
;
176.68C, DH=6.5 Jgꢀ1) these
samples
display
dendritic
growth, as shown in Figure 3a
and b, that quickly fills the field
the enantiomeric pairs 3a/3b and 4a/4b. These compounds,
held between two quartz plates, were heated to their iso-
tropic phase and cooled slowly, during which time the CD
spectra were recorded at selected temperatures; notably, all
the fluid phases displayed the Cotton effect, which signified
large chiral correlations in the molecular packing.[11] In par-
ticular, each of the enantiomeric pairs exhibited mirror-
image CD curves, as illustrated in Figure 2c and d for com-
pounds 4a and 4b as representative examples (see the Sup-
porting Information for the CD plots of 3a and 3b). It can
be seen that the intensity of the CD signals increases with
the decrease in temperature. Thus, from the results above, it
is evident that these dipeptides form helical aggregates in
solution and/or mesophase states governed by the nature of
the substituent and its orientation (stereochemistry) in the
peptide linkage.
of view and remains unaltered up to room temperature
(RT). The dendritic textural pattern suggests that the meso-
phase is a Col phase. The Col phase, once formed upon
melting, does not crystallize and instead it freezes into a
glassy state and exists for a wide thermal range, as estab-
lished by both optical and calorimetric studies; the textural
pattern obtained remains as such for any period of time and
no exothermic (from just below the Iso–Col phase transition
to ꢀ608C) or endothermic (from ꢀ608C to near the Col–Iso
phase transition) peaks were seen in the first cooling or
second heating DSC profiles, respectively. Figure 4 depicts
the DSC profiles obtained for compound 3a, in which the
above-mentioned features are seen. The 2D glassy state fea-
turing a helical order, as evidenced by CD measurements, is
of special importance because in such a state the charges
Table 1. Transition temperatures [8C] and enthalpies [Jgꢀ1] of peptides.[a]
Mesomorphic behavior: The
thermal behavior of dipeptides
1–4, ascertained with the aid of
optical polarizing microscopy
(OPM), differential scanning
calorimetry (DSC), and X-ray
diffraction (XRD) studies, is
summarized in Table 1. Com-
pounds 1 and 2, comprising of
dialkoxy and tetraalkoxy tails,
respectively, fail to exhibit LC
behavior. In contrast, dipepti-
Dipeptides
Phase sequence
Cooling
Heating
1
2
Cr1 136.6 (33.6) Cr2 142.1 (185.7) >320 I
Cr1 79.2 (3.6) Cr2 130.9 (32.9) I[b]
Cr 51.1 (35.8) Colob 181.1 (6.7) I
Cr 52.5 (29.9) Colob 181.1 (7) I
Cr 76 (6.6) Colr 102.3 (6.3) I
Cr1 49.7 (17.4) Cr2 220.1 (1.1) Col 228.1 (7) I
Cr 218.5 (2.6) Col 228.1 (14.8) I
Cr 90.9 (12.2) I
–
–
[c]
[c]
3a
3b
3c
4a
4b
4c
I 175.1 (5.9) Colob
I 176.6 (6.5) Colob
I 89.4 (6.3) Colrc
I 221 (6.4) Col 191 (2) Cr
I 224.6 (10) Col 197.8 (2.6) Cr
I 80 (6) M 78.1 (5) Colr 24.1 (1.4) Cr
[a] Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at
58Cminꢀ1. Colob =oblique columnar phase, Colr =rectangular columnar phase, M=unidentified mesophase,
I=isotropic liquid phase, Cr1 and Cr2 =crystal. [b] Upon cooling the isotropic liquid, the sample freezes into
des 3a–c and 4a–c, with hexaal- glassy state. [c] The phase exists (supercools) up to the limit of the DSC (ꢀ608C) instrument.
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C. V. Yelamaggad et al.
Table 2. Results of indexation of XRD profiles at a given temperature of
phases displayed by the dipeptides 3a–c and 4a–c. (The negative sign in
front of the miller index digit indicates an overline).
