Kinetics and Mechanism of Catalyzed Oxidation of L-Cysteine by Salen Schiff Base Complexes of Co(III), Fe (III) and Cr(III)

Kinetics of oxidation of L-cysteine by new series of substituted ONNO-donor salen-type Schiff base complexes of general formula [MIII(L)Cl] (M = Co, Fe, Cr; L = Schiff base ligand) have been studied in aqueous solutions. Measurements were run at constant temperature (25º C), constant ionic strength (0.20 M), and constant pH (7.0) under pseudo-first order conditions, in which the concentration of cysteine is around two orders of magnitude greater than that of metal complex. The observed rate constant was determined by following the change in absorbance of reaction mixture at a predetermined wavelength with time. Results show that the rate of oxidation depends on the type of metal center, with Co(III) complexes were found to have the highest rates due to higher reduction potential of Co(III). The oxidation rate was also found to depend on steric factor and the electron withdrawing / releasing ability of the ligand bound to the metal ion.