Remarkable shape-sustaining, load-bearing, and self-healing properties displayed by a supramolecular gel derived from a bis-pyridyl-bis-amide of L -phenyl alanine

Article abstract of DOI:10.1002/asia.201402053

A series of bis-amides decorated with pyridyl and phenyl moieties derived from L-amino acids having an innocent side chain (L-alanine and L-phenyl alanine) were synthesized as potential low-molecular-weight gelators (LMWGs). Both protic and aprotic solven

Full text of DOI:10.1002/asia.201402053

FULL PAPER
DOI: 10.1002/asia.201402053
Remarkable Shape-Sustaining, Load-Bearing, and Self-Healing Properties
Displayed by a Supramolecular Gel Derived from a Bis-pyridyl-bis-amide of
l-Phenyl Alanine
Uttam Kumar Das,[a] Subhabrata Banerjee,[b] and Parthasarathi Dastidar*[a]
Abstract: A series of bis-amides deco-
rated with pyridyl and phenyl moieties
derived from l-amino acids having an
innocent side chain (l-alanine and l-
phenyl alanine) were synthesized as
potential low-molecular-weight gelators
(LMWGs). Both protic and aprotic sol-
vents were found to be gelled by most
of the bis-amides with moderate to ex-
cellent gelation efficiency (minimum
gelator concentration=0.32–4.0 wt.%
and gel–sol dissociation temperature
gel =52–1108C). The gels were charac-
terized by rheology, DSC, SEM, TEM,
and temperature-variable 1H NMR
measurements. pH-dependent gelation
studies revealed that the pyridyl moiet-
ies took part in gelation. Structure–
property correlation was attempted
using single-crystal X-ray and powder
X-ray diffraction data. Remarkably,
one of the bis-pyridyl bis-amide gela-
tors, namely 3,3-Phe (3-pyridyl bis-
amide of l-phenylalanine) displayed
outstanding shape-sustaining, load-
bearing, and self-healing properties.
T
Keywords: gelators · load-bearing ·
self-healing · supramolecular gels ·
X-ray diffraction
Introduction
have remarkable self-standing or shape-sustaining, load-
bearing, and self-healing properties.[12–14] These properties
are expected to be useful in external coatings, architectural
materials, artificial cartilage, drug-delivery systems, biosen-
sors, etc.[15–22]
Low-molecular-weight gelators (LMWGs) are small mole-
cules (MWꢀ3000) capable of immobilizing a large volume
of solvent resulting in semi-solid viscoelastic materials called
gels.[1] Various supramolecular interactions such as hydrogen
bonding, halogen bonding, van der Waals forces, and p–p
stacking are responsible for forming one-dimensional (1D)
fibrous networks known as self-assembled fibrillar networks
(SAFINs),[1b] in which the solvent molecules are immobi-
lized, thus resulting in a gel. Due to the various potential ap-
plications of gels in oil recovery,[2] sensors,[3] conservation of
art,[4] electro-optics/photonics,[5] cosmetics,[6] structure-direct-
ing agents,[7] catalysis,[8] drug delivery,[9] biomedical applica-
tions,[10] etc., the research activity in the field of supramolec-
ular gels has been growing significantly in recent years. The
gel matrix can be used for crystallization as well.[11] Recent-
ly, efforts have been undertaken in developing gels that
The structural diversity of known gelators—from simple
organic salts[23] to bis-urea derivatives,[24] peptides,[25] carbo-
hydrates,[26] anthracene derivatives,[27] surfactants,[28] phthalo-
cyanines,[29] porphyrins,[30] cholesterol,[31] etc., and the lack of
understanding of the gel-forming mechanism at the molecu-
lar level make it difficult to design new gelator molecules.
Nevertheless, there have been few reports on the design of
LMWGs. For example, Weiss et al. have designed aromatic-
linker-steroid (ALS)-based gelators.[32] van Esch et al. have
designed C3-symmetric hydrogelators by balancing the hy-
drophobicity and the hydrophilicity of the gelling scaffold.[33]
Our group, on the other hand, made significant contribu-
tions to designing LMWGs following a supramolecular syn-
thon approach in the context of crystal engineering.[23] Re-
cently, we demonstrated that the supramolecular synthon
approach was indeed useful in developing gelators capable
of displaying a slow release of pheromones,[34] reverse ther-
mal gelation,[35] gel-sculpture, load-bearing, and self-healing
properties,[36] and in vivo self-delivery of a drug.[37] In this
report, we chose to explore well-studied supramolecular in-
teractions involving an amide functionality to develop supra-
molecular gelators having an innocent l-amino acid back-
bone. The amide functional group, being one of the most
studied supramolecular functionalities, is known to impart
gelation through hydrogen bonding. While a secondary
monoamide forms a 1D hydrogen bond network through
[a] Dr. U. K. Das, Dr. P. Dastidar
Department of Organic Chemistry
Indian Association for the Cultivation of Science (IACS)
2A and 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata-700032, West
Bengal (India)
Fax : (+91)33-2473-2805
E-mail: ocpd@iacs.res.in
parthod123@rediffmail.com
[b] Dr. S. Banerjee
Present address:
National Institute of Advanced Industrial Science and Technology
(AIST)
AIST Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402053.
