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

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 –NH3+ 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 –NH2) 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 k2=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=k2′=(1.32±0.11)×104 M−1 s−1. The relative values of the rate constants obtained are discussed, as is the bonding of cysteine to manganese(III).