Induction of supramolecular chirality in the self-assemblies of lipophilic pyrimidine derivatives by choice of the amino acid-based chiral spacer

Article abstract of DOI:10.1002/chem.201300605

A new family of supramolecular organogelators, based on chiral amino acid derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde, has been synthesized. L-alanine was incorporated as a spacer between the pyrimidine core and long hydrocarbon tails to compare the effect of chirality and hydrogen bonding to that of the achiral analogue. The role of aromatic moiety on the chiral spacer was also investigated by introducing L-phenyl alanine moieties. The presence of intermolecular hydrogen-bonding leading to the chiral self-assembly was probed by concentration-dependent FTIR and UV/Vis spectroscopies, in addition to circular dichroism (CD) studies. Temperature and concentration-dependent CD spectroscopy ascribed to the formation of β-sheet-type H-bonded networks. The morphology and the arrangements of the molecules in the freeze-dried gels were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques. Calculation of the length of each molecular system by energy minimization in its extended conformation and comparison with the small-angle XRD pattern reveals that this class of gelator molecules adopts a lamellar organization. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) indicate that the solid state phase behavior of these molecules is totally dependent on the choice of their amino acid spacers. Structure-induced aggregation properties based on the H-bonding motifs and the packing of the molecule in three dimensions leading to gelation was elucidated by rheological studies. However, viscoelasticity was shown to depend only marginally on the H-bonding interactions; rather it depends on the packing of the gelators to a greater extent. Copyright

Full text of DOI:10.1002/chem.201300605

DOI: 10.1002/chem.201300605
Induction of Supramolecular Chirality in the Self-Assemblies of Lipophilic
Pyrimidine Derivatives by Choice of the Amino Acid-Based Chiral Spacer
Sougata Datta,[a] Suman K. Samanta,[a] and Santanu Bhattacharya*[a, b]
Abstract: A new family of supramolec-
ular organogelators, based on chiral
amino acid derivatives of 2,4,6-tri-
chloro-pyrimidine-5-carbaldehyde, has
been synthesized. l-alanine was incor-
porated as a spacer between the pyri-
midine core and long hydrocarbon tails
to compare the effect of chirality and
hydrogen bonding to that of the achiral
analogue. The role of aromatic moiety
on the chiral spacer was also investigat-
ed by introducing l-phenyl alanine
moieties. The presence of intermolecu-
lar hydrogen-bonding leading to the
chiral self-assembly was probed by con-
centration-dependent FTIR and UV/
Vis spectroscopies, in addition to circu-
lar dichroism (CD) studies. Tempera-
ture and concentration-dependent CD
spectroscopy ascribed to the formation
of b-sheet-type H-bonded networks.
The morphology and the arrangements
of the molecules in the freeze-dried
gels were examined by scanning elec-
tron microscopy (SEM), transmission
electron microscopy (TEM), atomic
force microscopy (AFM), and X-ray
diffraction (XRD) techniques. Calcula-
tion of the length of each molecular
system by energy minimization in its
extended conformation and compari-
son with the small-angle XRD pattern
reveals that this class of gelator mole-
cules adopts a lamellar organization.
Polarized optical microscopy (POM)
and differential scanning calorimetry
(DSC) indicate that the solid state
phase behavior of these molecules is
totally dependent on the choice of
their amino acid spacers. Structure-in-
duced aggregation properties based on
the H-bonding motifs and the packing
of the molecule in three dimensions
leading to gelation was elucidated by
rheological studies. However, viscoelas-
ticity was shown to depend only mar-
ginally on the H-bonding interactions;
rather it depends on the packing of the
gelators to a greater extent.