À
N H···O interactions, a bis-amide generates a beta-sheet-
1
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Parthasarathi Dastidar et al.
like 1D hydrogen bond network. Both mono- and bis-
amides are known to impart gelation.[38] In this work, we
considered various bis-amides having terminal moieties like
various positional isomers of pyridyl moieties, which can
also take part in hydrogen bonding interactions; we also
studied the corresponding phenyl derivatives to probe the
role of pyridyl moieties in gelation (Figure 1). One such ge-
lator (3,3-Phe) displayed remarkable shape-sustaining, load-
bearing, and self-healing properties.
ed with the corresponding acid chloride to obtain the final
bis-amide derivatives with an average overall yield of about
65% (see the Experimental Section).
Gelation Studies
The compounds were scanned for gelation tests in 15 select-
ed polar and nonpolar solvents (Table 1). Gel formation was
confirmed by tube inversion experiments. All the gels ob-
tained were thermoreversible and stable for months at ambi-
ent temperature. While 3,3-Ala, 0,0-Phe, and 0,0-Ala acted
as a nongelator, hydrogelator, and ambidextrous gelator, re-
spectively, the remaining compounds were found to be orga-
nogelators. The terminal moieties and amino acid side chain
had a profound effect on the gelation ability of the corre-
sponding bis-amides; while 3,3-Phe (3-pyridyl bis-amide of
l-phenylalanine) was an organogelator, its phenyl analogue
(0,0-Phe) turned out to be a hydrogelator and was able to
gel DMF/water and DMSO/water. In a similar fashion, 3,3-
Ala was a nongelator whereas 0,0-Ala turned out to be an
ambidextrous gelator. The role of the pyridyl moieties in ge-
lation was also probed by pH-dependent gelation experi-
ments (see the Supporting Information). 3,3-Phe could not
gel 1,2-dichlorobenzene below pH 3, thus indicating an
active role of the pyridyl moieties. The minimum gelator
concentration (MGC) and gel–sol dissociation temperature
(Tgel) were in the range of 0.32–4.0 wt% and 52–1108C, re-
spectively, thereby indicating the significant gelation effi-
ciency and moderate to excellent thermal stability of the
gels. As shown in Figure 2, Tgel gradually increased with an
increase in gelator concentration. Such an increase in Tgel
may be attributed to the various non-covalent interactions
such as hydrogen bonding and hydrophobic interactions that
are responsible for forming SAFINs in the gels.
Figure 1. Various bis-amide derivatives of l-phenylalanine and l-alanine
studied herein.
Results and Discussion
Syntheses
The bis-amides investigated (see Figure 1) were synthesized
as follows: An N-Boc-l-amino acid was reacted with the
corresponding amine by DCC coupling. After deprotecting
the N-Boc group of the resultant mono-amide, it was react-
Differential scanning calorimetry (DSC) experiments for
two selected gel samples derived from 3,3-Phe and 3,4-Phe
clearly showed the thermoreversible nature of the gels
Table 1. Gelation data.
Sr. No Solvent
3,3-Ala
MGC Tgel MGC Tgel [8C] MGC Tgel MGC Tgel MGC Tgel MGC Tgel MGC Tgel MGC Tgel
[wt%] [8C] [wt%] [wt%] [8C] [wt%] [8C] [wt%] [8C] [wt%] [8C] [wt%] [8C] [wt%] [8C]
3,4-Ala
3,0-Ala
0,0-Ala
3,3-Phe
3,4-Phe
3,0-Phe
0,0-Phe
1
Acetonitrile
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L[a]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
NC
WP
WP
WP
WP
WP
WP
WP
NC
4
–
–
–
–
–
–
–
–
–
90
–
–
–
–
–
NC
2.86
1.82
2.22
WP
WP
WP
2.22
2.86
2.22
L
–
FC
4
GN
4
4
GN
L
–
80
–
84
78
–
–
–
90
–
85
96
WP
1.22
L
–
WP[a]
110 WP[a]
–
–
–
–
–
80
–
–
–
–
–
–
–
–
–
WP
WP
WP
WP
WP
WP
4
WP
WP
GN
WP
WP
WP
WP
NS
–
–
–
–
–
–
52
–
–
–
–
–
–
–
–
FC
–
–
–
–
–
–
–
–
–
–
2
3
4
5
6
7
8
o-Xylene
m-Xylene
p-Xylene
Chlorobenzene
1,2-Di chlorobenzene
Nitrobenzene
Toluene
68
56
70
–
–
–
66
58
62
–
–
–
WP
WP
WP
WP
WP
WP
WP
WP
WP
–
–
67
73
–
–
–
–
–
–
–
–
–
WP[a]
WP[a]
WP[a]
2.22
WP
L
0.91
0.47
WP
WP
WP
WP
L
WP
4
WP[a]
WP[a]
WP[a]
L
9
Mesitylene
Methyl salicylate
DMSO
DMF
EG
10
11
12
13
14
15
FC
4[b]
4[b]
L
L
0.95[b] 80
L
L
L
NS
L
L
WP[a]
L
1.5[b]
90
L[b]
L
0.95[b] 78
0.32[b] 78
DEG
H2O
–
–
4[b]
NS
82
–
L
L
0.95[b] 94
NS
WP[a]
WP[a]
NS
–
[a] Cosolvent dimethylsulfoxide (minimum) required to dissolve. [b] The corresponding solvents were added to dissolve the gelator in water. L, liquid;
FC, fibrous crystal; WP, white precipitate; NC, needle-shaped crystals; GN, gelatinous network; DMSO, dimethylsufoxide; DMF, N,N-dimethylforma-
mide; EG, ethylene glycol; DEG, diethylene glycol; NS, not soluble.
