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

The overall six-electron oxidation of water soluble porphyrin Fe(III)TPPS 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(III)TPPS --> Int(1)--> Int(2)--> 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 Int(1) is proposed to be the species (TPPS (+))Fe(IV)=O. Although no unambiguous proposal for the structure of Int(2) can be made, it is most probably the product of the four-electron oxidation of the original Fe(III)TPPS, 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.