Concerted Electron-Proton Transfer (EPT) in the Oxidation of Cysteine

Cysteine is the most acidic of the three common redox active amino acids with pK(a) = 8.2 for the thiol compared to pK(a) = 10.1 for the phenol in tyrosine and pK(a) approximate to 16 for the indole proton in tryptophan. Stopped-flow and electrochemical measurements have been used to explore the role of proton-coupled electron transfer (PCET) and concerted electronproton transfer (EPT) in the oxidations of L-cysteine and N-acetyl-cysteine by the polypyridyl oxidants M(bpy)(3)(3+) (M = Fe, Ru, Os) and Ru(dmb)(3)(3+) (bpy is 2,2'-bipyridine, and dmb is 4,4'-dimethyl-2,2'-bipyridine). Oxidation is rate limited by initial 1e electron transfer to M(bpy)(3)(3+), with added proton acceptor bases, by multiple pathways whose relative importance depends on reaction conditions. The results of these studies document important roles for acetate (AcO-) and phosphate (HPO42-) as proton acceptor bases in concerted electronproton transfer (EPT) pathways in the oxidation of L-cysteine and N-acetyl-cysteine with good agreement between rate constant data obtained by electrochemical and stopped-flow methods.