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Parthasarathi Dastidar et al.
Figure 2. Plot of Tgel as a function of gelator concentration; DCB=1,2-di-
chlorobenzene; NB=nitrobenzene; DMSO/W=N,N-dimethylsulfoxide/
water (8:5, v/v); MS=methylsalicylate; Mes=mesitylene.
(Figure 3). The gel-to-sol phase transition temperature
found in DSC experiments (129 and 1098C for the 6 wt.%
1,2-dichlorobenzene (DCB) gel of 3,3-Phe and 3,4-Phe, re-
spectively) matched well with the value determined by the
dropping ball method (126 and 1088C, respectively), high-
lighting that the Tgel data obtained from the dropping ball
method could be relied upon in case DSC experiments
failed to result in a noticeable phase transition.
Figure 4. Variable-temperature 1H NMR spectra of the hydrogel of 0,0-
Ala.
Microscopy
Scanning electron microscopy (SEM) micrographs of the xe-
rogels of some selected gels were collected (Figure 5). Most
of them showed highly aligned (for 3,0-Ala, 0,0-Ala, and
3,0-Phe) or entangled (for 3,4-Phe) fibers, whereas one of
them (3,3-Phe) displayed both fibers and colloidal particles,
both of which were also observed in the corresponding
transmission electron microscopy (TEM) images (Figure 5).
These data clearly indicated that in most of the cases 1D
growth of the gel fibers was promoted and accordingly, the
solvent molecules were immobilized within the entangled
network of fibers resulting in a gel.
Rheology
Dynamic rheology experiments of a few selected gels
(6 wt% DCB gels of 3,3-Phe and 3,4-Phe) were carried out
to study the viscoelastic nature or gel-like response of the
corresponding gels.[39] In a typical frequency sweep experi-
ment, the elastic modulus G’ and loss modulus G’’ were
plotted as a function of angular frequency (w) over a signifi-
cant time period at a constant strain of 0.05 and 0.01% (as
suggested by amplitude sweep experiments, see the Support-
ing Information); the data for 6 wt.% DCB gels of 3,3-Phe
and 3,4-Phe, respectively, are shown in Figure 6. The elastic
modulus G’ of the gel of 3,3-Phe was about 3.8 times higher
than that of the gel of 3,4-Phe, thus indicating that a subtle
change of the position of the pyridyl N atom had a profound
effect on the flow behavior of the gel.
Figure 3. DSC traces of an approximately 6.0 wt.% 1,2-dichlorobenzene
gel of 3,3-Phe and 3,4-Phe.
To probe the self-assembly process in the gelation process,
we performed temperature-variable 1H NMR spectroscopy
experiments. For this purpose, we chose 2.0 wt.% and
1.54 wt.% (w/v) DMSO/H2O (1:1 and 5:8 (v/v), respective-
ly) gels of 0,0-Ala and 0,0-Phe. In the case of 0,0-Ala, it was
observed that the aromatic protons and the proton attached
to the chiral carbon atom gradually shifted downfield (7.3–
8.5 and 4.7–5.5 ppm, respectively) with an increase in tem-
perature from 25 to 858C, thus signifying that these protons
experienced shielding during gelation, which could either be
À
due to C H···p or p–p interactions (Figure 4). On the other
Self-Sustaining, Load-Bearing, and Self-Healing
Experiments
hand, no such effect was observed in the case of 0,0-Phe,
thereby suggesting that such interactions were not contribu-
ting to hydrogelation (see the Supporting Information).
These data corroborated well with the single-crystal struc-
tures of the analogous molecules (see below).
The 1,2-dichlorobenzene gel of 3,3-Phe was found to be
a self-sustaining material even at a concentration as low as
1.0 wt.%. Thus, it could be kept undeformed on a glass slide
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into 4 pieces and allowed to self-heal, the pieces formed
a single piece within 15 min (Figure 7). Unfortunately, none
of the gels prepared from other compounds in the series
showed any self-sustaining, load-bearing, and self-healing
properties.