Dipeptides
T
[8C]
Phase
dexp
[ꢂ]
dcalcd
[ꢂ]
Miller
index
Lattice
parameters [ꢂ]
3a
100
Colob
25.69
23.66
21.27
19.88
16.21
14.41
13.38
10.69
4.51
25.65
23.37
21.31
19.80
16.06
14.36
13.57
10.71
20
21
ꢀ21
ꢀ12
ꢀ22
32
a=51.6
b=45.2
g=83.9
23
ꢀ33
3b
100
Colo
25.75
23.66
20.87
19.74
16.09
14.45
13.26
10.65
9.53
25.65
23.50
21.15
19.59
15.90
14.44
13.59
10.63
9.74
20
21
ꢀ21
ꢀ12
ꢀ22
32
a=51.7
b=45.6
g=82
Figure 3. Microphotographs of the optical textures obtained for the Colob
phase of 3a at 1708C (a) and 3b at 1778C (b); the Colr phase of 3c at
888C (c), 4a at 2058C (d), 4b at 2008C (e), and 4c at 608C (f).
23
ꢀ14
ꢀ51
4.57
3c
80
Colr
31.82
26.22
17.43
16.32
4.45
31.80
26.23
17.49
16.02
11
20
30
31
a=52.5
b=40
4a
4b
4c
210
210
70
Col
Col
Col
22.77
8.50
4.66
4.63
22.72
8.48
11
20
03
32
11
20
03
32
11
20
21
02
12
31
22
40
13
23
42
Figure 4. DSC thermograms of amphiphilic dipeptide 3a obtained at a
rate of 58Cminꢀ1
.
4.67
4.62
19.23
16.24
13.50
11.90
10.35
9.96
8.92
8.05
7.68
7.00
19.07
16.11
13.31
11.83
11.1
9.78
9.53
8.05
7.66
a=32.2
b=23.7
generated flow smoothly at the expense of the motion of the
ionic impurities.[6]
The powder XRD patterns obtained in the mesophase of
both compounds at different temperatures, including RT,
were found to be almost identical (see the Supporting Infor-
mation). In all of the profiles, a typical diffuse peak occur-
ring at high angles (2qꢁ208, spacing c=4.51 ꢂ) indicates
the fluid nature of the phase (Table 2). In the low-angle
region, a number of sharp and intense peaks were seen, a
feature that indicates long-range order of the 2D lattice. As
a representative example the 2D diffraction pattern (Fig-
ure 5a) and the derived one-dimensional intensity versus 2q
profile (Figure 5b) of an unoriented (powder) sample of the
Col phase formed by 3a are shown. First, we note that the
ratio of the two lowest angle peaks is 1.1, a value that is in-
7.08
6.66
6.67
4.53
3.86
Therefore, we assign the lattice (for both 3a and 3b) to be
an oblique one. The unit cell volume increases by approxi-
mately 4% when the temperature is increased from 100 to
1608C, and is accompanied by an increase in h as well as the
cell area. Assuming the density to be 1 gcmꢀ3 the lattice pa-
rameters suggest four molecules per unit cell. A possible
molecular configuration involving these features and with
polar properties is shown in Figure 6c.
Interestingly, this phase does not exhibit an X-ray peak
that is typical in systems possessing a finite, albeit short
range, interaction between the rigid cores of the neighboring
molecules of the same column. Therefore, the structure has
an extremely well-defined 2D lattice, but hardly any posi-
consistent with a hexagonal lattice for which the ratio
p
should be 1.73 ( 3). As can be seen from Table 2, such a de-
viation is true for all the dipeptides derived from alanine
studied herein and rules out the hexagonal lattice. Next, we
performed indexation of the reflections using a least-squares
method. Table 2 lists the d-spacing values obtained experi-
mentally and calculated from indexed lattices and it is seen
that deviations between the two are generally small. An ap-
propriate fitting of the data for compounds 3a and 3b point
to an oblique lattice. Forcing the fit to a rectangular lattice
results in a considerable increase in the cumulative error.
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Self-Assembly of Homomeric Dipeptides
FULL PAPER
number of sharp reflections
(Table 2 and the Supporting In-
formation) in the low-angle
region is reduced to four. While
the hexagonal lattice is ruled
out on the basis of the ratio of
the spacings of the first two re-
flections, fitting to an oblique
lattice (a=65.4 ꢂ, b=34.2 ꢂ
and g=76.68) and rectangular
columnar lattice with the unit
cell parameters a=52.5 ꢂ and
b=40 ꢂ were considered. How-
ever, fitting to the oblique lat-
tice yielded an increased cumu-
Figure 5. Two-dimensional XRD pattern (a) and the derived 1D-intensity versus 2q profile (b) for the Colob
phase of dipeptide 3a.
lative error. Therefore, we believe the lattice to be rectangu-
lar. Further, since the h and k values are such that h+k is
odd as well as even, a centered lattice is ruled out. There-
fore, we suggest that the space group is P21. Of course, a dif-
fuse reflection is seen at wide angles centered at a spacing
of around 4.4 ꢂ, indicating liquidlike ordering.
