Mechanistic studies of the amine-catalysed decarboxylation of oxaloacetic acid

Abstract

Oxaloacetic acid is decarboxylated enzymically by oxaloacetic decarboxylase in many organisms. It was hoped that by study of amine-catalysed decarboxylation as a model system for the enzyme, the nature of the less easily studied enzymic mechanism could be elucidated. Amine-catalysed decarboxylation of oxaloacetic acid, HO2C.CO.CH2.CO2H → HO2 C.CO.CH3 + C02, has been studied. The cyanomethylamine-catalysed decarboxylation in aqueous solution and catalysis by aniline and its derivatives in methanol have been investigated at 25°. The nature of the intermediates in both solvents has been investigated. The rate of intermediate formation in methanol was compared with the observed rate of decarboxylation and results indicated that intermediate formation was the rate-determining step. Amine-catalysed decarboxylation of oxaloacetic acid was found to be first-order with respect to amine and pseudo first-order with respect to substrate. Results suggest that both a carbinolamine and Schiff's base are intermediates in aqueous solution and it appears conclusive that in methanol a Schiff's base is the reactive intermediate.