Kinetic studies on oxidation of l-cysteine and 2-mercaptoethanol by a trinuclear Mn(IV) species in aqueous acidic media

In aqueous media, the trinuclear MnIV complex, [MnIV3(μ-O)4(phen)4(H2O)2]4+ (phen = 1,10-phenanthroline) (1) behaves like a monobasic acid, equilibrates with its deprotonated conjugate base (2) ( 1 ⇌ 2 + H+, pKa = 4.00 (± 0.15) at 25.0 °C, I = 1.0 M, maintained with NaNO3). An aqueous solution of 1 in the pH range 2.2–4.0 smoothly oxidizes 2-mercaptoethanol (mercap) and l-cysteine (cys) to their respective disulfides; itself being reduced to [Mn(phen)3]2+ in presence of excess 1,10-phenanthroline which also masks Cu2+, if any, present in the reaction media as impurity as well as acting as a buffer that controls pH range during the reaction. The observed rate constants in H2O media were found to be considerably lowered in media enriched with D2O and a proton-coupled single electron transfer rate step is proposed. Interestingly, we found that deprotonated oxidant (2) reacts faster than the protonated species (1) in oxidizing both the thiols.