Figure 5. A–E) SEM images of selected xerogels. All xerogels were pre-
pared from the gel of different solvents given within parentheses. A) 3,0-
Ala (o-xylene); B) 0,0-Ala [DMSO/H2O (37%)]; C) 3,0-Phe (nitroben-
zene); D) 3,4-Phe (nitrobenzene); and E) 3,3-Phe (1,2-dichlorobenzene).
F) TEM image of the xerogel of 3,3-Phe prepared from 1,2-dichloroben-
zene.
Figure 7. A) Free-standing and B) load-bearing DCB gel (1 wt%) of 3,3-
Phe. C) Gel slice with the abbreviation ꢁIACSꢂ carved. D) Self-healing of
several gel blocks of 3,3-Phe.
Structure–Property Correlation
A detailed insight into the structure is the key to any design
aspect. Thus, it is highly desirable and important to under-
stand the detailed structure of the gel network or SAFINs.
However, determination of the structure of SAFINs is chal-
lenging; understandably, single-crystal X-ray diffraction is
not an option because of the submicron size of the gel
fibers. To determine the structure of the gel fiber in a gel
sample, ab initio structure determination from diffraction
data of a gel sample collected in a synchrotron beam-line
should be employed. However, limited access to such
a beam-line and the non-routine nature of ab initio structure
determination from powder X-ray diffraction (PXRD)
data[40] compelled researchers to develop other, relatively
easy alternative avenues. Thus, an indirect method was de-
veloped originally by Weiss et al.,[41] wherein the various
powder X-ray diffraction patterns (PXRDs) (simulated from
single-crystal X-ray data, bulk solid, gels/xerogels) are com-
pared; a near perfect match of these patterns would indi-
rectly determine the structure of the SAFINs. In principle,
Figure 6. Dynamic rheology of a 6 wt.% DCB gel of 3,3-Phe and 3,4-Phe
at 258C. The elasticity modulus G’ and viscosity modulus G’’ are shown
as a function of the angular frequency w. The strain used was 0.1%;
DCB=1,2-dichlorobenzene.
without any container. The self-sustaining behavior of this
gel was also demonstrated by carving the word ꢁIACSꢂ on
a slice of the gel (1.0 wt.% DCB gel). The load-bearing abil-
ity of this gel (2.5 mL) was also extraordinary: 14 Indian 5
Rupee coins weighing 125.0 g could be kept on a cylindrical
piece (1.0ꢃ0.9 cm) of this gel with no visible deformation or
solvent leaching. This gel also displayed highly efficient self-
healing properties; when a slab (3.0ꢃ1.1ꢃ0.3 cm) was cut
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Parthasarathi Dastidar et al.
À
the comparison of these PXRDs should be strictly restricted
to simulated, bulk, and gel. However, obtaining a meaningful
PXRD pattern for a gel sample is often difficult because of
the scattering contribution of the solvent and the generally
poor diffraction (due to low crystallinity and/or a low
amount of the gelator present in a gel). On the other hand,
during xerogel formation (usually done by evaporating a gel
sample), a new nucleation event might occur from the al-
ready dissolved gelator molecules present in the bulk sol-
vent of a gel, thus leading to a mismatch of the PXRD pat-
terns due to the formation of a new crystalline phase. How-
ever, there is no guarantee that such an event would always
take place. Thus, comparing simulated, bulk solid, and xero-
gel PXRD patterns is a reasonable compromise in determin-
ing the gel network structure. Our best efforts resulted in
crystallization of two gelator molecules, namely 3,3-Phe and
3,4-Ala, which were subjected to SXRD.
a beta-sheet-like hydrogen bond network through N H···O
À
interactions
[N···O=2.977(4)–3.017(4) ꢄ;
aN H···O=
156(3)–161(3)8], thereby leading to a 1D hydrogen-bonded
strand. Such 1D strands were further packed in a parallel
fashion (Figure 8).
The crystals of 3,3-Phe were grown from a DMSO/water
(1:5, v/v) mixture and were crystallized in the noncentro-
symmetric monoclinic space group P21 (Table 2). The asym-
metric unit contained two molecules of 3,3-Phe. The dihe-
dral angles between the two pyridyl moieties in these mole-
cules were 63.2 and 61.28. Contrary to general wisdom, the
pyridyl N atoms did not participate in hydrogen bonding
whereas the amide functionalities, as expected, displayed
Figure 8. Illustration of the crystal structure of 3,3-Phe; 1D hydrogen
Table 2. Crystallographic parameters.
À
bond network through N H···O interactions (A) and overall parallel
À
packing (B). All hydrogen atoms (except N H) were omitted for clarity.
Crystallographic parameters
3,4-Ala
3,3-Phe
Empirical formula
CCDC No.
