Optically transparent hydrogels from an auxin-amino-acid conjugate super hydrogelator and its interactions with an entrapped dye

Article abstract of DOI:10.1002/chem.201103757

Low-molecular-weight organic hydrogelators (LMHGs) that can rigidify water into soft materials are desirable in various applications. Herein, we report the excellent hydrogelating properties of a simple synthetic auxin-amino-acid conjugate, naphthalene-1-

Full text of DOI:10.1002/chem.201103757

FULL PAPER
DOI: 10.1002/chem.201103757
Optically Transparent Hydrogels from an Auxin–Amino-Acid Conjugate
Super Hydrogelator and its Interactions with an Entrapped Dye
Amarendar Reddy, Aashish Sharma, and Aasheesh Srivastava*[a]
Dedicated to Professor Santanu Bhattacharya
Abstract: Low-molecular-weight organ-
ic hydrogelators (LMHGs) that can ri-
gidify water into soft materials are de-
sirable in various applications. Herein,
we report the excellent hydrogelating
properties of a simple synthetic auxin–
amino-acid conjugate, naphthalene-1-
acetamide of l-phenylalanine (1-NapF,
Mw =333.38 Da), which gelated water
even at 0.025 wt%, thereby making it
the most-efficient LMHG known. Opti-
cally transparent gels that exhibited
negligible scattering in the range 350–
900 nm were obtained. A large shift
from the theoretical pKa value of the
gelator was observed. The dependence
of the minimum gelator concentration
(MGC) and the gel-melting tempera-
tures on the pH value indicated the im-
portance of H-bonding between the
carboxylate groups on adjacent phenyl-
alanine molecules in the gelator assem-
bly. FTIR spectroscopy of the xerogels
showed a b-sheet-like assembly of the
lengths that exceeded a few microns.
Rheology studies showed the formation
of stable gels. The entrapment of
water-soluble dyes afforded extremely
fluorescent gels that involved the for-
mation of J-aggregates by the dye
within gel. A strong induced-CD band
established that the RhoB molecules
were interacting closely with the chiral
gelator aggregates. H-bonding and
electrostatic interactions, rather than
intercalation, seemed to be involved in
RhoB binding. The addition of chao-
1
gelator. Variable-temperature H NMR
spectroscopy demonstrated that
p
stacking of the aromatic residues was
also partly involved in the gelator as-
sembly. TEM of the xerogel showed
the presence of a dense network of
thin, high-aspect-ratio fibrillar assem-
blies with diameters of about 5 nm and
AHCTUNGTRENNtGUN ropic reagents, as well as increasing
the pH value, disassembled the gel and
promoted the release of the entrapped
dye with zero-order kinetics.
Keywords: amino acids
· dyes ·
gels · hormones · hydrogen bonds
Introduction
ing blocks can be degraded enzymatically.[6] The simplified
molecular architectures of these gelators allow for their
straightforward synthesis and scale-up. There is also a con-
stant impetus to convert drugs and other bioactive mole-
cules into hydrogels.[7–9] The direct application of these hy-
drogels in biology and materials science provides the neces-
sary fillip for this exploration.[10] A stimulus-responsive gela-
tion process (in which the gel could be unraveled under an
external stimulus, such as pH value, enzymes, etc.) offers an
added advantage in drug entrapment and controlled re-
lease.[11–13]
The assembly of dipeptide and tripeptide-based gels has
been extensively studied.[14] Miravet and co-workers per-
formed NMR investigations to highlight the exchange dy-
namics between discrete gelator molecules and the gel-net-
work in valine-based gelators.[15] They also showed that vari-
ous additives interacted differently with the gel network. By
varying the protecting groups on lysines, Hirst et al. correlat-
ed the thermal stability of the gels with the solubility and
cooperative assembly of the gelator molecules.[16] A proline-
based supramolecular hydrogel exhibited efficient, highly
stereoselective, and heterogeneous organocatalysis for the
direct aldol reaction.[17] Lysine-based gelators of both organ-
ic and aqueous media have been explored in detail by
Suzuki and Hanabusa.[18] The release of small (model) drug
Gels contain 3D, continuous- and interpenetrated solid- and
liquid phases. Even though the gelation of water is challeng-
ing for low-molecular-weight organogelators (LMOGs),
a wide variety of hydrogelators have been reported, often
based on biomolecules, such as peptides, sugars, and nucleo-
bases.[1–3] These gelators utilize various supramolecular inter-
actions to self-assemble into fibrous aggregates that can
entrap and immobilize water.[4,5] Gels that are obtained
from low-molecular-weight amino acid derivatives may have
important applications in drug delivery because of their con-
trolled-release characteristics and because the peptide build-
[a] A. Reddy,+ A. Sharma,+ Dr. A. Srivastava
Department of Chemistry
Indian Institute of Science Education and
Research Bhopal
ITI Gas Rahat Building, Govindpura
Bhopal, 462023 Madhya Pradesh (India)
Fax : (+91)755-4092392
E-mail: asrivastava@iiserb.ac.in
[+] These authors contributed equally to this work.
