Formation of superoxide-metal ion complexes and the electron transfer catalysis

The formation of a variety of superoxide-metal complexes and their catalytic functions have been reviewed. The g₌₌-values of the EPR spectra of superoxide-metal ion complexes vary significantly depending on the type of metal ions. From the deviation of the g₌₌-value from the free spin value are determined the energy splitting values (ΔE) of π_g levels due to complex formation. The ΔE values correlate well with the catalytic reactivities of the metal ions in the electron transfer reduction of oxygen. Such a complex formation between superoxide ion and metal ions has been shown to play an essential role in the enzymatic function of superoxide dismutase, formation of active oxygen-copper complexes and the novel catalytic effect of O₂ in back electron transfer of zinc porphyrin-fullerene linked molecules.