Homomeric dipeptides 4a and 4b, constructed from two
l-leucine and two d-leucine residues, respectively, exhibit an
identical but enantiotropic Col phase, as revealed by optical,
calorimetric, and XRD studies. The optical texture was a
combination of pseudofocal conic fan-shaped defects, and
homeotropic digitated stars (Figure 3d and e), which is typi-
cal of the Col phase; we faced a problem indexing the latti-
ces, since only two sharp reflections were seen with spacings
of 22.77 and 8.5 ꢂ for 4a, and 22.72 and 8.48 ꢂ for 4b.
Therefore, no assignment is made to the lattice structure for
the Col of dipeptides 4a and 4b. Further, an unusual broad
peak occurring at around 2q=68 was seen (Figure 7). Note
that such a feature has also been reported for hexaazatri-
naphthylene[20] and polycatenar[21] mesogens, and attributed
to the height of a Col segment.
Figure 6. a) Molecular (with a schematic) model of 3a in its all-trans con-
formation (front view), b) schematic representation of a supramolecular
column (lateral view) in which peptide molecules organize in a helical
fashion, and c) a possible lattice (the arrows indicate the direction of the
dipole moment).
tional order perpendicular to the lattice plane. The reason
for the absence of positional ordering could be that unlike
in conventional discotics the central region in the present
molecule is not a rigid core.[18] The fact that a Cotton effect
could be observed in the Col phase establishes that the
structural organization is due to intermolecular hydrogen
bonding between the hexacatenar-like mesogens (Figure 6a)
within the same column causing a helical array as shown in
Figure 6b (in this model, for convenience, the two terminal
phenyl rings are sketched to be planar; needless to say,
these two rings are otherwise free to rotate). Further, such
indefinitely long columns in turn self-organize to form a Col
phase with the cell parameters discussed above (a possible
lattice is shown in Figure 6c, which is also one of the possi-
bilities suggested for another ferroelectric Col phase[22c]). It
may be recalled that such Col structures with a helical ar-
rangement are seen in chiral molecules self-assembled in a
solvent medium.[19a] It may be emphasized that in the pres-
ent case the self-assembly occurs in a solvent-free medi-
um.[19b]
Figure 7. One-dimensional intensity versus 2q profile derived from the
two-dimensional XRD pattern obtained at 2108C for the Col phase of
compound 4a.
Dipeptide 3c resulting from coupling d- and l-alanine res-
idues displayed a mesophase with different textural and
XRD patterns than those obtained for its diastereomers 3a
and 3b. An enantiotropic mesophase, which on cooling from
the Iso phase shows a noncharacteristic texture (Figure 3c)
for any surface-anchoring condition, exists over a wide ther-
mal range and does not crystallize. In the XRD profile, the
Dipeptide 4c, possessing d- and l-leucine residues, stabil-
izes two monotropic mesophases unlike its diastereomers 4a
and 4b. When the thin film of an isotropic liquid of com-
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C. V. Yelamaggad et al.
pound 4c held between clean glass slides was cooled slowly,
a transition to the mesophase (hereafter referred to as M)
occurs with uncharacteristic grainy texture of low birefrin-
gence; this was true even when slides treated for planar or
homeotropic alignments were used. Because of the short
thermal range of the phase, its structural elucidation by X-
ray investigations was not successful. However, the identity
of the Col phase occurring below the M phase was con-
firmed by OPM and XRD studies. It showed an optical tex-
ture pattern consisting of both pseudofocal conic fan-shaped
defects and a homeotropic region typical of the Col phase.
In the low-angle region, a set of sharp reflections was ob-
tained and here again we rule out the hexagonal lattice
owing to the small ratio of the first two low-angle reflec-
tions. This is supported by the presence of the 210 reflection,
which is inconsistent with the hexagonal symmetry. A hex-
agonal cell can be treated as equivalent to a C-centered or-
ascribed to occurrence of multiple hydrogen bonds between
ꢀ
N H and C=O groups and subtle microphase segregation
among the heterogeneous molecular fragments.
