Unpaired Electron Migration between Aromatic and Sulfur Peptide Units

Cysteine thiyl radicals (Cys/S') were found capable of one-electron oxidation of tyrosine. Equilibration occurred, using Cys and Gly-Tyr, with an equilibrium constant of K₅ = 20 ± 4 at pH 9.15: Cys/S- + Tyr = Cys + Tyr/O

Hence the reduction potentials of these couples differ at pH 9.15 by E(Cys/S', Cys) - E(Tyr/O^r, Tyr) = 80 mV. Oxidation of Trp-Gly by Cys/S' was not detectable from pH 7 to 12. The methionyl radical cation (Met/S'N), formed via 'OH-attack on Met-Gly, reacts with Trp-Gly to generate the indolyl radical (Trp/N'). New results on intramolecular Trp/N' → Tyr/O' transitions indicate that the reaction requires direct contact between the two redox centers. Various possible pathways for migration of unpaired electrons between peptide units are compiled in a scheme.