Studies on the reactivity of malondialdehyde. I. Effects of acid concentration and solvent composition on deuterium exchange reactions

Abstract

The rates of deuterium exchange reactions of malondialdehyde (MDA) and deuterated malondialdehyde (MDA d) have been studied as a function of acidity and the content of dimethyl sulfoxide (DMSO) in binary mixtures with D 2O . MDA incorporates deuterium from D 2O solutions in a first-order reaction with a rate constant ( k obs) that depends on the acid concentration. From this dependence, a catalytic constant, k cat, can be derived ( k cat MDA = 2.25 × 10 5 M − s −1). Similar kinetic behavior was found for MDA d in H 2O solutions, and in this case, k cat MDA = 1.56 × 10 5 M −1 s −1. Results from reactions of MDA and MDA d in identical H 2OD 2O mixtures show that primary and secondary isotope effects are small ( k H / k D = 1.13) and that solvent isotope effects cause most of the differences found between reactions in D 2O and H 2O. Reactions in binary DMSO d 6D 2O mixtures show a six-fold rate increase as the proportion of DMSO d 6 increases from 50% to 90%. These results also illustrate the relatively high reactivity of MDA at pH values well above its p K a and the importance of medium composition on its reaction rate.