Salt, pH and thermoresponsive supramolecular hydrogel of N-(4-n-tetradecyloxybenzoyl)-l-carnosine

Article abstract of DOI:10.1039/b914665b

A carnosine based amphiphilic hydrogelator has been developed which efficiently gelates water and exhibits salt, pH and thermoresponsive gelation properties. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914665b

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COMMUNICATION
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Salt, pH and thermoresponsive supramolecular hydrogel
of N-(4-n-tetradecyloxybenzoyl)-L-carnosinew
Amrita Pal, Saurabh Shrivastava and Joykrishna Dey*
Received (in Cambridge, UK) 21st July 2009, Accepted 18th September 2009
First published as an Advance Article on the web 6th October 2009
DOI: 10.1039/b914665b
A carnosine based amphiphilic hydrogelator has been developed
which efficiently gelates water and exhibits salt, pH and thermo-
responsive gelation properties.
Stimuli-responsive hydrogels of low-molecular-weight-
gelators (LMWGs) have attracted tremendous attention in
the recent literature because of their potential applications in
the pharmaceutical industry as soft materials for drug delivery
and tissue engineering.1 In the field of biological applications,
desired responsiveness of the hydrogel to one or more external
stimuli, such as pH, light and salts is essential. Also for these
applications, biocompatibility of the gel-forming materials is
Chart 1 Chemical structure of N-acyl-L-carnosine amphiphiles.
very important. Various LMWGs capable of forming hydrogels
have been reported in the literature.2,3 Many of these LMWGs
are derivatives of sugars, amino acids and nucleotides. Some
of the hydrogels formed by these gelators have also exhibited
temperature and pH-responsive character, and tolerance to
additives, such as salt and surfactants, etc.4 Peptide-based
gelators have gained importance because of biocompatibility
of the constituent materials (amino acids).5 Recently, the
gelation ability of N-acyl peptide amphiphiles also have been
reported.6
(B303 K) for months when preserved under a constant
condition (pH and temperature). The gelation was observed
in all heating–cooling cycles attempted suggesting thermore-
versibility of the hydrogel. Gelation was studied using buffers
of various pH. Gelator 1 was found to gelate water in both
alkaline (7.0 o pH o 11.0) and acidic (0.3 o pH o 2) pH
ranges. However, under similar conditions, compounds 2 and
3 failed to gelate water. The gelation ability of the amphiphile
at different pH as measured by minimum gelator concentration
(MGC), which is defined as the minimum amount of gelator
required to gelate a given volume of solvent, are listed in
(Table 1). The gelation of water by the amphiphile at
concentrations less than 1% (w/vol) (3000–11000 molecules
of water per molecule of gelator) is of particular interest. Data
in Table 1 show that MGC value is lowest at neutral pH and
increases with both decrease and increase of pH. It is clear that
the acid–base reactions involving the carboxylic acid group
(COOÀ) and the imidazole ring of the histidine residue of the
headgroup modulate the gelation ability of the amphiphile.
The protonation of the carboxylate (COOÀ) group reduces
hydration of the headgroup thereby increasing the MGC
value. On the other hand, protonation of the nitrogen atom
of the imidazole ring of the histidine residue at pH o 0.3
transforms the gel to a sol or viscous dispersion as a result of
increased repulsion among positively charged ionic headgroups
of the amphiphile, which prevents growth of supramolecular
aggregates. The inability of the amphiphiles 2 and 3 to gelate
L-Carnosine is a dipeptide normally found in mammalian
skeletal and brain tissues and consists of two amino acids
L-histidine and b-alanine. L-carnosine has been found to
(i) scavenge peroxyl radicals,7 (ii) inhibit copper-catalyzed
oxidative reactions,8 and (iii) exhibit efficient singlet oxygen
quenching.9 On the other hand, N-(long-chain-acyl)carnosine
derivatives are known to have excellent emulsifying activity
and possess antioxidant activity toward lipid peroxidation.10
This attracted our attention toward self-organization of
N-(long-chain-acyl)carnosine in water. In this communication,
we report the gelation behavior of N-acyl-L-carnosines 1–3
(Chart 1) at different pH in the absence as well as in the
presence of inorganic and organic additives.
It was observed that when aqueous dispersions of N-(4-n-
tetradecyloxybenzoyl)-L-carnosine, 1, were allowed to stand at
room temperature, after dissolution of the gelator in
phosphate buffer (pH 7.0) by heating at B368 K, opaque
hydrogels were formed. The gelation was confirmed by
resistance to flow upon inversion of the screw-capped vials.
The hydrogels thus formed are stable at room temperature
Table 1 Minimum gelator concentration (MGC, Æ0.1 g LÀ1) at 25 1C
of hydrogelator 1 in different pH solutions. Gel-to-sol transition
temperatures (TGS, Æ1 K) of the hydrogels containing 0.02 M gelator
are also listed
Department of Chemistry, Indian Institute of Technology, Kharagpur,
Kharagpur-721 302, India. E-mail: joydey@chem.iitkgp.ernet.in;
Fax: (+) 91-3222-255303
pH
1
2
7
8
9
10
11
w Electronic supplementary information (ESI) available: Details
regarding synthesis, identification of chemical structures, and the
experimental techniques employed. See DOI: 10.1039/b914665b
MGC/g LÀ1
TGS/K
6.5
334
5.5
344
3.4
338
4.8
337
5.5
335
6.1
333
6.8
330
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Chem. Commun., 2009, 6997–6999 | 6997