Keywords: amino acid · cotton ef-
fect · helical nanofiber · hydrogen
bonding · self-assembly · viscoelas-
ticity
Introduction
dering as a result of multiple non-covalent interactions (i.e.,
H-bonding, p–p stacking, solvophobic effects, van der Waals
forces, charge-transfer interactions, metal–ligand coordina-
tion, etc.). For example, the main driving force among
amides,[9] urethanes,[10] ureas,[11] and sugar derivatives[12] is
hydrogen bonding, with van der Waals forces dominant[13]
among steroid derivatives. Some gelators utilize two or
more of these interactions.[14] The reversible systems may be
potentially applied to drug-delivery systems[15] and supramo-
lecular “switch” systems with appropriate memory func-
tions.[16] Self-assemblies of the LMOGs are inherently re-
sponsive toward thermal stimulus and show reversible sol-
to-gel transitions when their heated solutions are cooled
below the respective gelation temperatures.[17]
Naturally occurring pure chiral amino acid-based LMOGs
have received special attention because of their potential
applications in the pharmaceutical industry as soft materials
for drug-delivery and tissue engineering. Amino acid- or
peptide-based hydrogelators have gained importance be-
cause of the biocompatibility of the constituent materials
(amino acids).[18] We have previously reported that the orga-
nogelators of fatty acid amides of l-alanine can gelate oil se-
lectively from oil–water mixtures.[19] Since then, several re-
ports have appeared in the literature addressing this impor-
tant problem of phase-selective gelation.[20,21]
The spontaneous formation of self-assembly is a remarkable
phenomenon in nature that helps in the transcription of in-
teresting features from molecular level to complex supramo-
lecular structures through specific molecular interactions.[1]
In this context, low molecular-weight organogelators
(LMOGs)[2] have received remarkable interest because of
the reliable formation of various self-assembled three-di-
mensional superstructures. They have proven to be attrac-
tive candidates for diverse range of applications in a number
of different fields, including the design and synthesis of
nanomaterials,[3] biomaterials,[4] sensors,[5] stimuli-responsive
materials,[6] and in molecular electronics,[7] and catalysis.[8]
Self-assembly, which underpins a nanoscale structure, re-
flects the individual components and the supramolecular or-
[a] S. Datta, Dr. S. K. Samanta, Prof. Dr. S. Bhattacharya
Department of Organic Chemistry, Indian Institute of Science
Bangalore 560 012, Karnataka (India)
Fax : (+91)80-2360-0529
E-mail: sb@orgchem.iisc.ernet.in
[b] Prof. Dr. S. Bhattacharya
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore 560 064, Jakkur (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300605.
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Attaching one amino acid moiety through amide coupling
moieties has been varied to see the effect this has on the
self-assembly in solution. Thus, the incumbent chiral phe-
nomenon could be compared with the corresponding achiral
analogue. The gelation property of these compounds was ex-
amined in hydrocarbons such as, n-hexane, n-heptane, and
n-dodecane. The thermal and mechanical properties were
investigated in the solid and gel phase, respectively, to find
out the role of H-bonding through the l-amino acid-based
spacer on the self-assembly process.
À
creates H-bonding donor (N H) and -acceptor (C=O) sites
in the resulting molecule. This enhances their propensity to
self-assemble in solution to produce various types of supra-
molecular structures (helices, b-sheets, fibers, nanosheets,
nanotapes, nanotubes, etc.), a number of which lead to gel
formation.[21,22] The fibers inside such gel networks could
template different kinds of nanomaterials to give rise to new
nanocomposites with interesting properties.[3] In this respect,
the role of the solvent in which the gelator forms the self-as-
sembled structures is also crucial; the solvent–gelator inter-
actions play a great role to produce different morpholo-
gies.[23]
Results and Discussion
Recently, Fenniri and co-workers have introduced an ex-
cellent molecular template based on 4-chloro-2,6-bis(octade-
cylamino) pyrimidine-5-carbaldehyde, which self-assembles
into a fibrous lamellar structure in both the dry solid and
the gel state.[17c] However, the self-assemblies of 2,4,6-tri-
chloro-pyrimidine-5-carbaldehyde derivatives containing
two dissimilar residues are still unknown. It occurred to us
that grafting such molecular templates with natural chiral
amino acids might produce interesting supramolecular or-
ganizations. Here, we investigate the corresponding chiral
self-assemblies in solution by circular dichroism (CD) spec-
troscopy. These studies offer an opportunity to acquire val-
uable information about the nature of packing in the molec-
ular level and provide new ways of designing useful soft ma-
terials.
Herein, we introduce a family of organogelators based on
l-amino acid derivative of 2,4,6-trichloro-pyrimidine-5-car-
baldehyde. We took the advantage of this molecule, which
allows incorporation of more than one amino moiety by suc-
cessive substitution of the chloride groups by different
amines.[24] l-Alanine has been selected for the incorporation
as one of the simplest chiral counterparts. Further substitu-
tion with l-phenylalanine introduces an element of inter-ar-
omatic interactions. Also, the number of the amino acid
A series of organogelators were synthesized based on 2,4,6-
trichloro-pyrimidine-5-carbaldehyde as the building block
and chiral amino acid moieties were inserted in the side
chains by using amide bond formation. Compared with the
simple achiral analogue 5 (Figure 1), we have introduced an
l-alanine spacer as the simplest chiral moiety (1 and 3) and
further extended this to l-phenylalanine (2 and 4) to under-
stand the effect of p–p interaction in the self-aggregation
process. The gelators were synthesized by using chlorination
of barbituric acid followed by nucleophilic attack with corre-
sponding amines to obtain the final gelators in moderate to
high yields (see Scheme S1 in the Supporting Information).
Each new compound and the intermediates were character-
1
ized by H and 13C NMR spectroscopy, ESI-MS, matrix-as-
sisted laser desorption/ionization mass spectrometry
(MALDI MS), FTIR, and elemental analysis (see the Ex-
perimental Section in the Supporting Information).
