Kinetics and mechanism of oxidation of L-cysteine, DL-homocysteine and glutathione by trans-(diaqua)(salen)manganese(III) complex in aqueous medium: Structure optimization at the DFT level: Influence of externally added copper(II)

Abstract

The kinetics of oxidation of L-cysteine (cys) by trans-Mn^III(salen)(OH₂)₂⁺ (H₂salen = N,N′-bis(salicylidene)ethane-1,2-diamine) is studied at 30.0–45.0 °C, 1.90 ≤ pH ≤ 7.46, I = 0.3 mol dm⁻³, and the same is investigated for DL-homocysteine (Hcys) at 35 °C for comparison. Similarly, kinetic studies are performed for glutathione (GSH) at 30.0–45. 0 °C, 4.35 ≤ pH ≤ 7.46. The product analysis indicated the formation of corresponding disulphides, and Mn^III is reduced to Mn^II. The same products are also formed when the oxidation is carried out in the presence of externally added Cu²⁺ ions. Although the oxidation is moderately catalyzed in the presence of Cu²⁺ ions but it is retarded by the chelating ligand EDTA. The stoichiometric ratio $∆{\mathrm{Mn}}^{\mathrm{III}}\left(\text{salen}\right){\left(H_{2}O\right)}_2^{+}:∆\left(X\right)=$ 1:1 (X = cys, Hcys, and GSH). The reaction proceeds via fast equilibrium pre-association inner-sphere complexes between trans-Mn^III(salen)(OH₂)₂⁺ and X, followed by very slow intramolecular electron transfer steps. The kinetic parameters for various steps for cys, Hcys, and GSH are presented. The results indicate an outer-sphere electron transfer mechanism for the reduction of Mn^III to Mn^II. The DFT-optimized structures supported the carboxylate mode of binding and the ground state structural trans effect for all the species. In addition, it supports a proton-controlled electron transfer process (PCET).