Supporting information for this article, including experimental details,
is available on the WWW under http://dx.doi.org/10.1002/
chem.201103757.
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molecules from the gels of N,N’-dibenzoyl-l-cystine (DBC)
was studied by Friggeri et al.[19] Gels that were derived from
N-acetyl-l-cysteine were shown to be responsive to redox
reactions in addition to changes in pH value and tempera-
ture.[20] Simple fluorenylmethoxycarbonyl (Fmoc) deriva-
tives of many amino acids and peptides have also resulted in
hydrogelation.[6,21]
Amongst the amino acid derivatives that have been con-
sidered, diphenylalanine (FF) has emerged as a potent as-
sembly unit and various FF derivatives have provided access
to a range of peptide nanomaterials.[22] Hydrogels that were
derived from the self-assembly of Fmoc-FF and Fmoc-pro-
tected tripeptide arginylglycylaspartic acid (RGD) have
been used for culturing cells by Ulijn and co-workers.[23]
This result exemplified the biocompatibility of these gels.
The entrapment and release of various biomolecules, such
as cytochrome C, salicylic acid, and vitamin B12, from F-
based hydrogels have been reported.[24,25] Adams and co-
workers studied the controlled release of Naphthol Yellow
and Direct Red as model drugs from Fmoc-based hydro-
gels.[6] They also investigated the gelation ability of naphtha-
lene derivatives of FF.[26]
We have previously reported a tetrameric sugar-based hy-
drogelator and we have also investigated the interactions of
nanoparticles with organogels that were derived from ala-
nine derivatives.[2,27] Herein, we report the exceptional hy-
drogelating ability of amide conjugate of naphthalene-1-
acetic acid and l-phenylalanine (1-NapF). Naphthalene-1-
acetic acid (1-NAA) is a well-known plant hormone; it is
a synthetic auxin that promotes shoot-growth in plants by
the elongation of cells.[28] This structurally simple synthetic
auxin–amino-acid conjugate acts as a “supergelator” and
can gel acidic water at concentrations ꢀ0.025 wt% (equiva-
lent to one gelator molecule holding ꢁ74000 water mole-
cules). This hydrogelation was possible in, and influenced
by, the presence of common salts and buffers in aqueous
media. The molecular contributors to the gelation process
were delineated through various variable-temperature and
variable-pH studies. The ensuing transparent gel was used
to entrap, and subsequently release, Rhodamine B (RhoB)
dye upon exposure to a chaotropic agent or by changing the
pH value. RhoB was chosen as a model drug because it al-
lowed “visualization” of its interactions with the gel through
spectroscopic techniques.
