Kinetics and mechanism of the oxidation of water soluble porphyrin FeIIITPPS with hydrogen peroxide and the peroxomonosulfate ion

Abstract

The overall six-electron oxidation of water soluble porphyrin Fe^IIITPPS by hydrogen peroxide and peroxomonosulfate ion was studied by the stopped-flow method with UV-vis detection. A three-step consecutive reaction was observed with two intermediates: Fe^IIITPPS → Int₁ → Int₂ → products. The products were identified as the iron(III) complex of the biliverdin analog formed from TPPS and 4-sulfobenzoic acid. All the rate constants with both oxidizing agents were determined. Intermediate Int₁ is proposed to be the species (TPPS˙⁺)Fe^IVO. Although no unambiguous proposal for the structure of Int₂ can be made, it is most probably the product of the four-electron oxidation of the original Fe^IIITPPS, contains an iron-oxo center and has a dissociable proton with a pK of around 3.1. In spite of the protolytic equilibria occuring in the pH region 2–4, the kinetic observations do not show pH dependence.