Kinetic isotope effect studies of some carboxylic anhydrides useful as models for certain enzymatic processes

Abstract

Solvent and β-secondary kinetic isotope effects were used to study the hydrolysis mechanisms of some carboxylic anhydrides and a thiolester. The studies were carried out in order to obtain model system data useful in interpreting kinetic data obtained in studies of enzymes. Proton inventory studies of propionic anhydride hydrolysis were carried out at 5, 25, and 45°C. The solvent isotope effects obtained at the three temperatures are 3.07 ± 0.09 (25°C), 2.85 ± 0.09 (5°C), and 3.05 ± 0.09 (45°C). The γ curvature parameter values obtained are 0.35 ± 0.09 (25°C), 0.38 ± 0.11 (45°C), and 0.57 ± 0.14 (5°C). The proton inventory for succinic anhydride water-catalyzed hydrolysis at 25°C yielded a (gamma) value of 0.39 ± 0.05. Proton inventories of S-ethyltrifluorothiolacetate hydrolysis at 25 and 30°C yielded (gamma) values of 0.38 ± 0.03 and 0.50 ± 0.04, respectively. The β-secondary isotope effects (k[subscript H]/k[subscript D]) for the water-catalyzed hydrolysis of acetic anhydride was determined at four temperatures in the range 3.7 to 45.2°C and was found to be about 0.95 in all cases. The solvent and β-secondary deuterium isotope effects on the formate-catalyzed hydrolysis of acetic anhydride at 25°C were determined and found to be 1.20 ± 0.04 and 0.97 ± 0.03, respectively. The pH rate profiles for the water-catalyzed hydrolysis of 3-acetylaminophthalic anhydride and of quinolinic anhydride were found to be flat from pH 1 to pH 6 at 25°C. The formate, acetate, and water-catalyzed rates were determined and used to construct a Bronsted plot for the hydrolysis of 3-acetylaminophthalic anhydride hydrolysis, the slope of which is 0.34. The solvent isotope effect on the formate-catalyzed reaction was determined to be 1.96 ± 0.04. The above data, when coupled with other spectroscopic and structural data from the literature, is interpreted in terms of transition state models for hydrolysis which are a function of the anhydride being acyclic and conformationally mobile or cyclic and conformationally rigid. Implications of this idea concerning enzyme-catalyzed reactions are discussed.