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 Fe^II, or may deprotonate at a rate dependent on pH to yield the Fe^II-complexed Trp˙. The bimolecular rate constants for oxidation of Fe^II(aq) and Fe^II/tryptophan complexes by both uncomplexed TrpH^˙+ and Trp˙ have also been determined. For histidine complexes, the Fe^II-imidazole centre was found to be more reactive than either histidine or Fe^II(aq) to Br₂^˙–. Again, oxidation of Fe^II is believed to be the likely outcome of the reaction.