Formula weight
Crystal size/mm
Crystal system
Space group
a [ꢄ]
b [ꢄ]
c [ꢄ]
a [8]
b [8]
C14H14N4O2
953874
270.29
0.20ꢃ0.16ꢃ0.08
Monoclinic
P21
7.9724(5)
8.8830(5)
9.6390(6)
90.00
110.274(2)
90.00
640.33(7)
2
1.402
284
C20H18N4O2
953873
246.38
0.24ꢃ0.18ꢃ0.12
Monoclinic
P21
17.9316(17)
5.1648(5)
18.9905(19)
90.00
103.643(3)
90.00
1709.1(3)
4
1.346
728
The crystals of 3,4-Ala were grown from an acetonitrile/
water (1:20, v/v) mixture and were found to be in the non-
centrosymmetric monoclinic space group P21 (Table 2). The
asymmetric unit contained one molecule of 3,4-Ala. The di-
hedral angle between the two pyridyl moieties was 88.68,
that is, they are almost orthogonal to each other. Unlike 3,3-
Phe, one of the pyridyl N atoms participated in hydrogen
bonding with one of the amide moieties through N-H···N in-
g [8]
À
Volume [ꢄ3]
Z
teractions [N···N=2.967(3) ꢄ; aN H···N=174.28]; the
other amide moiety displayed the usual amide···amide hy-
Dcalc [gcmÀ3
]
drogen bonding through N-H···O interactions [N···O=
F
N
À
2.871(2) ꢄ; aN H···O=170.08], thus resulting in an overall
m MoKa [mmÀ1
T [K]
]
0.098
298(2)
0.0196
À8/8, À8/9, À9/9
0.090
298(2)
0.0345
À
2D HBN. In this case, C H···p interactions (3.249 and
3.386 ꢄ) were also observed (Figure 9). It may be noted
here that the temperature-variable 1H NMR data (see
above) of an analogous compound, namely 0,0-Ala, dis-
Rint
Range of h, k, l
À20/20, À6/5,
À22/22
qmin/max [8]
2.25/22.47
4823/866/861
1.10/24.45
15254/3154/2660
À
played C H···p or p–p interactions.
Reflections
collected/unique/
observed
ACHTUNGTRENNUNG[I>2s(I)]
Data/restraints/
We carried out PXRD experiments on selected gels of
3,3-Phe and 3,4-Ala as depicted in Figure 10. It was clear
from the plots that in the case of 3,4-Ala the PXRD pat-
terns obtained under various conditions were nearly super-
imposable, which meant that the single-crystal structure in
this case actually represented the bulk as well as the
SAFINs found in the xerogels. However, in the case of 3,3-
Phe, the PXRD patterns of the bulk and simulated matched
very well but did not match with the PXRD pattern of the
866/1/182
3154/1/485
parameters
Goodness of fit on F2
Final R indices [I>2s(I)]
1.082
1.038
R1 =0.0235
wR2 =0.0584
R1 =0.0236
wR2 =0.0585
R1 =0.0410
wR2 =0.1063
R1 =0.0504
wR2 =0.1141
R indices (all data)
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Parthasarathi Dastidar et al.
tion of pyridyl and phenyl moieties as terminal groups as
potential low-molecular-weight gelators. Out of the 8 bis-
amides synthesized, 7 turned out to be gelators of both or-
ganic and aqueous solvents. Both 1D and 2D hydrogen
bond networks could be seen in the single-crystal structures
of two gelator molecules (3,3-Phe and 3,4-Ala, respectively),
thus indicating the importance of 1D and 2D hydrogen
bond networks in gelation. Temperature-variable 1H NMR
experiments on one such gel (0,0-Ala) confirmed the pres-
À
ence of either p–p or C H···p interactions during gelation,
in good agreement with the single-crystal structure of an
analogous gelator (3,4-Ala). Remarkably, a 1,2-dichloroben-
zene gel of 3,3-Phe displayed excellent shape-sustaning,
load-bearing, and self-healing properties, which are not so
common in supramolecular gels.
Experimental Section
Materials and Physical Measurements
All the starting materials and reagents were commercially available and
were used without further purification. Solvents were of LR grade and
1
used without further distillation. Both H and 13C NMR spectra were col-
lected using a 500 MHz spectrometer (Bruker Ultrashield Plus-500) and
a 300 MHz spectrometer (Bruker Avance DPX-300). FT-IR spectra were
obtained using an FTIR instrument (FTIR-8300, Shimadzu). The elemen-
tal compositions of all the purified compounds were confirmed by ele-
mental analysis using a PerkinElmer 2400, Series-II, CHNO/S elemental
analyzer. SEM images of the selected xerogels were recorded using
a JEOL JMS-6700F field-emission scanning electron microscope. TEM
images of selected samples were recorded by using a JEOL instrument
and a 300 mesh copper TEM grid. DSC data were recorded using Perki-
nElmer Diamond DSC system. All the rheology studies were carried out
using an SDT Q series advanced rheometer AR 2000. X-ray powder dif-
Figure 9. Illustration of the crystal structure of 3,4-Ala; 2D hydrogen
À
À
bond network through N H···O and N H···N interactions (A) and over-
À
all parallel packing (B). All hydrogen atoms (except N H) were omitted
for clarity.
fraction patterns were recorded using
a Bruker AXS D8 Advance
powder X-ray diffractometer (CuKa1 radiation).
