Gel catalysts that switch on and off

Abstract

We report development of a polymer gel with a catalytic activity that can be switched on and off when the solvent composition is changed. The gel consists of two species of monomers. The major component, N-isopropylacrylamide, makes the gel swell and shrink in response to a change in composition of ethanol͞water mixtures. The minor component, vinylimidazole, which is capable of catalysis, is copolymerized into the gel network. The reaction rate for catalytic hydrolysis of p-nitrophenyl caprylate was small when the gel was swollen. In contrast, when the gel was shrunken, the reaction rate increased 5 times. The activity changes discontinuously as a function of solvent composition, thus the catalysis can be switched on and off by an infinitesimal change in solvent composition. The kinetics of catalysis by the gel in the shrunken state is well described by the Michaelis-Menten formula, indicating that the absorption of the substrate by the hydrophobic environment created by the N-isopropylacrylamide polymer in the shrunken gel is responsible for enhancement of catalytic activity. In the swollen state, the rate vs. active site concentration is linear, indicating that the substrate absorption is not a primary factor determining the kinetics. Catalytic activity of the gel is studied for substrates with various alkyl chain lengths; of those studied the switching effect is most pronounced for p-nitrophenyl caprylate.N atural enzymes catalyze chemical reactions, and equally importantly, regulate them by reversibly and repeatedly switching on and off the catalytic activities. To mimic this special feature of enzymes, we designed a polymer gel consisting of two species of monomers, each having a specific role. The major component allows the gel to reversibly swell and shrink in response to changes in environmental parameters such as temperature and solvent (1-9). On swelling and shrinking, the local density of the hydrophobic moiety changes and the affinity to substrate molecules is altered accordingly. The minor component capable of catalysis of substrate decomposition is copolymerized with the responsive monomers into the gel network. The catalytic activity of the gel is expected to be switched on and off as the substrate is reversibly bound and released during the cycle of gel swelling and shrinking. Materials and MethodsTo demonstrate the feasibility of the idea, we chose hydrolysis of p-nitrophenyl esters with different alkyl chain lengths as the chemical reaction to examine (Fig. 1a). This reaction is known to be catalyzed by 4(5)-vinylimidazole (10). We, therefore, used it as the minor catalytic monomer component of the gel (Fig. 1b). As the major component, we chose the widely used hydrophobic monomer N-isopropylacrylamide (NIPA). The substrate and gel monomer components were selected because (i) the substrate and one of the hydrolysis products, p-nitrophenol, were easily detected by using UV spectroscopy; (ii) the NIPA gel underwent a discontinuous swelling͞shrinking transition in response to changes in environmen...

Gels with sense: supramolecular materials that respond to heat, light and sound

“…169 The usefulness of such changes was demonstrated by Wang et al, who found that basic vinylimidazole moieties incorporated into a pNIPA gel are more active towards the hydrolysis of pnitrophenyl esters when the gel is in its collapsed, hydrophobic high-temperature state. 170 By contrast, osmium-based complexes tethered to hydrogels of poly(vinylpyridine) were found by Heller et al to undergo electron-transfer reactions less readily after deswelling, because aggregation disfavours the folded conformations needed to bring together neighbouring metal centres. 25 Intuitively, swelling processes should be particularly useful for controlling reactions that are diffusion-limited.…”

Role of mechanical factors in applications of stimuli-responsive polymer gels – Status and prospects

“…Second, reactions that commonly do not proceed well enough in aqueous media may be possible in a hydrogel environment because reactants can be activated through interactions with the hydrogel. Finally, reactants can be concentrated within the hydrogel because of affinity between the reactants and the hydrogel network [63]. Approaches for entrapping man-made catalysts and enzymes in a gel have been investigated and developed since the early Bernfeld and Wan report [64] on physical immobilization of enzymes into a cross-linked synthetic acrylamide gel.…”

Multiple point adsorption in a heteropolymer gel and the Tanaka approach to imprinting: experiment and theory

“…A polymer gel can perform some of the fundamental functions of proteins [45,74]. Like proteins, a heteropolymer gel can exist in four thermodynamic phases: (1) Swollen and fluctuating, (2) Shrunken and fluctuating, (3) Shrunken and frozen in a degenerate conformation, and (4) Shrunken and frozen in the global minimum energy conformation.…”