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Fig. 3 XRD Pattern of the hydrogel of 1 in 20 mM phosphate buffer,
Fig. 1 Variation of MGC value of 1 with [NaCl] (’) and [EtOH] (m)
pH 2.0.
in phosphate buffer (pH 7.4) at 298 K.
water suggest that removal of the phenyl ring from the
hydrocarbon tail destroys its gelation ability. Thus it can be
concluded that p–p stacking interactions play an important
role in the promotion of gelation.
Gelation of buffers by the gelator also occurred in the
presence of additives, such as NaCl, and alcohol (EtOH).
However, the MGC value of 1 increased with the increase of
[NaCl] or [EtOH] as shown by the plots in Fig. 1. Hydrogelation
also occurred in the presence of 60% EtOH. This might be due
to the increased solubility of the gelator in the presence of
NaCl or EtOH. In the presence of salt, the electrostatic
interaction between the dipoles of the headgroup of the
amphiphile is partially screened making the molecule partially
charged which increases its aqueous solubility.
Fig. 4 Molecular arrangement of 1 in the lamellar structure.
planes. This suggests that gelator 1 assembles into an ordered
lamellar structure as shown in Fig. 4. This bilayer thickness of
the lamella, which is equal to the interlayer distance (d) of the
001 plane, is 3.97 nm. It is important to note that the bilayer
thickness is smaller than twice the extended molecular length
of the hydrophobic tail of 1 (2.52 nm)11 molecule, but larger
than the length of a single hydrophobic tail. This implies that
the aggregate has interdigitated hydrocarbon tails, which
facilitates hydrogen-bonding interactions at the headgroup
of the amphiphile.
The microstructures (Fig. 2A and B) of the hydrogel at pH 2
and 8 were investigated by field-emission scanning electron
microscopy (FE-SEM). As shown by the micrograph in
Fig. 2A the hydrogel appears to be comprised of 3-D networks
of intertwined ribbons that result in immobilization of water.
A similar morphology of the hydrogel can also be observed at
pH 8.0 (Fig. 2B). A close look at the micrograph also shows
twisting of the ribbons, which are bilayer assemblies of the
gelating molecule. The existence of bilayer structures is
confirmed by X-ray diffraction patterns (Fig. 3) of the gel cast
film at pH 2, which exhibits periodical reflection peaks. The
XRD pattern of the hydrogel at pH 8 (not shown) was closely
similar. The relative peak position q1 : q2 : q3 is 1 : 2 : 3 : 4,
which is typical for a Bragg scattering pattern from a 1-D
lamellar structure corresponding to 001, 002, 003 and 004
The thermal stability of the hydrogel was investigated by
measuring gel-to-sol transition temperature (TGS), which
depends on the concentration, as shown by the representative
plot in Fig. 5. The higher thermal stability at higher concentration
can be attributed to tight packing of the gelator molecules due
to enhanced hydrogen-bonding and hydrophobic interactions
as a result of incorporation of a larger number of gelator
molecules in the aggregate. In other words, the increase of TGS
is linked to growth of the one-dimensional aggregates, which
makes them more flexible, causing more entanglement of the
fibres, and thereby increasing the TGS value. A known amount
of the gelator (10.8 g LÀ1) was employed for determination of
TGS in different pH solutions and the data are listed in Table 1.
As observed, the hydrogel at pH 2 has the highest melting
temperature suggesting its highest stability in acidic pH. The
gel melting temperature decreases slightly with increase of pH
above 7.0. This must be associated with the acid–base reaction
involving histidine residues as discussed above. At lower pH,
the COOÀ groups become protonated, and thus intermolecular
hydrogen-bonding interactions involving COOH groups is
facilitated, thereby increasing the melting temperature; at
Fig. 2 FESEM pictures of air-dried hydrogels of 1 in 20 mM
phosphate buffer of pH 2.0 (A) and pH 8.0 (B).
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In summary, we have developed a peptide based LMWG
that efficiently gelates water at a concentration less than 1% (w/v).
Also the hydrogelation ability in a wide pH range shows its
versatility. The hydrogels entrap a relatively large amount of
water in a thermoreversible manner. The p–p stacking inter-
action of the phenyl rings in the hydrocarbon chain of the
amphiphiles was found to be essential for the gelation. The
gelation occurred as a result of entanglement of twisted
ribbons that are formed by the self-assembly of the amphiphilic
molecules. These ribbons are comprised of interdigitated
bilayer aggregates. Although the hydrogels are quite stable
in the presence of salt and alcohol the gelation properties can
be modulated by changing pH, salt and alcohol concentration.
The hydrogels in different pH media have melting temperatures
higher than the physiological temperature. Thus the hydrogel
may have potential application in drug delivery.
We thank Department of Science and Technology,
New Delhi (SR/S1/PC-18/2005) for financial support of this
work. A. P. thanks IIT, Kharagpur for a research fellowship.
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Chem. Commun., 2009, 6997–6999 | 6999