A general approach is presented for creating polymer gels that can recognize and capture a target molecule by multiple-point interaction and that can reversibly change their affinity to the target by more than one order of magnitude. The polymers consist of majority monomers that make the gel reversibly swell and shrink and minority monomers that constitute multiple-point adsorption centers for the target molecule. Multiple-point interaction is experimentally proven by power laws found between the affinity and the concentration of the adsorbing monomers within the gels.

Weakly cross-linked heteropolymer gels that memorize molecular pairs have been designed and synthesized. The polymer consists of a main monomer component responsible for volume phase transition, methacrylic acid that adsorbs one divalent ion as a pair, and cross-links. The memory of pairing of methacrylic acids within the gels was encoded in the primary sequence of main monomers, methacrylic acids and cross-links within the gels, which was achieved by “imprinting”, namely, by synthesizing gels while methacrylic monomers were paired prior to polymerization. The control gels, where methacrylic monomers were randomly distributed, showed frustration in forming pairs, whereas such frustration was completely diminished in the imprinted gels allowing the memory of pair formation.

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.

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...

Polyampholyte gel, frustrated by the conflicting interactions between its positive and negative monomers, can undergo a sharp discontinuous phase transition driven by changing either gel composition or solvent quality and /or permeability. This finding completes the investigation of all four major interactions involved in the biomolecular machinery with respect to their role in the behavior of polymer gels, considered here as a model of biopolymer system. The result also sheds light on the possible role of frustrations in biomolecular systems, such as proteins. [S0031-9007(99)09334-5] PACS numbers: 61.41. + e, 82.70.Gg, 87.14.Ee, 87.15.Cc In the quest of the principle behind the marvelous functions of biopolymers, it is crucial to understand the role of all four major molecular interactions, including hydrogen bonds, hydrophobic, van der Waals, and Coulomb interactions. To address this problem experimentally, one of the most attractive avenues is to examine simpler polymer systems, such as gels, specifically designed to demonstrate the role of certain interactions. Along this line, three of the above mentioned major interactions-hydrogen bonds, hydrophobic, and van der Waals interactionsare known to induce phase transition in gels, in which an infinitesimal change of environment causes the polymer network to reversibly change volume up to several thousands times [1][2][3][4][5][6]. Gel transition is a gas-liquid type condensation phase transition and results from a competition between attraction and repulsion of monomers and polymer rubber elasticity, i.e., the same factors that are at play in biopolymer systems. The fourth interaction, Coulombic, is fundamentally different because attraction occurs only between different kinds of monomers. This makes polyampholytes, polymers with positively and negatively charged monomers, frustrated: They cannot have a particular microstructure with perfectly minimized interaction energy but instead take many microstructures of locally minimal energies with high barriers between them [7-9]. In this sense, polyampholyte is not similar to a regular vapor-liquid system, but rather to proteins where frustrated interactions control folding, molecular recognition, and other functions. We demonstrate in this work for the first time that frustrated polyampholyte interactions can cause a discontinuous gel phase transition.To seek the frustration-driven phase transition in polyampholytes, we synthesized gels made of negatively charged monomer, acrylamidopropyl-sulfonic acid (AMPS), and positively charged monomers, methacrylamidopropyl-trimethyl-ammonium chloride (MAPTA) [10,11]. Their chemical structures are shown in Fig. 1a. They were mixed in water in various compositions with total monomer concentration fixed at 2 M.An amount of 10 mM of N, N 0 -methylene-bis-acrylamide was added as a cross-linker. The monomers were polymerized by free radical polymerization in a glass tube of 300 mm inner diameter at 60 ± C. After taken out, and washed with water, the gels were immersed in a large...