In the following sections, we report the physical character-
ization of different modes of aggregation and gelation prop-
erties of the compounds shown in Figure 1, in a number of
aromatic and aliphatic hydrocarbons and investigate the
role of the amino acid-based chiral spacer on the self-assem-
bly process.
Figure 1. Molecular structures of various amino acid based gelators derived from 2,4,6-trichloropyrimidine-5-carbaldehyde.
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S. Bhattacharya et al.
Gelation studies: The gelation of each of these compounds
(1–5) was tested in different aromatic hydrocarbons, lower
molecular weight aliphatic alcohols, and aliphatic hydrocar-
bons. Their attempted gelation in toluene, CH2Cl2, CHCl3,
and THF led to sol formation in every instance. However,
they were found to be insoluble in polar aliphatic alcohols,
for example, ethanol and n-butanol, due to a polarity mis-
match. Achiral compound 5, having no amino acid moiety,
formed an opaque gel only in aliphatic hydrocarbons and
failed to gelate aromatic hydrocarbons. Hydrolysis of gelator
5 produced the di-acid derivative (COOH-C10-Pym-C10-
COOH), which remained insoluble in each of the hydrocar-
bon and aromatic solvents undertaken for gelation study,
perhaps due to the high polarity of the di-acid. However,
other members formed clear, transparent gels in hydrocar-
bons (Figure 2). Also, increasing the chain length of the ali-
phatic hydrocarbons from n-heptane to n-dodecane, the
moiety was capable of immobilizing aliphatic hydrocarbons.
In contrast, compound 2, containing two l-phenylalanine
spacers, resulted in sol formation in both aliphatic and aro-
matic hydrocarbons. This suggests that the, presence of aro-
matic ring may allow a p–p stacking interaction among
them but an excess of which may disfavor the intermolecu-
lar aggregation process necessary for gelation.
UV/Vis and circular dichroism (CD) spectroscopy: To
obtain further insights into the intermolecular interaction in
the supramolecular aggregates, concentration- and tempera-
ture-dependent UV/Vis spectra were recorded in n-dodec-
ane. The concentration-dependent UV/Vis spectra of 1 in n-
dodecane show a broad absorption in the range 250–350 nm
and a sharp peak near 220 nm at higher concentrations (>
0.2 mm). When the concentration was decreased to 0.05 mm,
two new bands near 265 and 310 nm appeared with the ex-
isting band at 220 nm (Figure 3a). The UV absorption peak
observed near 220 nm may be attributed to the n–p* transi-
À
tion of the CONH group, C=N, and C=C bonds. The UV
absorption peaks at 265 and 310 nm most likely originated
from the p–p* of the aromatic ring and n–p*transition of
the formyl substituent, respectively.[25] The intensity of these
Figure 2. Photograph showing organogels of 1 (A), 3 (B), 5 (C), and 4
(D) in n-dodecane.
minimum gel concentration (mgc) value of each of these ge-
lators decreased indicating better van der Waals interactions
between the aliphatic chains of the gelators and the solvent
(Table 1). It was observed that compound 3, which was dec-
orated with one l-alanine-based spacer, formed most effi-
Table 1. Gelation ability of compounds 1–5 in different solvents.[a]
Compound
Toluene
n-Heptane
n-Dodecane
C16-A-Pym-A-C16 (1)
C16-F-Pym-F-C16 (2)
C16-Pym-A-C16 (3)
C16-Pym-F-C16 (4)
MeO2C-C10-Pym-C10-CO2Me (5)
S
S
S
S
S
TG (2.8)
S
TG (2.8)
TG (8)
OG (12.6)
TG (2.8)
S
TG (2)
TG (7.8)
OG (8.7)
[a] OG=opaque gel, TG=transparent gel; S=soluble. Parentheses con-
tain the mgc (in mm) of the gelator molecules.
cient transparent gel in n-dodecane and the mgc value
(2 mm) of 3 was found to be the lowest among all the mem-
bers in this family of gelators. However, the di-substituted
derivative 1 showed higher mgc compared with that of the
monosubstituted compound 3. Interestingly, compounds 1
and 3 did not show any difference in their mgc values
(2.8 mm) in n-heptane probably because van der Waals inter-
actions between the aliphatic chains of the gelators and the
solvent is less prominent in n-hexane relative to that in n-
dodecane. Moreover, compound 4 with one l-phenylalanine
Figure 3. a) Concentration-dependent changes in the UV/Visible spectra
of 1 in n-dodecane at 258C; inset shows the plot of absorbance versus
concentration at 310 nm. b) Temperature-dependent changes in the UV/
Vis spectra of 1 in n-dodecane at the concentration of 0.03 mm; inset
shows the plot of absorbance versus temperature at 310 nm.