Measurements of the Gel-Sol Dissociation Temperature (Tgel
)
Tgel values were determined using the dropping ball method. A locally
made glass ball that weighed 306.0 mg was kept on the top of the gel
(0.5 mL) prepared in a test tube (10ꢃ100 mm). The tube was immersed
in an oil bath and then placed on a magnetic stirrer to ensure uniform
heating. The temperature at which the ball reached the bottom of the
tube was considered to be Tgel
.
SEM and TEM Imaging
Dilute (0.3–0.5 wt.%) solutions of the corresponding gelators were drop-
cast on a glass plate fixed with a standard metallic SEM stub and allowed
to dry under ambient conditions. All the samples were coated with plati-
num prior to recording of SEM images. The sample for TEM was pre-
pared by depositing a drop of a dilute solution (on the order of ~10À3 m)
on a carbon-coated Cu (300 mesh) TEM grid. Subsequently, the grid was
dried under vacuum at room temperature overnight and used for record-
ing TEM images using an accelerating voltage of 200 kV.
Figure 10. PXRD patterns of 3,4-Ala (A) and 3,3-Phe (B) under various
conditions.
Single-Crystal X-ray Diffraction
xerogels, which meant the existence of a new crystalline
phase other than the phase present in the single crystal.
Single crystals suitable for X-ray diffraction were obtained by slow evap-
oration at room temperature from the corresponding solvent systems
mentioned in the structure descriptions. Single-crystal X-ray data were
collected with MoKa radiation (l=0.7107 ꢄ) using a SMART APEX-II
diffractometer equipped with a CCD area detector. All the data collec-
tion, data reduction, and structure solution and refinement were carried
out using the SMART APEX-II software package. All the structures
were solved by direct methods and refined in a routine manner. In all
Conclusions
A new series of bis-amides of l-alanine and l-phenyl ala-
nine have been synthesized by considering various combina-
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Parthasarathi Dastidar et al.
cases, non-hydrogen atoms were treated anisotropically. Whenever possi-
ble, the hydrogen atoms were located on a difference Fourier map and
refined. In other cases, the hydrogen atoms were geometrically fixed at
their idealized positions. CCDC 953873 (3,3-Phe) and CCDC 953874
(3,4-Ala) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3289, 1667, 1638, 1533, 1445, 690 cmÀ1; elemental anal. calc. (%) for
C22H20N2O2: C 76.72, H 5.85, N 8.13; found: C 76.60, H 5.82, N 7.93; MS:
m/z (%): 367 [M+Na]+ (100).
3,3-Ala: 1H NMR (300 MHz, MeOD-d4): d=9.04 (s, 1H), 8.76
ACHTUNGTRENNUNG(s, 1H),
8.72–8.58(d, J=4.8 Hz, 1H), 8.32–8.13 (m, 2H), 8.13–7.92 (d, J=8.97 Hz,
1H), 7.56–7.40 (dd, J=7.8, J=4.8, 1H), 7.40–7.10(dd, J=7.8, J=4.8, 1H),
4.76–4.64ACTHNUTRGNEUNG
(q, 1H),1.59–1.57 ppm (d, J=7.2 Hz, 3H); 13C NMR (300 MHz,
MeOD-d4): d=173.95, 167.90, 152.79, 149.43, 145.35, 142.01, 137.30,
131.43, 129.17, 125.0, 125.31, 125.09, 51.9, 17.83 ppm; FT-IR (KBr pellet):
n˜ =3269, 1643, 1547, 1482, 1421, 1286, 706 cmÀ1; MS: m/z (%): 271
[M+H]+ (45), 293 [M+Na]+ (100).
Synthesis Procedure
Pyridin-3-amine (0.72 g, 7.5 mmol) was dissolved in dry THF (50 mL) in
a 250 mL two-neck round-bottomed flask under nitrogen atmosphere
and then DCC (1.53 g, 7.5 mmol, 1 equiv) was added. Subsequently,
a pre-prepared solution of N-Boc-l-alanine (1.42 g, 7.5 mmol, 1 equiv) or
phenylalanine (2 g, 7.5 mmol, 1 equiv) in dry THF (50 mL) was added
slowly to the amine solution cooled in an ice bath and stirred overnight.
The reaction mixture was then filtered and the filtrate evaporated under
reduced pressure to afford a white residue (compound 1). This residue
was dissolved in a 15% acidic solution (HCl/dry methanol, v/v) and
stirred overnight. Subsequently, dry Et2O was added to yield white crys-
tals (compound 2), which were then dissolved in CH2Cl2 through the ad-
dition of dry triethylamine. One equivalent of acid chloride was dissolved
in dry CH2Cl2 with the addition of dry triethylamine and added to the so-
lution of compound 2 in CH2Cl2. Following stirring for overnight, the
compounds were purified by column chromatography (methanol/chloro-
form, 1:50) to afford the product as a white powder.
