Abstract
The kinetics of oxidation of l-methionine by chromic acid in acidic medium (pH=0.83-2.2) has been studied spectrophotometrically. The effect of l-methionine and chromium(VI) concentrations on the rate of the reaction was determined. The reaction rate decreases with increasing the pH of the medium. The kinetics of the formation of a chromium(III) complex conform to the rate law: with k1=7.5×10-2s-1 and Kes1=43.85 at constant [H+]=1.9×10-2moldm-3 and [l-methionine]T= 2.0×10-2moldm-3. The same values were found with [l-methionine]T variation at constant [H+]. The reaction proceeds through formation of chromium(VI)-l-methionine ester in a rapid pre-equilibrium step, followed by a slower redox reaction of the ester. The present study provides kinetic evidence for formation of a complex ion (ester). One mole of Cr2O72- oxidizes four moles of l-methionine, which acts as a monodentate ligand and binds to chromium(VI) through the sulfur atom. The coordinated sulfur atom of l-methionine-chromium(VI) ester is responsible for the oxidative degradation (breaking of the C1-C2 bond) of l-methionine. Coordinated oxygen of the carboxylate group inhibits the cleavage of the C1-C2 bond.[PUBLICATION ABSTRACT]