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Induction of Supramolecular Chirality
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bands decreased steadily on further dilution (inset, Fig-
ure 3a).
It is evident from the concentration-dependent UV/Vis
spectra of 1 in n-dodecane that the peaks are broadened at
higher concentrations. Therefore, we recorded its tempera-
ture-dependent UV/Vis spectra at a lower concentration
(0.03 mm). The peak intensities at 220, 265, and 310 nm in-
creased monotonically with the increase of temperature and
became saturated at approximately 758C (Figure 3b). This
was a consequence of the weakening of the non-covalent in-
teractions between the chromophores. This resulted in the
breaking of the larger aggregates to form aggregates of
shorter length.[17a] The thermotropic transition temperature
(ca. 538C) of the aggregated species of 1 in UV/Vis spectro-
scopy was calculated by plotting peak intensity at 310 nm
versus temperature (inset, Figure 3b). Other members in
this family of gelators followed similar pattern in their con-
centration- and temperature-dependent UV/Vis spectra (see
Figures S1 and S2 in the Supporting Information).
The self-assemblies of the gelator were investigated by
using CD spectroscopy to understand the role of chirality of
the individual molecules due to the presence of the chiral
amino acid moieties. CD spectra as a function of concentra-
tion and temperature were recorded to unveil the mecha-
nism and growth of the self-assembly process. As the gelator
1 showed aggregation-dependent CD signals, it was subject-
ed to concentration variation in n-dodecane to probe its
concentration effect at 258C. The CD spectra of 1 showed
an intense negative band near 220 nm, ascribed to the n–p*
Figure 4. a) Concentration-dependent changes in the CD spectra of 1 in
n-dodecane at 258C; inset shows the plot of CD (in millidegrees (mdeg))
versus concentration at 326 nm. b) Temperature-dependent changes in
the CD spectra of 1 in n-dodecane at the concentration of 0.03 mm; inset
shows the plot of CD (mdeg) versus temperature at 326 nm.
À
transition of the CONH group (Figure 4a). This signature
is similar to the CD signature of peptides adopting the b-
sheet secondary structure.[26] This observation indicates that
extensive hydrogen-bonding is playing an important role to
build up this supramolecular assembly of 1. The characteris-
tic CD signal at 220 nm was always observed during the con-
centration variation. The most notable phenomenon was ob-
served near 328 nm. Only one negative peak at 220 nm was
observed at a high concentration of 1 (3.27 mm) in n-dodec-
ane (Figure 4a). When the concentration was decreased to
0.41 mm, the solution exhibited a new positive Cotton effect
at 304 nm and a new negative Cotton effect near 328 nm
with the existing negative band at 220 nm, indicating an
anti-clockwise orientation of dipole moments in the supra-
molecular aggregates.[12b] This molecular helical arrangement
of 1 was reflected in the mesoscopic left-handed helicity ob-
servable under the AFM study (discussed later). A further
decrease in the gelator concentration up to 0.1 mm intensi-
fied these new peak intensities. At the concentration of
0.1 mm, the peak at 328 nm shifted to 326 nm along with
broadening of the peak observed at 220 nm. However, the
peak at 304 nm did not show a significant shift during the
concentration variation. Interestingly, the intensity of the
peaks at 304 and 326 nm steadily diminished when the con-
centration was further decreased from 0.1 to 0.002 mm with-
out showing any blueshift (inset, Figure 4a). The concentra-
tion variation CD plots of 2 in n-dodecane were also similar
to the behavior of gelator 1. The 335 nm band at 2.72 mm
shifted to 320 nm at 0.02 mm. The intensity of the peak near
335 nm increased on decreasing concentration up to 0.34 mm
and steadily decreased on further dilution, which is clearly
evident from Figure S3a in the Supporting Information. This
behavior showed that aggregation was induced by 2 in the
n-dodecane although it did not form a physical gel. Similar-
ly, concentration-dependent CD spectra 3 and 4 showed a
significant decrease in the intensity of 220 nm peak due to
the weakening of H-bonding (see Figure S3b and c in the
Supporting Information). However, the existence of the neg-
ative band at 220 nm indicates that they also adopt b-sheet-
type arrangements in the aggregates.