3,3-Phe: m.p.=1768C; 1H NMR (300 MHz, [D6]DMSO): d=10.44 (s,
1H), 9.08–9.02 (d, J=8.1 Hz, 1H), 8.94 (s, 1H), 8.74 (s, 1H), 8.72–8.65
(d, J=3.6 Hz, 1H), 8.30–8.23 (d, J=3.6 Hz, 1H), 8.17–8.10 (d, J=7.8 Hz,
1H), 8.06–7.99 (d, J=7.8 Hz, 1H), 7.52–7.44 (dd, J=7.8 Hz, J=4.8 Hz,
1H), 7.43–7.30 (m, 3H), 7.30–7.22 (t, J=7.2 Hz, 2H), 7.22–7.12 (t, J=
7.2 Hz, 2H), 4.93–4.80 (m, 1H), 3.25–3.00 ppm (m, 2H,); 13C NMR
(300 MHz, [D6]DMSO): d=171.26, 165.68, 152.61, 152.53, 149.17, 149.06,
144.99, 141.62, 141.52, 138.33, 135.96, 135.72, 129.87, 129.72, 128.69,
126.99, 124.17, 123.91, 56.26, 37.65 ppm; FT-IR (KBr pellet): n˜ =3345,
1655, 1599, 1578, 1533, 1495, 1439, 1323, 750, 716, 691 cmÀ1; elemental
anal. calc. (%) for C20H18N4O2: C 69.35, H 5.24, N 16.17; found: C 69.26,
H 5.34, N 15.97; MS: m/z (%): 347 [M+H]+ (100), 369 [M+Na]+ (40).
3,0-Ala: m.p.=1388C; 1H NMR (500 MHz, [D6]DMSO): d=10.28 (s,
1H), 8.79–8.76 (d, J=2 Hz, 1H), 8.79–8.76 (d, J=6.5 Hz, 1H), 8.28–8.25
(d, J=4.5 Hz, 1H), 8.08–8.04 (d, J=8.5 Hz, 1H), 7.94–7.91(d, J=7.0 Hz,
2H), 7.57–7.53(t, J=7.0 Hz, 1H), 7.50–7.46 (t, J=7.5 Hz, 2H), 4.65–4.59
(m, 1H), 3.47–3.43 ppm (d, J=7.5, 3H); 13C NMR (500 MHz,
[D6]DMSO): d=172.61, 166.89, 144.76, 141.45, 136.28, 134.42, 131.90,
128.72, 128.10, 126.72, 124.13, 50.42, 18.15 ppm; FT-IR (KBr pellet): n˜ =
3308, 3262, 1668, 1609, 1553, 1425, 1325, 1194, 837, 509 cmÀ1; elemental
anal. calc. (%) for C15H15N3O2: C 66.90, H 5.61, N 15.60; found: C 66.83,
H 5.42, N 15.27; MS: m/z (%): 270 [M+H]+ (100), 292 [M+Na]+ (60).
3,4-Ala: m.p.=1548C; 1H NMR (300 MHz, [D6]DMSO): d=10.29 (s,
1H), 9.04–8.96 (d, J=6.6 Hz, 1H), 8.78–8.69 (t, J=5.4 Hz, 3H), 8.28–8.23
(d, J=4.5 Hz, 1H), 8.00–8.07 (d, J=8.1 Hz, 1H), 7.84–7.78(d, J=4.8 Hz,
2H), 7.38–7.30(q, J=5.1 Hz, 1H), 4.66–4.56 (t, J=7.2 Hz, 1H), 1.48–
1.40 ppm (d, J=6.6, 3H,); 13C NMR (300 MHz, [D6]DMSO): d=172.24,
165.47, 150.78, 144.92, 141.40, 136.26, 126.86, 124.25, 122.13, 50.57,
18.12 ppm; FT-IR (KBr pellet): n˜ =3254, 1678, 1657, 1541, 1485, 1329,
1287, 638 cmÀ1; elemental anal. calc. (%) for C14H14N4O2: C 62.21, H
5.22, N 20.73; found: C 63.12, H 5.14, N 20.33; MS: m/z (%): 270.85
[M+H]+ (100), 292.8 [M+Na]+ (30).
0,0-Ala: m.p.=1458C; 1H NMR (500 MHz, MeOD-d4): d=10.03 (s, 1H),
8.64–8.56 (d, J=11.5 Hz, 1H), 7.93–7.85 (d, J=11.5 Hz, 2H), 7.57–7.49
(m, 5H), 7.35–7.20(t, J=13, 2H), 7.09–6.95(t, J=12.5, 1H), 4.68–4.54ACHTUNGTRENNUNG(m,
1H), 3.19–3.11 (d, J=9 Hz, 1H), 1.49–1.35 ppm (d, J=12 Hz, 3H);
13C NMR (300 MHz, MeOD-d4): d=171.92, 166.71, 139.55, 134.38,
131.77, 129.12, 128.62, 127.98, 123.63, 119.63, 50.33, 18.25 ppm; FT-IR
(KBr pellet): n˜ =2893, 1668, 1601, 1582, 1537, 1522, 1445, 1256, 1117, 752,
696 cmÀ1; elemental anal. calc. (%) for C16H16N2O2: C 71.62, H 6.01, N
10.44; found: C 72.20, H 6.50, N 9.85; MS: m/z (%): 291 [M+Na]+ (100).
