IMIDAZOLE CATALYSES IN AQUEOUS SYSTEMS .I. ENZYME-LIKE CATALYSIS IN HYDROLYSIS OF A PHENYL ESTER BY IMIDAZOLE-CONTAINING COPOLYMERS

Hydrolyses of 3-nitro-4-acetoxybenzoic acid catalyzed by copolymers of 1-vinyl-2-methylimidazole with 1-vinylpyrrolidone (copolymer A) and with acrylamide (copolymer B) were studied at 30° and mostly at pH 8.0 in 1.0 M aqueous KCl. The rate of the catalytic hydrolysis could be described by Michaelis-Menten kinetics, showing substrate saturation phenomena at high substrate concentrations, as in enzymatic reactions. Copolymer A of low (<20%) imidazole contents showed the same kinetic pattern, independent of the copolymer composition, and gave Km (dissociation constants of the catalyst-substrate complex) of 9.3 mM and k3 (first-order rate constant of the pseudo-intramolecular product formation) of 0.038 min-1. Copolymer B gave Km of 63 mM and k3 of 0.11 min-1. When, however, copolymer A contained higher amounts of the imidazole unit, the kinetic pattern was not the simple Michaelis-Menten type and the over-all catalytic efficiency decreased, suggesting the presence of the catalytic site of a differing nature. The substrate binding was attributed to hydrophobic interaction, since the electrostatic interaction between catalyst and substrate was found negligible. The kinetic characteristics of copolymer A were reasonably explained by assuming the loop formation of the polymer segment surrounding the substrate molecule. Finally, probable reasons for the relatively small k3 value for copolymer A were discussed, taking the nature of the catalytic site into consideration. © 1969, American Chemical Society. All rights reserved.