Electro-optical switching: From the symmetry arguments, all
the tilted columns formed from chiral disklike molecules are
expected to show ferroelectric behavior as the columns are
spontaneously polarized perpendicular to the plane contain-
ing the column axis and the disk normal. Surprisingly, the
majority of tilted columns consisting of chiral disklike mole-
cules are apolar (nonswitchable); except for a few exam-
ples,[22–25] all known discotics exhibiting a polar columnar
phase possess O-alkyl-lactic acid chains that create strong
tilt-induced dipoles.[22e,f] Given the fact that the dipeptides of
the present investigation exhibit a columnar phase com-
posed of chiral supramolecules, they were tested for ferro-
electric behavior. For this purpose, samples contained be-
tween ITO-coated glass plates were cooled slowly from the
isotropic phase. Then the electro-optic switching features
were observed under the OPM. To ascertain the ferroelec-
tric nature of the mesophase, as well as to determine the
spontaneous polarization (Ps), a triangular wave field was
used. Only the Colob phase of 3a–3b could be switched,
which we describe as follows: On application of a triangular
wave field (64 V, 10 Hz; thickness of the cell 8.5 mm) the Col
phase of 3a–3b showed a single peak per half cycle of the
field (Figure 8a), indicating the ferroelectric nature; the
peak vanishes in the isotropic phase ruling out any ionic
origin. When the sample cell was oriented such that one of
the states has minimum intensity between crossed polarizers,
there is a change in the birefringence when the polarity re-
versal of the field switches the other state (see Figure 8b
and c). To further support the argument that the switching is
due to the ferroelectric character, we monitored the light
transmission from the sample kept between crossed polariz-
ers. Upon application of a square wave field, a sharp varia-
tion in the sample transmission is seen as shown in Fig-
ure 8d.
thorhombic cell with the lattice constants satisfying the con-
p
dition a=b 3. But for a C-centered lattice the reflections
with an odd value for h+k must be systematically absent, a
feature not corroborated by the data in Table 2 (also note
that the (21) reflection is quite intense). This feature is, how-
ever, possible if there is an alternation in the tilt direction
so as to give a herringbone-type of arrangement, with the
space group P21a and two molecules per unit cell. Unlike
the compounds derived from alanine, the leucine derivatives
exhibited, in the wide-angle region, two broad diffuse peaks
well separated in the case of 4c. While the slightly lower
angle one with a spacing of approximately 4.5 ꢂ is due to a
correlation between aliphatic chains, the higher angle one
indicates the correlation of the molecular cores within the
same column. It is interesting to see that diastereomer 4c
has the closest packed (3.8 ꢂ) cores.
It is quite prominent that enantiomers 3a,b and 4a,b and
their respective diastereomers 3c and 4c profoundly differ
not only in their phase-transition temperatures, but also in
the extent of lattice formation of the columnar phase. Most
strikingly, the Col–Iso transition temperatures are quite high
(>1808C) for enantiomeric pairs 3a,b and 4a,b, when com-
pared with their respective diastereomers 3c and 4c (Col–
Iso<1008C). The important difference between these supra-
molecules is the stereochemical feature of the peptide chain
(spacer) resulting from the size and orientation (chirality) of
the substituent of the amino acid components present,
which affects the extent of hydrogen-bond formation in the
resulting supramolecular structure. The presence of two sub-
stituents on either side of the peptide chain enforces thor-
ough and strong intermolecular hydrogen bonding for com-
pounds 3a,b or 4a,b leading to interactions between the su-
pramolecules of the column that reflects on the overall col-
umnar structure. Thus, it is noteworthy that these novel ho-
momeric dipeptides form oblique or rectangular columnar
structures, indicating that these hexacatenar architectures
possessing three sufficiently incompatible subunits provide a
powerful design principle for creating new and diverse fluid
superstructures from supermolecules. In essence, the forma-
tion of supramolecular columns with form chirality can be
A feature observed is that the switching is actually bista-
ble; the field-achieved state would remain intact after the
removal of the field. As the CD measurements show that
the columns are helical, the above observation suggests that
the helix would have unwound, perhaps due to a phenomen-
on similar to surface stabilized for the smectic ferroelectrics.
The combined area under the current response peak is a
direct measure of the Ps and was found to be 440 nCcmꢀ2
;
this is analogous to the chiral smectic (SmC*) phase exhibit-
ed by chiral rodlike molecules or their mixtures.