Concentration-dependent CD spectra in n-dodecane re-
vealed that all the peaks were not resolved at high concen-
tration (>0.41 mm) due to spectral broadening. Since aggre-
gation is still retained at the lower concentrations, we re-
corded the temperature-dependent CD spectra of 1 in a
dilute solution (0.03 mm), in which each peak was clearly
visible.[17a,26b] The variable-temperature CD spectra of 1
showed changes in the intensity of the CD bands upon in-
creasing temperature from 15 to 858C (Figure 4b). An inter-
esting phenomenon was observed near 326 nm in which the
CD spectra of gelator 1 in n-dodecane showed a negative
Cotton effect. The negative CD band at 326 nm and positive
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S. Bhattacharya et al.
signal at 304 nm started to quench with increasing tempera-
ture up to 858C because of the destruction of supramolecu-
lar chirality, which was clearly visible when we plotted the
intensity of the CD peak at 326 nm as a function of temper-
ature (Inset, Figure 4b). The negative CD band at 326 nm
quenched in a more pronounced fashion compared with the
positive band at 304 nm. Additionally, the positive peak at
304 nm at 158C shifted to 308 nm at 858C. However, the
326 nm band did not shift during the thermotropic transi-
tion. The transition temperature (ca. 538C) was calculated
from the plot of the CD intensity at 326 nm versus tempera-
ture for the disruption of the self-assembly to smaller aggre-
gates. Even at 858C, the CD signal at 220 nm did not disap-
pear completely, which indicated that even when the mole-
cules were in the sol state, few small aggregates still persist-
ed. This clearly indicates that the supramolecular chirality of
the self-assembled aggregates in the gel state is responsible
for the observed helicity (see below).[27] Compound 2 in n-
dodecane, containing two l-phenylalanine moieties in place
of l-alanine of 1 showed a similar behavior in the CD spec-
troscopy upon temperature variation. With increasing tem-
perature, the intensity of the negative peaks at 220, 270, and
320 nm decreased. However, an interesting phenomenon
was observed at 658C. The CD band intensity at 270 and
320 nm diminished followed by the appearance of two posi-
tive bands at 280 and 330 nm possibly due to the secondary
structural transformation (see Figure S4a in the Supporting
Information).[28] Gelators 3 and 4 contained l-alanine and l-
phenylalanine residue only at one branch. Both of these
compounds showed a temperature-variable CD spectral pat-
tern unlike compound 1 and 2 (see Figure S4b and c in the
Supporting Information). Only a little change was observed
at 220 nm during thermotropic transition indicating the exis-
tence of strongly bound H-bonded network in the monosub-
stituted amino acid derivatives.
Figure 5. AFM images of the xerogels derived from a) 1, b) 3, c) 4, and
d) 5, respectively, in n-dodecane.
aspect ratio. Parquette and co-workers observed similar re-
sults with certain naphthalene diimide (NDI)-appended di-
peptides.[29] Such NDI-dipeptides formed amyloid-like heli-
cal nanofibers and twisted nanoribbons depending on the
placement of the NDI group, in which b-sheet-type hydro-
gen bonding and p–p association played important roles in
directing the assembly process.
FTIR spectroscopy: The presence of extensive H-bonding
was also confirmed by concentration-dependent IR spectro-
scopy. Figure 6 shows the IR-stretching frequencies of the
Morphological features: The superstructures created by the
organogel networks were examined under TEM, SEM, and
AFM. The freeze-dried gels of 5 revealed the presence of fi-
brous networks with a fiber width of 1–2 mm (see Figure S5
in the Supporting Information). A comparatively larger ag-
gregated morphology was observed for 1, 3, and 4 under
SEM because the introduction of an extra amide interaction
brings the smaller aggregates close to each other to form ag-
gregates of longer length. The TEM images of gelator 1, 3,
and 4 in n-dodecane showed entangled nanofibers of very
high aspect ratio and diameters ranged from 20–30 nm for 1
and 100–500 nm for 3 and 4 (see Figure S6 in the Supporting
Information). However, compound 5 exhibited microtapes
with diameters ranging from 0.5–1.0 mm.
“strong” gel, the “weak” gel, and the “sol” of 3 in dodecane.
À1
À
The N H stretching frequency at 3280 cm in the “strong”
gel (2 mm) shifted to 3290 cmÀ1 in the “weak” gel (1 mm)
and 3295 cmÀ1 in the “sol” state (0.5 mm). Thus, the stretch-
Figure 5a–d shows the AFM images of dodecane gels of
1, 3, 4, and 5. The gel of 5 exhibited uniform tapes with di-
ameters ranged from 1–5 mm. However, entangled nanofib-
ers (50–100 nm) were obtained in the n-dodecane gel of 4.
Interestingly, gels from 1 and 3 containing di- and monosub-
stituted l-alanine spacers showed left-handed and right-
handed helical nanofibers (50–100 nm), respectively, of high
Figure 6. Concentration-dependent changes in the FTIR spectra of 3 in
n-dodecane.