all of these gelators (Fmoc-FF, Fmoc-F, etc.), the aromatic
groups provided rotational freedom through an intervening
methylene residue. These compounds were often better ge-
lators than their conjugated counterparts, which emphasized
the importance of rotational freedom of the aromatic
moiety for efficient hydrogelation. Although stacking inter-
actions were undoubtedly important for gelation, the rota-
tional freedom of the aromatic moiety seemed to accentuate
the gelation process. This ac-
centuation was presumably due
to an expansion of the confor-
mational space for the aromatic
rings to achieve the most-opti-
mal packing. Considering these
key features, we designed 1-
NapF to have large rotational
Figure 1. Synthetic
amino-acid conjugate superge-
lator; rotational freedom
(double-sided arrows) of the
aromatic residues was possible
auxin–
freedom for the
p
surface
(Figure 1). MM2 energy-mini-
mization studies on 1-NapF in-
dicated that it possesses a bent
L-shape in which intramolecu- in 1-NapF.
À
lar CH p interactions were
dominant (see the Supporting
Information, Figure S1).
Apparent pKa of the gelator: The titration of a solution of
1-NapF (10 mm) in dilute NaOH (pH 11) was performed as
described earlier by Tang et al.[31] The apparent pKa of 1-
NapF was about 5.3 (Figure 2), which indicated a significant
Figure 2. Titration curve for the determination of the pKa value of 1-
NapF; solid line: 10 mm 1-NapF, dashed line: H2O (control). The appar-
ent pKa value for the gelator was 5.3.
Results and Discussion
Design of the gelator: The F moiety seemed to be important
À
for gelation because it gave access to p stacking or CH p
pKa shift of about 1.8 compared to the theoretical pKa of N-
protected phenylalanine (3.5). As reported by Chen et al.,
the apparent pKa value of this class of molecules is strongly
dependent on their hydrophobicity, denoted by their logP
value.[26] The calculated logP value for this compound was
1.963 and hence the calculated pKa value was 5.6, which was
quite close to the observed value. However, during the titra-
tion, the aqueous solution of anionic 1-NapF started to
interactions through the phenyl side-chain. The simple
Fmoc-F only achieved hydrogelation within a restricted pH
region.[29] Xu and co-workers prepared many other amide
derivatives of F, including the 2-naphthalene acetamide de-
rivative of F that showed striking similarity to the 1-NapF
reported herein, except that the linkage at the naphthalene
unit was at the 2-position rather than at the 1-position.[30] In
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Optically Transparent Hydrogels with Entrapped Dyes
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become translucent much above the pKa value (in the pH
range 9–7). At a pH value below the pKa, a white precipi-
tate started to appear and complete phase-separation oc-
curred at pH<4.5. Based on these observations, we sur-
mised that 1-NapF molecules started aggregating through
À
the formation of NH CO H-bonds at higher pH. However,
the equilibrium was more towards the solubilized form. As
the carboxylate groups became neutralized, aggregation do-
À
minated through H-bonding interactions between the NH
CO and the carboxylic acid residues. However, as discussed
below, the concentration of 1-NapF was much higher than
the minimum gelator concentrations (MGCs) for 1-NapF.
The high concentration of 1-NapF, in conjunction with
strong stirring, resulted in precipitation.
Gelation properties: Upon addition of 1-NapF to boiling
water, it underwent slow dissolution and the resulting sol
cooled to form an optically clear, thermoreversible gel at
room temperature with a MGC of <0.5 mgmLÀ1. However,
we observed that, by decreasing the pH value of the aque-
ous medium to pHꢂ4 by using dilute HCl, the MGC could
be lowered to ꢀ0.025 wt%, which meant that only 1 mg of
1-NapF could stabilize ꢁ4 mL of dilute HCl solution against
inversion. Gelation was only observable at pH values >3.5,
and precipitation of the gelator was observed below this pH
value. The MGC values kept increasing with the pH of the
medium. The gel-melting temperatures (measured by the
tube-inversion method) showed a consequent decrease with
an increase in the pH value, thereby indicating a weakening
of the aggregation propensity and increased solubility of the
gelator as the pH value was increased. These data are sum-
marized in Table 1.
