Kinetics of Oxidation of L-Cysteine by Cobalt (III), Iron(III), and Chromium(III) Complexes of Salicylaldiminato Ligands

Kinetics of oxidation of L-cysteine by penta-coordinated Co (III), Fe(III), and Cr(III) complexes of salicylaldiminato ligands have been studied in aqueous solutions. Metal complexes of Nsalicylidinecyclohexylamine, N-salicylidinisopropylaniline, N-salicylidinnaphthylamine, N-(4- sulfonitsalicylidine)cyclohexylamine, and N-(4-sulfonatesalicylidine)-2-isoprpylaniline were used in the studies. Measurements were run at constant temperature (25° 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 steric factor and the electron withdrawing / releasing ability of the ligand bound to the metal ion. It also depends on the metal center. The Co(III) complexes were found to have the highest rates due to higher reduction potential of Co(III).