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Induction of Supramolecular Chirality
FULL PAPER
ing frequency shifted to higher wavenumbers on dilution in-
dicating a progressive decrease of the intermolecular H-
bonding by the participation of gelators in the self-assem-
bly.[2c,17a,23] A similar behavior observed in other instances
under the concentration variation and the shifts in the
stretching frequencies were 26 for 1, 32 for 4, and 12 cmÀ1
for 5 (see Figure S7 in the Supporting Information).
decane showed three peaks corresponding to the d value at
3.66 nm, 1.83 nm, which is in the ratio of 1:0.5, and 1.22 nm
(1:0.33), thus indicating a lamellar pattern of the aggregates
of gel 5 with an interlayer spacing of 3.66 nm.[30] Similarly
the d spacing was observed for xerogel of 1 at 3.52 nm,
1.76 nm (1:0.5), 1.15 nm (1:0.33); xerogel of 3 at 3.42 nm,
1.71 nm (1:0.5), 1.14 nm (1:0.33) and xerogel of 4 at 3.54 nm,
1.74 nm (1:0.5), 1.18 nm (1:0.33) indicating the presence of a
lamellar structure (see Figure S8 in the Supporting Informa-
tion). The WAXD profiles showed sharp reflections in the
2q range of 18–308, which were typical of a crystalline ar-
rangement of long alkyl side-chains (see Figure S9 in the
Supporting Information).[30b]
Small-angle and wide-angle X-ray diffraction: Small-angle
X-ray diffraction (SAXD) was carried out to acquire infor-
mation about the packing pattern in the supramolecular as-
sembly. The diffraction patterns of each of these xerogels
showed three peaks, which were in the ratio of 1:0.5:0.33 in-
dicating a lamellar-type aggregation pattern (Figure 7a).
The SAXD pattern of the xerogel of 5 obtained from n-do-
Energy minimization and the proposed model of self-assem-
bly: Figure S10a in the Supporting Information shows the
ground-state geometry of the symmetrical 1 optimized by
using B3LYP/6-31G* program in the gas phase. The results
indicate that the long aliphatic chains are directed opposite
to each other with longest molecular length of 2.7 nm start-
ing from the central aromatic core. The molecular lengths in
their optimized geometries are comparable in all the analo-
gous compounds (see Figure S10b–e in the Supporting In-
formation).
SAXD studies of 1 revealed the presence of periodic la-
mellar structures with repeat distances of 3.52 nm, which is
less than the double of the molecular lengths. This indicates
that the long alkyl chains in 1 interdigitate with each other
through the van der Waals interactions among them.[31] This
in turn, brings the pyrimidine rings into close proximity to
form extended hydrogen-bonded networks. Pyrimidine de-
rivatives are well-known to form complementary H-bonded
structures.[17c,32] The molecules described herein also possess
complementary H-bonding functions, which occur through
the participation of sp2-hybridized ring nitrogen atoms,
NH, and aldehyde groups (Figure 7b). These 2D-supramo-
lecular aggregates propagate in the orthogonal direction of
the molecular plane through interlayer H-bonding between
À
À
the CONH groups (Figure 7c) to form a 3D self-assembly.
Such a system is indeed supported from their CD signatures,
which are very similar to b-sheet-type structures that are
formed in peptides. Based on the results from CD and IR
spectroscopies, SAXD analysis and energy minimization cal-
culations we have proposed a packing model of the self-as-
sembly (Figure 7b and c).[29] Interestingly, the optimized
structure of 5 shows some differences from the other mem-
bers. The lack of a methyl/phenyl group near to the pyrimi-
dine ring favors the residues to be in close proximity. This
results in a geometry which is much closer to a “V”-shaped
conformation. SAXD analysis of 5 confirmed lamellar type
of arrangements with a repeat distance of 3.66 nm, which is
fairly close to double the molecular lengths calculated from
the optimized molecular geometry. This precludes the prob-
ability of interdigitation of the hydrophobic chains in 5 per-
haps due to the presence of dipole–dipole interactions
among the ester groups.
Figure 7. a) Small-angle XRD plot of xerogel of 1, 3, and 5 obtained
from n-dodecane. Inset showing magnification of 1 for clarity. b) Sche-
matic illustration of the lamellar structure of 1 in the xy plane through
complementary intermolecular hydrogen-bonding. c) Proposed molecular
packing of propagation of the aggregates of 1 along the direction (z axis)
of the growth of supramolecular fibrous networks through interlayer hy-
À
drogen-bonding of the CONH groups.
Chem. Eur. J. 2013, 19, 11364 – 11373
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S. Bhattacharya et al.
Rheology experiment: Rheological studies of each supramo-
lecular gel confirmed their viscoelastic properties. Since
each gelator exhibited a lower mgc in n-dodecane compared
with that in n-heptane, rheological measurements were per-
formed in n-dodecane. Oscillatory frequency sweep and am-
plitude sweep experiments were performed to find out the
storage and loss moduli (G’ and G’’). In principle, the stor-
age (or elastic) modulus G’ represents a solid-like character,
that is, the ability of the material to resist deformation
under stress. In contrast the loss (or viscous) modulus G’’ re-
flects the liquid-like behavior, that is, the propensity of the
material to flow. Generally, the gels show higher magnitude
of G’ values than the G’’ values, demonstrating an evidence
of the viscoelastic behavior. In an oscillatory amplitude
sweep experiment, the crossover point of G’ and G’’ is
termed as yield stress (sy) in which the transition of the elas-
tic solid-like character to the viscous liquid-like character
occurs.
