title: Unpaired electron migration between aromatic and sulfur peptide units.
abstract: 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 in equilibrium Cys + Tyr/O. (5) Hence the reduction potentials of these couples differ at pH 9.15 by E(Cys/S., Cys) - E(Tyr/O., 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.
mesh_terms: D003545:Cysteine; D005609:Free Radicals; D009842:Oligopeptides; D010084:Oxidation-Reduction; D014364:Tryptophan; D014443:Tyrosine; D014867:Water
chemical_list: D005609:Free Radicals; D009842:Oligopeptides; D014867:Water; D014443:Tyrosine; D014364:Tryptophan; D003545:Cysteine