Figure 3. UV/Vis spectrum for the hydrogel in 0.1m acetate buffer
(pH 5); note the low scattering value, even at 400 nm. Inset: image of the
resulting transparent gel.
tion, such as UV/Vis and CD spectroscopy, of the entrapped
dye (see below).
Effect of the salt on the thermal stability of the gels: The
thermal stability of the 1-NapF hydrogels was also investi-
gated in different salt solutions (Table 2). The chosen salts
Table 2. Tm for the hydrogels in different salt solutions at pHꢂ6.5.
Salt solution
Tm[a] [8C]
Na2SO4
NaCl
>94
85
NaNO3
NaClO4
H2O
80
83
80
[a] Gelator concentration: 1 mgmLÀ1, salt concentration: 0.5m.
Table 1. MGC and Tm for the hydrogels at different pH values.
pH
MGC[a] [wt%]
Tm [8C][b]
4
5
6
0.1
0.1
0.5
>80[c]
67
61
had a common cation (Na+) and the anions were chosen ac-
cording to the Hofmeister series wherein ions are arranged
according to their efficiency to precipitate proteins.[32] It is
known that anions that have higher charge-density (e.g., sul-
fate) have stronger interactions with water compared to the
anions that have lower charge-density (e.g., perchlorate).
Thus, above the isoelectric point of proteins, sulfate ions ac-
celerate protein-aggregation and act as kosmotropes whilst
perchlorate ions act oppositely as chaotropes to increase the
solubility of proteins. The thermal stability of 1-NapF hydro-
gels in salt solutions followed the Hofmeister series: the gels
melted at higher temperature in the presence of sulfate ions
compared to chloride or nitrate ions. The equilibrium be-
tween discrete, solubilized gelator molecules and those that
were involved in aggregates was shifted in the presence of
these salts. The solubility of the gelator was lowered in the
presence of sulfate anions (the salting-out effect) and the
gel-assembly remained stable, even at higher temperatures.
[a] Minimum gelator concentration in 0.1m acetate buffer at a particular
pH value; [b] gel-melting temperature at 0.5 wt% in 0.1m acetate buffer;
[c] at pH 4, the gel underwent a lot of sineresis above 808C but did not
“melt”.
A representative UV/Vis spectrum and digital image of
the hydrogel is shown in Figure 3. The optical clarity was
probably due to the minimal amount of gelator required
and to the formation of thin aggregates (as confirmed by
TEM, see below). Under these conditions, the scattering
grain-boundaries would be very few. Furthermore, there
were no chromophores in 1-NapF that absorbed in the visi-
ble region, although the gels absorbed very strongly in the
near-UV range (below 325 nm) due to the presence of naph-
thalene groups in 1-NapF (for the UV/Vis spectrum of a so-
lution of 1-NapF in MeOH, see the Supporting Information,
Figure S2). Owing to their transparency in the visible-light
region, these gels were amenable to spectroscopic investiga-
Characterization of the hydrogel: Variable-temperature
1H NMR spectroscopy: This study was performed on a gel
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(Figure 5, top). In the bottom
image of Figure 5, these fibers
formed mesh-like aggregates.
However, we were unable to
obtain good quality images at
high magnification to rule out
the possible formation of nano-
tubes. We believe that the as-
sembly process was similar to
the amyloid fibrillation. This
premise was supported by
FTIR and dye-adsorption stud-
ies (see below). However, in
our case, the fibrillar assembly
was observed at pH values
below the pKa value, in contrast
to the work by Tang et al., who
observed large flat ribbons with
Fmoc-FF under these condi-
tions.[31] The reason for this dif-
ference was in the molecular
structures of Fmoc-FF and 1-
NapF. Our gelator had a smaller
p surface as well as fewer H-
bonding elements than Fmoc-
1
Figure 4. Selected regions in the variable-temperature H NMR spectra of the hydrogel; as the temperature in-
creased, a downfield shift was observed for all of the peaks (dashed lines).
