Comparison of the oxidation of two porphyrin complexes by bromate with respect to wave propagation

The kinetics and mechanism of the oxidation of iron(III) protoporphyrin IX (hemin) and iron(III) meso-tetra(4-sulfonatophenyl)porphine (TPPSFe) were compared. Both reactions exhibit sigmoid-shaped time dependence of the absorbance of the porphyrin complexes during the course of the reaction. The oxidation of hemin was proved to be an autocatalytic reaction resembling very much the autocatalytic subset reaction of the BZ oscillators. The oxidation of TPPSFe was proved not to be autocatalytic. It consists of consecutive reaction steps without any feedback. One of the intermediates absorbing in the same region as TPPSFe could be identified as an iron(V)-oxo complex of TPPS. According to the kinetic differences, single autocatalytic wave fronts could be observed in the autocatalytic hemin-bromate reaction if hemin was fixed and stabilized in silica gels, while no wave formation appeared in the TPPSFe-bromate system. The kinetic differences are in agreement with the different structures of the two porphyrin complexes.