Imidazole Catalyses in Aqueous Systems. VI. The Enzyme-Like Catalysis in the Hydrolysis of a Phenyl Ester by Phenylimidazole-Containing Copolymers. The Correlation between Binding Capacity and Catalytic Activity

Hydrolyses of p-acetoxybenzoic acid catalyzed by copolymers of N-[p-4(5)-imidazolylbenzyl]acrylamide (PI) were studied at 15-40° and mostly at pH 8.0 in 1.0 M aqueous KC1. The rate of the catalytic hydrolysis could be described by Michaelis-Menten kinetics showing leveling off of the rate at high substrate concentrations. On the other hand, the catalytic hydrolysis with a model compound of the catalytic unit, p-acetamidomethylphenyI-4(5)-imidazole, followed second-order kinetics. The PI-vinylpyrrolidone (PI-VP) copolymers showed smaller dissociation constants of the catalyst-substrate complex and smaller intracomplex rates than the PI-acrylamide (PI-AA) copolymers, reflecting an increased hydrophobic property of the catalytic site in PI-VP copolymers. The substrate binding with the polymer catalyst showed large negative entropy changes, supporting the supposition that the substrate binding is based on hydrophobic forces. The entropies of activation of the intracomplex process were unusually large negative values (- 58 to -60 eu). When the reactivity difference of the imidazole group due to different pK_a. values was corrected according to Bruice and Schmir, the decrease in the free energy due to substrate binding was mostly offset by the increase in the free energy of activation of the intracomplex process. The interpretation of this result is that the hydrophobic interaction in the catalyst-substrate complex becomes destroyed in the transition state of the intracomplex reaction.