OXIDATION OF L-METHIONINE WITH AQUEOUS CHROMIC-ACID - A KINETIC-STUDY

The kinetics of oxidation of L-methionine by chromic acid in acidic me dium (pH = 0.83-2.2) has been studied spectrophotometrically. The effe ct of L-methionine and chromium(VI) concentrations on the rate of the reaction was determined. The reaction rate decreases with increasing t he pH of the medium. The kinetics of the formation of a chromium(III) complex conform to the rate law: d[Complex]/dt = k(1)K(es1)[H+][L-meth ionine](T)[Cr-VI]T/1 + K-a + K-es1[L-methionine](T) with k(1) = 7.5 x 10(-2) s(-1) and K-es1 = 43.85 at constant [H+] = 1.9x10(-2)mol dm(-3) and [L-methionine](T) = 2.0 x 10(-2) mol dm(-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 ra pid pre-equilibrium step, followed by a slower redox reaction of the e ster. The present study provides kinetic evidence for formation of a c omplex ion (ester). One mole of Cr2O72- oxidizes four moles of L-methi onine, which acts as a monodentate ligand and binds to chromium(VI) th rough the sulfur atom. The coordinated sulfur atom of L-methionine-chr omium(VI) ester is responsible for the oxidative degradation (breaking of the C-1-C-2 bond) of L-methionine. Coordinated oxygen of the carbo xylate group inhibits the cleavage of the C-1-C-2 bond.