Bifunctional catalysis by a metal ion and a carboxylate anion in hydrolysis of an alkyl ester and an alkyl amide in dimethyl sulfoxide: A carboxypeptidase a model

Abstract

Kinetics of the hydrolysis of methyl 2-carboxy-6-(2-imidazoleazo)benzoate ( 1) and N,N-dimethyl-2-carboxy-6-(2-imidazoleazo)benzamide ( 2) were investigated in the presence or absence of the Cu(II) or Ni(II) ion in water or in dimethyl sulfoxide (DMSO) containing 5% (v/v) water. In 95% (v/v) DMSO, potassium chloroacetate and chloroacetic acid were used as the buffer species. Potentiometric and spectral titration of chloroacetic acid, 1, M(II) 1 (metal-complexed 1), 2, and M(II) 2 in 95% (v/v) DMSO indicated that the carboxyl groups of M(II) 1 and M(II) 2 ionize almost fully over the pH range examined and that the imidazolyl NH groups of M(II) 1 and M(II) 2 are nearly as acidic as chloroacetic acid. The kinetic data obtained for the hydrolysis of Ni(II) 1 and Cu(II) 2 in 95% (v/v) DMSO revealed that the ester and the amide bonds are readily hydrolyzed through the catalytic action of the bivalent metal ion and the anionic form, instead of the acid form, of the carboxyl group. The cooperative catalytic action of the carboxylate anion and the metal ion in the hydrolysis of alkyl ester and alkyl amide bonds, which is characteristic of carboxypeptidase A action, is achieved with 1 and 2 by using 95% (v/v) DMSO as the reaction medium. Mechanisms of the model reactions are analyzed, and implications of the present results on the mechanism of carboxypeptidase A are discussed.