Superoxide general facts

The oxidation number of oxygen in its compounds is -2 except in peroxides (where it is -1), superoxides (where it is - ), or in OF2 and 02F2 (where it is positive). The peroxides and superoxides generally occur only with other elements in their maximum oxidation states. You will be able to recognize peroxides or superoxides by the presence of pairs of oxygen atoms and by the fact that if the compounds were normal oxides, the other element present would have too high an oxidation number (Sec. 13.3). 

Besides the superoxide dismutation mechanism, the reactivity of metal centers, in particular manganese complexes, toward NO is very much dependent on the possibility for binding a substrate molecule. As it will be shown later, the possibility that MnSOD enzymes and some mimetics can react with NO has been wrongly excluded in the literature, simply based on the known redox potential for the (substrate) free enzymes, mimetics, and NO, respectively. Therefore, the general fact that, upon coordination, redox potentials of both the metal center and a coordinated species are changed should be considered in the case of any inner-sphere electron-transfer process as a possible reaction mechanism. 

In this scheme the k values represent rate constants, which are generally potential dependent. It must be emphasized that the four-electron pathway does not imply the transfer of four electrons in a single step rather, it underscores the fact that all intermediate species, such as, but not restricted to peroxide, remain bound to the electrode surface yielding, upon further reduction, water as the sole product. Also depicted in Scheme 3.1 are mass transport processes (diff) responsible for the replenishment of 02 and removal of solution phase peroxide next to the interface, and the adsorption and desorption of the peroxide intermediate, for which the rate constants are labeled as ks and k6, respectively. Not shown, for simplicity, is the one-electron reduction of dioxygen to superoxide, a radical species that exhibits moderate lifetime in strongly alkaline electrolytes [15]. 

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