sample that was prepared at the MGC in sodium phosphate
buffer (pD=4.5); this gel exhibited a macroscopic “melting”
at 528C. As a result, the spectra in the gel phase between 25
and 508C were too broad to be of use (not shown). Above
528C, a sudden sharpness in the peaks was observed
(Figure 4). As discussed earlier, there was an equilibrium
between the solubilized and aggregated gelator molecules,
and the solubility of the gelator increased on increasing the
temperature. For the peaks that are highlighted by dashed
lines in Figure 4,
a uniform downfield shift of Dd=
0.009 ppm8CÀ1 was observed (from d=8.24 to 8.40ppm and
from d=4.25 to 4.40 ppm). This downfield shift was due to
de-stacking of the aromatic groups on heating. In the gel
À
state, CH p interactions between the aromatic groups pro-
vided a somewhat-shielded environment around these pro-
tons, which unraveled slowly and non-cooperatively upon
heating. However, the shifts in the d values were not as
large as those previously observed in other systems.[33] Thus,
it was likely that the gelation process involved greater con-
tributions from H-bonding interactions between the carbox-
À
ylic acid residues, and was assisted by the CH p interactions
of the aromatic groups. Moreover, the two parameters that
could disrupt the gel, an increase in the pH value and the
use of chaotropic agents, such as urea, indicated a greater
role for hydrogen-bonding interactions. Through H-bonding,
the 1-NapF molecules assembled into fibrous assemblies, as
observed by TEM.
Figure 5. TEM images of gel fibers that were stained with phosphotungs-
tic acid. Top: Thin, high aspect-ratio fibrillar assemblies with diameters
of about 5 nm and lengths of a few microns; bottom: a dense mesh of the
fibers that entrapped water molecules.
Transmission electron microscopy: Thin, high-aspect-ratio fi-
brils were formed by the gelator at pH 5 (Figure 5); the fi-
brils were about 5 nm in diameter and a few microns long
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Optically Transparent Hydrogels with Entrapped Dyes
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FF. Hence, the propensity to assemble was weaker. As evi-
dent from the bottom image in Figure 5, the fibrillar assem-
blies formed porous mesh-like structures that could entrap
water through H-bonding and surface-tension effects. This
network-formation by 1-NapF was further confirmed by the
rheology discussed below.
both the G’ and G’’ values were weakly dependent on the
frequency (Figure 6, bottom), which was indicative of an en-
tangled network-like system, as corroborated by TEM anal-
ysis. The G’ value for the hydrogel from 1-NapF was about
300 Pa. Also, it was noted in the work by Tang et al. that
gels that were formed through the cooling of hot sols were
homogeneous but weak; this observation was mirrored in
our system too.[31] It has been noted recently that, for Fmoc-
FF, the strength of the gel was intimately pH-dependent.[35]
Rheology: The rheological properties of a gel system pro-
vide the portfolio for determining its practical utility.[34]
With 1-NapF, the rheology was performed at pH 5 (just
below the apparent pKa). This gel system showed a G’ value
that was approximately 10 times larger than G’’; this differ-
ence remained almost constant up to about 16% strain
(Figure 6, top). As the strain was increased beyond this
FTIR spectroscopy: Solid 1-NapF and the xerogel of 1-NapF
in water and D2O were also analyzed by FTIR spectroscopy
to investigate the H-bonding interactions between the gela-
tor molecules. Figure 7 shows the amide I and amide II
Figure 7. FTIR spectra (amide I and amide II regions) of solid 1-NapF
(dash–dot), the xerogel from H2O (solid line), and the xerogel from D2O
(dashed line); the spectra were recorded as KBr pellets, which indicated
a b-sheet-like structure in the xerogels.