Gelation study: Gels, formed by the noncovalent interaction
between the gelator (organic or inorganic) and solvent mol-
ecules, are of great significance in both life science and ma-
terials chemistry.[26–29] Owing to their promising features in
various applications, a considerable amount of work focus-
ing on the design and preparation of a variety of functional
gels has been witnessed. For example, the gels derived from
LC phases (nematic or smectic)[27] or mesogens (calamitics
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Self-Assembly of Homomeric Dipeptides
FULL PAPER
tides was carried out; it was found that they readily form
stable gels in ethanol. The aprotic solvents do not seem to
favor the formation of gels. Herein, we describe the gelation
property of dipeptide 3b as a representative example. A
mixture of dipeptide 3b (50 mg) and absolute ethanol
(1 mL) was warmed in a capped vial until the solid dis-
solved. The undisturbed clear solution obtained was allowed
to cool to room temperature during which time the gel was
formed. The formation of the gel was ascertained by the test
tube inversion method, as illustrated in Figure 9a. The sol-
to-gel phase transition was found to be thermally reversible.
The FTIR spectrum of the gel showed the stretching vibra-
ꢀ1
ꢀ
tion of N H and C=O at 3369 and 1624 cm , respectively;
additionally, the absorption band arising from the asymmet-
ꢀ
ꢀ
ric nasACHTNUGTRENNUG(C H) and symmetric nsHCATUNGTRENN(UGN C H) stretching vibrations of
the alkyl chains at 2974 and 2849 cmꢀ1, respectively, were
also observed. These results clearly indicate a strong interac-
tion of the dipeptide (gelator) and the protic solvent
through hydrogen bonds. The gel, sandwiched between two
quartz plates, was investigated by CD spectroscopy at ambi-
ent temperature to learn about whether the soft macroscop-
ic structure features the form chirality: the helical organiza-
tion. Indeed, the Cotton effect was observed; the CD spec-
tra consisted of two broad negative bands at 301
(ꢀ4.9 mdeg) and 225 (ꢀ16 mdeg) suggesting the intermolec-
ular interactions between two or more electronic transition
dipoles arranged in a helical order (see the Supporting In-
formation for details).
The scanning electron microscopic (SEM) images (Fig-
ure 9b, c) were obtained to learn about the morphology of
the formed gel qualitatively. The images revealed the occur-
rence of a 3D network comprising fibers featuring a right-
handed twist. Therefore, it may be reasonable to presume
that the formation of a 3D network (gel) of such helical
fibers is accountable for the gelation of amphiphile 3a in
ethanol. Additional support for this assumption came from
the fact that the SEM images of the solid revealed the pres-
ence of fibers with an identical helical twist. Of course, fur-
ther detailed studies covering various aspects of these gels
Figure 8. a) Ferroelectric switching current response trace obtained for
the Colob phase of 3a (triangular wave field, 64 V, 10 Hz). b, c) Micropho-
tographs of the two bistable states. d) Optical response showing a sharp
change in the intensity associated with a change in the sign of the applied
square wave field (143 V, 1 Hz, c: sample response, d: applied volt-
age).
or discotics) are currently in the
limelight, and importantly, their
usage as a possible media for
different applications has been
demonstrated.[26b] Particularly,
LC physical gels composed of
supramolecules, namely, amino
acid derivatives (as gelators)
Figure 9. a) The demonstration of the formation of a stable organogel by dipeptide 3b in ethanol. Note that
are of special significance for the sample does not flow when its container is inverted. b, c) SEM images obtained for the gel.
various
technological
rea-
sons.[26b,27] In this context, it
would be interesting to know whether amino acid deriva-
tives 3a–c and 4a–c, which are amphiphilic systems, form
stable organogels. Thus, the preliminary investigation on the
gelation ability of all the synthesized LC homomeric dipep-
are under way. These results suggest that the molecules in
their solid, LC, and gel states self-assemble in macroscopic
chiral (helical) structures.
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C. V. Yelamaggad et al.
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Conclusion
We have demonstrated that several homomeric dipeptides
derived from proteinogenic a-amino acids can be made to
exhibit electrically switchable novel fluid supramolecular ag-
gregates, in which the ordering in the columns facilitated by
hydrogen bonding, are essentially controlled by the stereo-
chemistry encoded (chirality) in the peptide chain. Interest-
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of diastereomers are nearly half of those noted for the cor-
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the amino acid (alanine and leucine) residues. The creation
of such low molar mass, monodisperse, mesogen oligopepti-
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Received: August 4, 2008
Published online: October 1, 2008
Chem. Eur. J. 2008, 14, 10462 – 10471
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