Figure 8. Rheological measurements show oscillatory amplitude sweep
experiment of the gels derived from 1, 3, 4, and 5 in n-dodecane (concen-
tration=10 mm in each case).
The flow behavior of the gel was measured by using a
simple rotational flow curve experiment. With increasing
shear rate, the viscosity of the n-dodecane gel decreased
almost linearly indicating that the gels behaved as viscoelas-
tic solid-like and shear-thinning material (see Figure S11a in
the Supporting Information). The frequency sweep experi-
ment of dodecane gels of 5 and 3 demonstrated comparable
elastic storage modulus, G’ (ca. 27000 Pa), which was always
greater than the associated loss modulus, G“, in the angular
frequency range 0.1–500 radsÀ1 under an initial strain of
0.001%, thereby indicating the viscoelastic nature of the gel
over the entire angular frequency range (see Figure S11b in
the Supporting Information).[2f] The G’ value of gel of 1 was
approximately 9000 Pa and it decreased further in the gel
derived from 4 (G’ꢀ200 Pa). This result clearly indicates
that the presence of the l-phenylalanine-based spacer re-
duces the viscoelasticity of the present system.
The viscoelastic behavior of the n-dodecane gels was
found to be similar under the amplitude sweep experiment
as with the frequency sweep experiment. There was no sig-
nificant difference in the G’ of 5 and 3 and the value was ap-
proximately 26000 Pa; whereas in 1 (G’ꢀ4000 Pa) and 4
(G’ꢀ600 Pa), the values were significantly less. These data
indicate that the viscoelasticity of the gelator is greater
when there is no chiral spacer (compound 5) or when the
gelator is functionalized only at one end by one l-alanine
residue (compound 3). However, when the gelator was craft-
ed with l-alanine at both sides (compound 1), the viscoelas-
ticity decreased further. When l-phenylalanine was inserted
at one end of the gelator (compound 4), there was a de-
crease in the viscoelasticity probably due to the larger size
of the phenyl side-chain, which most probably induced steric
crowding to compromise with the compact packing required
for the manifestation of higher viscoelasticity.
tively. However, compounds 3 and 5 began to lose their
solid-like flow behavior only at a shear stress of approxi-
mately 100 Pa. These results indicate that the trend in vis-
coelastic behavior of these gelators is 5ꢀ3>1>4. The
trends in their yield stress values signify that the gelator 3
(containing one l-alanine spacer) and the achiral compound
5 are most resistant to flow under an applied mechanical
force. Whereas, gelator 4 (one l-phenylalanine spacer)
showed least yield stress value. This may be again attributed
to the steric crowding of the side chains as discussed before.
Differential scanning calorimetry (DSC) and polarized opti-
cal microscopy (POM): Solid phase DSC and POM experi-
ments are well-known and useful techniques for the investi-
gation of the thermotropic behavior.[33] The DSC profile of
compound 3 showed a small endothermic transition at 858C
and a larger endothermic peak at 958C (Figure 9a). Howev-
er, compound 4 showed a large, broad endothermic transi-
tion at 1168C. In the cooling cycle, both of them showed
double exothermic transitions at 79 and 538C for 3 and 98
and 788C for 4. Larger peaks appeared at higher tempera-
tures and the smaller exothermic transitions appeared at
lower temperature during cooling in both the cases. On the
other hand, compound 5 showed a single and sharp peak at
808C during heating and at 538C on the cooling cycle. It has
been observed that amino acid-bearing molecules have
higher melting temperatures compared with the analogous
compound (5) without any amino acid moiety. An interest-
ing phenomenon was observed in the DSC profiles of 1 and
2 (see Figure S12 in the Supporting Information). These
compounds contained identical amino acid branches on both
sides. The kinetics of the crystallization process of such com-
pounds was so slow that they did not show any exothermic
peak in the cooling cycle immediately upon melting. There-
fore, they were kept at room temperature for 24 h for crys-
tallization. After annealing at room temperature for 24 h,
they were allowed to undergo one more heating and cooling
cycle. The heating cycle showed broad endothermic transi-
tions and again the cooling cycle did not show any peak.[34]
Each gel has its own characteristic yield stress (sy) accord-
ing to its viscoelasticity to an external oscillating stress
(Figure 8). When the n-dodecane gels made from 1, 3, 4,
and 5 succumbed to the applied stress, only those of 1 and 4
started to flow at a shear stress of 10 Pa and of 1 Pa, respec-
11370
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Induction of Supramolecular Chirality
FULL PAPER
to the thermotropic behavior of 4-chloro-2,6-bis(octadecyla-
mino)pyrimidine-5-carbaldehyde reported by Fenniri
et al.[17c] However, compound 5 did not show any biphasic
behavior, which supported the single peak appeared at 538C
in the DSC experiment (see Figure S13 in the Supporting In-
formation). Interesting observations were noted in the POM
experiment of 1 and 2 (see Figure S13 in the Supporting In-
formation). Each of these compounds contained two identi-
cal l-amino acid branches. No crystalline phase appeared
immediately during the cooling of the isotropic melt. How-
ever, small crystals started appearing after the sample re-
mained at room temperature for 24 h. These phenomena
also explain the absence of peak in their cooling curve of
their DSC profiles. The presence of an l-alanine or l-phe-
nylalanine substituent on both the sides of the di-substituted
derivatives (1 and 2) presumably drives the aliphatic chains
apart from each other, which probably impedes the compact
packing of the molecules leading to the slow crystallization
of the isotropic melt.