bands in the FTIR spectra. It is well-known that the location
of the amide I absorption is primarily determined by the
conformation of the backbone and is independent of the
amino acid sequence.[36] It was observed that, in solid 1-
NapF, the amide I bond resonated at 1661 cmÀ1, thereby in-
dicating the presence of b-turn-like conformations. Howev-
er, the xerogel from water showed a bifurcated amide I
band with resonances at 1642 and 1618 cmÀ1. This result in-
dicated that the b-sheet-like conformations were dominating
in the xerogel. Furthermore, the amide II band in the xero-
gel was broadened, which pointed to the presence of both
parallel- and antiparallel b-sheet conformations. The xerogel
from D2O also showed a rather broad amide I resonance
that was centered at about 1620 cmÀ1, which also indicated
the presence of b-sheet conformations. The rotational free-
dom of aromatic groups would allow the adoption of a varie-
ty of conformations, which was also indicated in the broad-
ness of the amide frequencies of the xerogel. In this regard,
the aggregation of 1-NapF was analogous to the aggregation
of tau protein fragments wherein b-sheet-like conformations
were observed.
Figure 6. Rheological study of the hydrogel. Top: the amplitude-sweep
study shows that the storage modulus (Gꢁꢂ300 Pa) was higher than the
loss modulus (G’’ꢂ37 Pa), as expected for gels; bottom: the frequency-
sweep study shows the characteristic features of a gel that was formed by
an entangled network of fibers.
point, both the storage modulus (G’) and the loss modulus
(G’’) started to fall because intermolecular forces that held
the gel together started being overcome by the applied
strain and the fibrils were unable to withstand large defor-
mations. The frequency-sweep experiments showed that
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Entrapment of RhoB: When a hot sol of 1-NapF in acetate
buffer was mixed with a solution of RhoB or fluorescein in
the same buffer, the resulting sol cooled to form a uniform-
ly-colored, optically clear, highly-fluorescent gel (see the
Supporting Information, Figure S5a). The optical clarity of
these gels allowed us to probe the interactions between the
dye and the gelator molecules through spectroscopic investi-
gations in the visible-light region. We examined whether the
entrapment of RhoB was purely physical or whether the dye
molecules were interacting chemically with the gelator ag-
gregates. As noted in the NMR study by Miravet and co-
workers, entrapped molecules showed interactions with gel
fibers.[15]
ure S5b). However, the melting temperature of the gels did
not increase, even with increasing concentration of RhoB.
This observation negated the intercalative mode of binding
of RhoB in the gel, and was similar to the observation by
Rodrꢂguez-Llansola et al. on the thermal stability of gels in
the presence of methylene blue.[39] Thus, dye-assembly along
the gel aggregate was more likely, owing to H-bonding and
electrostatic interactions, as corroborated by the dye-release
studies (see below).
The release of the dye from the gel aggregates was inves-
tigated by repeated exposure to acetate buffer at pH 5, as
well as to a buffer that contained different concentrations of
urea (see the Supporting Information, Figure S6). The dye
was released from the gel aggregates with a zero-order rate.
However, the gel itself dissolved within 24 h when exposed
repeatedly to fresh buffer. Degradation of the gel and the
consequent release of entrapped dye were accelerated by
the presence of urea (a chaotropic agent) in the buffer; it
also allowed a faster—but controlled—release of the entrap-
ped dye. The rate of release increased with the concentra-
tion of urea, but it still followed zero-order kinetics. Chang-
ing the pH value to about 9 also prompted the release of
the entrapped material by dissolution of the gel, albeit with-
out any control of the release kinetics. In addition, the pres-
ence of salt (0.1m NaCl) in the buffer also accelerated the
release of the dye. Taking all of these observations into con-
sideration, it was clear that H-bonding and electrostatic in-
teractions were involved between RhoB and the gel. Based
on these data, a model that indicated the molecular-level in-
teractions between the gelator and the dye molecules was
proposed (see the Supporting Information, Figure S7).
UV/Vis spectroscopy indicated that the lmax of RhoB in
the gel state was red-shifted by about 6 nm compared to the
solution state (553 nm in a buffer solution versus 559 nm in
the gel, Figure 8). This result was indicative of the formation
of J-aggregates. The adsorption of RhoB on solid inorganic
Conclusion
Figure 8. UV/Vis and CD spectra of rhodamine B in the solution state
(dashed line) and upon entrapment in a gel (solid line); note the red-
shift in the lmax value and the induced-CD band that was formed upon
entrapment in the gel.