3,4-Phe: m.p.=1608C; 1H NMR (500 MHz, [D6]DMSO): d=10.46 (s,
1H), 9.17–9.12 (d, J=8.0 Hz, 1H), 8.77 (s, 1H), 8.75–8.71 (m, 2H), 8.32–
8.28 (d, J=4.5 Hz, 1H), 8.08–8.03 (d, J=8.0 Hz, 1H), 7.77–7.72ACHTNUTGRNEUNG(m, 2H),
7.44–7.35 (m, 3H), 7.33–7.27 (t, J=8.0, 2H), 7.23–7.17 (t, J=7.5 Hz, 1H),
4.94–4.87 (m, 1H), 3.25–3.19 (dd, J=4.5 Hz, J=9.0 Hz, 1H), 3.15–
3.08 ppm (dd, J=10.5 Hz, J=12.5 Hz, 1H); 13C NMR (500 MHz,
[D6]DMSO): d=170.46, 164.95, 150.10, 144.41, 141.01, 140.67, 137.65,
135.32, 129.09, 128.07, 126.38, 123.55, 121.31, 55.66, 36.10 ppm; FT-IR
(KBr pellet): n˜ =3300, 1670, 1637, 1605, 1553, 1529, 1483, 1431, 1292,
696 cmÀ1; elemental anal. calc. (%) for C20H18N4O2: C 69.35, H 5.24, N
16.17; found: C 68.64, H 5.19, N 16.01; MS: m/z (%): 347 [M+H]+ (100),
369 [M+Na]+ (40).
3,0-Phe: m.p.=1568C; 1H NMR (500 MHz, [D6]DMSO): d=10.42 (s,
1H), 8.80–8.75 (m, 2H), 8.30–8.26 (d, J=4.5 Hz, 1H), 8.06–8.03 (d, J=
8.5 Hz, 1H), 7.85–7.83 (d, J=7.5 Hz, 2H), 7.55–7.51 (t, J=7.5 Hz, 1H),
7.48–7.44 (t, J=7.5 Hz, 2H), 7.42–7.40 (d, J=7.5 Hz, 2H), 7.37–7.34 (q,
J=8.5 Hz, J=4.5 Hz, 1H), 7.31–7.27 (t, J=7.5 Hz, 2H), 7.21–7.17(t,
J=7.5 Hz, 1H), 4.90–4.85 (m, 1H), 3.21–3.10 ppm (m, 2H,); 13C NMR
(500 MHz, [D6]DMSO): d=171.57, 167.09, 144.93, 141.56, 138.52, 136.05,
134.38, 131.93, 129.73, 128.73, 128.66, 126.93, 126.90, 124.16, 56.32,
37.62 ppm; FT-IR (KBr pellet): n˜ =3300, 1672, 1549, 1485, 1288,
702 cmÀ1; elemental anal. calc. (%) for C20H17N3O2: C 72.49, H 5.17, N
12.68; found: C 72.92, H 5.66, N 11.95; MS: m/z (%): 346 [M+H]+ (100),
368 [M+Na]+ (40).
Acknowledgements
U.K.D. and P.D. thank CSIR, New Delhi for their Senior Research Fel-
lowship (SRF) and financial support, respectively. S.B. thanks IACS for
his Senior Research Fellowship (SRF). All the single-crystal X-ray dif-
fraction data were collected at the DBT funded Single Crystal Diffrac-
tometer facility at the Department of Organic Chemistry, IACS, Kolkata.
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7
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Received: February 13, 2014
Revised: April 18, 2014
Published online: && &&, 0000
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ÝÝ These are not the final page numbers!

FULL PAPER
Healing is a matter of time: A series
of bis-amides derived from l-phenyla-
lanine and l-alanine displayed moder-
ate to good gelation ability with vari-
ous solvents. Structure–property corre-
lation using single-crystal and powder
X-ray diffraction on selected gels was
attempted. One of the gelator resulted
in a gel with outstanding load-bearing
and self-healing properties. The 1,2-
dichlorobenzene gel of this gelator was
so strong that letters could be carved
into a thin slice of the gel.
Supramolecular Gels
Uttam Kumar Das,
Subhabrata Banerjee,
Parthasarathi Dastidar*
&&&& — &&&&
Remarkable Shape-Sustaining, Load-
Bearing, and Self-Healing Properties
Displayed by a Supramolecular Gel
Derived from a Bis-pyridyl-bis-amide
of l-Phenyl Alanine
9
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Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