Anaerobic oxidation of cysteine to cystine by manganese(III) in aqueous acetic acid

Abstract

The anaerobic oxidation of cysteine, Cys, by Mn(III) in acetic acid solutions has been followed by use of a stopped-flow spectrophotometric method at a temperature of 20 °C. The formation and disappearance of the [Mn(OAc) 2Cys] − complex was monitored at 350 nm. The rate depends strongly on the acetic acid concentration (and hence also on pH) and led to the conclusion that more than one cysteine-containing species was involved. These mono-cysteinyl complexes are formed by the loss of two protons from the cysteine – one from the – SH and the other from either the –NH 3 + or, more likely, the –COOH which is partially protonated at the low pH values involved (0.5–2.5). The rate-determining reprotonation of the bound –COO − (or –NH 2) is then accompanied by internal electron transfer yielding Mn(II) and the cysteinyl radical, Cys•, which then dimerises to form (inactive) cystine. At high acetic acid concentrations (60–90% AcOH) the tris-acetato species, [Mn(OAc) 3], predominates together with some of the bis-complex, [Mn(OAc) 2] +, and the active species is [Mn(OAc) 2Cys] − which decomposes with a rate constant of k 2=16.8±0.9 M −1 s −1. At low acetic acid concentrations (20–30% AcOH) the mono-acetato species predominates and the reactive species is [Mn(OH)Cys] for which the rate of decomposition= k 2 ′=(1.32±0.11)×10 4 M −1 s −1. The relative values of the rate constants obtained are discussed, as is the bonding of cysteine to manganese(III).