Iron(III)–salen ion catalyzed s-oxidation of l-cysteine and s-alkyl-l-cysteines by H2O2: Spectral, kinetic and electrochemical study

Abstract

The H2O2 oxidation of l-cysteine and s-alkyl-l-cysteines (s-met-l-cys, s-et-l-cys & s-pro-l-cys) catalyzed by iron(III)–salen (salen = N,N′-bis(salicylidene)ethylenediaminato) complexes in aqueous CH3CN proceeds through Michaelis–Menten kinetics. The rate constant (k) values correlate well with Hammett σ constants, which gives the positive reaction constant (ρ = 1.5–1.9) value. The CV of oxoiron(IV)-salen ion shows a clear oxidation peak at 1.28 V in 0.1 M phosphate buffer (PB) solution using 0.1 M tertiary butyl ammonium perchlorate (TBAP) as supporting electrolyte at 266 K. The rate of the reaction is highly sensitive to the length of the alkyl chains present in the l-cysteines, pH and solvent composition of the medium. The calculated binding constant values (Kf) in the range of 117–613 M−1, indicate that iron(III)–salen complexes carrying electron donating substituents in the salen ligand have higher binding constant values compared to those carrying electron withdrawing substituents. Product analysis shows the conversion of l-cys to its disulfide and s-alkyl-l-cys to the corresponding sulfoxides. Based on the spectral and kinetic data the plausible mechanism has been proposed.