We synthesized a structurally simple and extremely efficient
hydrogelator, 1-NapF. This 1-naphthaleneacetyl derivative
was about 20 times more-efficient in hydrogelation (MGCꢀ
0.025 wt% under optimal conditions) compared to the pre-
viously reported 2-naphthaleneacetyl derivative. Optically
transparent gels were obtained from 1-NapF. Gelation was
effected at pH>3.5, and the MGC value increased with the
pH value of the medium. A Hofmeister-series-type influence
of the added salts was observed on the gel-melting tempera-
tures. H-bonding seemed to be the dominant interaction
compared to the aromatic stacking interactions between the
gelator molecules, as elucidated by variable-pH and -tem-
perature NMR studies. Gelation involved the assembly of
the gelator into high-aspect-ratio fibers, as confirmed by
TEM and rheological studies. Entrapment of water-soluble
dyes afforded highly fluorescent gels. RhoB formed J-aggre-
gates upon entrapment in the hydrogel and exhibited
a strong induced-CD signature. The aggregates of 1-NapF
were easily disrupted by either changing the pH value of
medium or by the addition of chaotropic agents, with the
latter allowing for a more-controlled disruption. Based on
these observations, a model was proposed for the chemical
surfaces is also known to yield J-aggregates.[37] A circular di-
chroism (CD) study of the gel-entrapped RhoB showed
a strong induced-CD band in the region 500–600 nm, in
which RhoB is known to absorb (Figure 8). This result fur-
ther indicated the formation of close chemical interaction
between RhoB and the gelator aggregate; as a result, the
dye molecules experienced the chiral environment of the ag-
gregate. This observation was reminiscent of the helical as-
sembly of cyanine dye along the polynucleotide template.[38]
CD studies were not possible in the UV regions in which
the naphthalene and amide groups absorbed owing to ex-
tremely high absorptions in these regions.
Confocal microscopy of the gel-entrapped RhoB showed
the formation of highly fluorescent assemblies, as expected
for J-aggregates. A cottony, fibrous morphology of the ag-
gregate was observed (see the Supporting Information, Fig-
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[16] A. R. Hirst, I. A. Coates, T. R. Boucheteau, J. F. Miravet, B. Escud-
er, V. Castelletto, I. W. Hamley, D. K. Smith, J. Am. Chem. Soc.
2008, 130, 9113–9121.
[17] F. Rodrꢂguez-Llansola, J. F. Miravet, B. Escuder, Chem. Commun.
2009, 7303–7305.
interactions between the dye and gelator molecules. Because
1-NapF was prepared from the synthetic auxin 1-naphtha-
lene acetic acid (1-NAA), it may have biological significance
that needs further exploration.
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Acknowledgements
A.R. and A.S. thank the IISER Bhopal for institute fellowships. This re-
search was supported by the Rapid Grants for Young Investigator
(RGYI) Scheme of the Department of Biotechnology (India) (No. BT/
PR15143/GBD/27/334/2011). We thank Dr. Vimlesh Kumar (IISER
Bhopal) for his help with the confocal microscopy. SAIF-AIIMS New
Delhi is acknowledged for TEM imaging of the samples. We acknowl-
edge the very important suggestions and comments from the reviewers.
One of the reviewers is also thanked for the calculation of the logP and
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Received: November 29, 2011
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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A. Srivastava et al.
Flexible gels better: Amide conjugate
of 1-naphthaleneacetic acid and l-phe-
nylalanine (Mw =333 Da) was as an
exceptional hydrogelator owing to the
flexibility of the aromatic residues. The
entrapment of fluorescent dyes yielded
highly fluorescent gels (see figure).
Gels
A. Reddy, A. Sharma,
A. Srivastava* ................ &&&& — &&&&
Optically Transparent Hydrogels from
an Auxin–Amino-Acid Conjugate
Super Hydrogelator and its Interac-
tions with an Entrapped Dye
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!