Conclusion
In this work, we have described the syntheses and properties
upon the self-assembly exhibited by a new family of 2,4,6-
trichloropyrimidine-5-carbaldehyde derivatives tailored with
number of H-bonding motifs depending on the nature and
number of chirally pure amino acid moieties inserted as a
spacer. The role of amino acid moieties on their self-assem-
bly leading to the formation of b-sheet type structure of the
chiral supramolecular aggregates was explored by using UV/
Vis and CD spectroscopies. Generally, the b-sheet type ar-
rangement is quite regular in the self-assemblies of the pep-
tides in aqueous solution. However, here we show a rare
manifestation in which the extensively H-bonded self-assem-
bly of small organic molecule in aliphatic hydrocarbons
mimics the b-sheet type organization. The effect of H-bond-
ing on the self-assembly by temperature and concentration
variation was also demonstrated by using UV/Vis and CD
spectroscopies. Due to the presence of extra pair of H-bond-
ing donor and acceptor sites of the amide linkages of the
amino acids, the extent of H-bonding increased as probed
by using FTIR studies. This complementary H-bonding led
them to exist as periodic lamellar structure, which was con-
firmed by a SAXD study.
Interestingly, gelator 1 and 3 containing di- and monosub-
stituted l-alanine residues exhibited helical nanofibers of
high aspect ratio as discerned from AFM studies. Thus, in-
corporation of l-alanine has been found to be useful to ex-
aggerate the chiral signature from the molecular to the mi-
croscopic level. However, incorporation of l-phenylalanine
as spacer produced only simple nanofibers. This shows the
importance of subtle balance in packing imposed by the side
chains of the l-amino acid spacers. Depending on the amino
acid moiety these molecules exhibited interesting thermo-
tropic behavior, which is reflected in the POM and DSC
studies. Hence, this work sheds a new light in the self-assem-
Figure 9. a) Solid phase DSC profiles of 3, 4 and 5. Inset shows magnifi-
cation of the exothermic scan of 3 for clarity. The circled zone appears as
a clear peak in the inset. POM images of 3 were captured at b) 808C and
c) 558C. POM images of 4 at d) 1008C and e) 758C. Magnification=20ꢁ.
To illustrate more about the thermotropic behavior, we
investigated the samples under a polarized optical micro-
scope. The sample was first heated to get an isotropic melt
and then slowly cooled down to room temperature at the
same rate of the DSC experiment (58C minÀ1). A thermo-
tropic transition was observed during cooling and different
crystalline birefringent textures appeared upon changing
temperature. Birefringent phases of 3 started appearing at
808C, at which a large crystalline phase was observed that
was associated with a large exothermic transition in the
DSC profile (Figure 9b). The birefringent texture shifted
drastically to a new type of phase with a small crystalline
morphology when cooled to 558C, corresponding to the
small exothermic peak in the DSC curve; the texture was re-
tained upon cooling further to 308C (Figure 9c). A similar
type of biphasic behavior was also observed under the POM
images of 4, in which the birefringent texture started appear-
ing at 1008C followed by the appearance of a different type
of texture at 758C, which are in accordance with the DSC
thermotropic profile (Figure 9d and e). This kind of biphasic
transition of monosubstituted derivatives (3 and 4) is similar
Chem. Eur. J. 2013, 19, 11364 – 11373
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Acknowledgements
This work was supported by the Department of Science and Technology
(J. C. Bose Fellowship to S.B.), and the Government of India, New Delhi,
India. S.D. thanks CSIR for a senior research fellowship.
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Received: February 16, 2013
Published online: June 28, 2013
Chem. Eur. J. 2013, 19, 11364 – 11373
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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