One-electron oxidation of iron(II) complexes of tryptophan and histidine. A pulse-radiolysis study
Abstract
Iron(II) complexes of tryptophan and histidine, as models of some metallo-enzymes, have been oxidised by Br2˙– in the pH range 7–11. For the tryptophan complex, the rate of oxidation is controlled by the ligand through the formation of metal-complexed TrpH˙+. This species may either undergo intramolecular transfer at rates > 1.7 × 106 s–1 resulting in oxidation of FeII, or may deprotonate at a rate dependent on pH to yield the FeII-complexed Trp˙. The bimolecular rate constants for oxidation of FeII(aq) and FeII/tryptophan complexes by both uncomplexed TrpH˙+ and Trp˙ have also been determined. For histidine complexes, the FeII-imidazole centre was found to be more reactive than either histidine or FeII(aq) to Br2˙–. Again, oxidation of FeII is believed to be the likely outcome of the reaction.