Oxidation of methionine residue in Gly-Met dipeptide induced by [Au(en)Cl2]+ and influence of the chelated ligand on the rate of this redox process

Spectroscopic (H-1 NMR and UV-vis) and electro-chemical (CV) techniques have been applied to study the reaction of Gly-Met dipeptide with [Au(en)Cl-2](+) complex having bidentate coordinated ethylenediamine (en) ligand. The final product in the investigated reaction was Gly-Met sulfoxide, formed by the oxidation of methionine residue with [Au(en)Cl-2](+). The formation of Gly-Met sulfoxide proceeds through two steps including the monodentate coordination of thioether sulfur atom of methionine residue and release of en ligand from Au(III). In order to investigate the influence of the pH and Cl-concentration on this redox process, the reaction was performed in solution of KCl with the concentration varying from 0.01 to 0.50 M and in the pH range 2.00-5.00. The obtained kinetic data showed that oxidation of methionine residue in Gly-Met was 200 times slower in comparison to the same process with previously investigated [AuCl4]-complex. Difference in the oxidation rate of Gly-Met between these two complexes was attributed to the presence of chelated ethylenediamine ligand in [Au(en)Cl-2](+). The finding that the oxidation of sulfur-containing peptides and proteins can be inhibited by structural modifications of gold(III) complexes introducing polydentate nitrogen-containing ligands can contribute in designing new gold(III) complexes with lower toxic side effects.