Studies on an iron(iii)-peroxo porphyrin. Iron(iii)-peroxo or iron(ii)-superoxo?

Abstract

We demonstrate that a one electron reduced product of the heme iron dioxygen adduct exists in solution not only as the commonly accepted iron(III)-peroxo species, but coexists with its isomeric iron(II)-superoxo form. This unusual reduced metal-superoxide adduct [M(II)-O₂⁻] is recently reported as a reactive intermediate in the case of non-heme extradiol dioxygenases and could also be generated by cryoreduction of a heme Fe^II-O₂ adduct. The existence of iron(II)-superoxo species in solution is consistent with IR, EPR, mass and Mössbauer spectra. The equilibrium between heme iron(III)-peroxo and iron(II)-superoxo forms is supported by density functional theory and explains our previous finding that upon release of coordinated (su)peroxide a corresponding iron(II) complex remains. These results shed new light on the nature of heme iron(III)-peroxo species that are key intermediates in the metalloenzyme-catalyzed dioxygen and hydrogen peroxide activation.