Radical anions, continued superoxide

The lower valency states of many metal ions and their complexes are oxidized by molecular oxygen. The initial reaction may involve electron transfer to give the superoxide radical anion or co-ordination of the O2 to form a complex. Studies of short-lived species resulting from the former process have been described and extensive investigations continue in the latter area. Synthetic models for oxygenbinding haemoproteins have been reviewed. ... 

With the semiconductor oxidation catalyst, however, the surface becomes activated only upon photoexcitation. At low light intensities, the possibility that many holes are formed in the valence band is remote, so that the irradiated semiconductor powder becomes an effective one-electron oxidant. Now if the same chemistry ensues on the photochemically activated TiC>2 surface, then the reaction will proceed as in the bottom route of eqn 9. Thus, the carboxy radical is formed, producing an alkyl radical after loss of carbon dioxide. Since the semiconductor cannot continue the oxidation after the first step, the radical persists, eventually recapturing the conduction band electron, either directly or through the intervention of an intermediate relay, perhaps superoxide. The resulting anion would be rapidly protonated to product. 

Despite the orderly process of enzyme biosynthesis and the denaturation and degradation of older enzymes, there are problems of stability of proteins in solution. There is continual endogenous generation of peroxides, ammonia, hydrogen ions, carbon monoxide, superoxide, singlet oxygen, and hydroxyl radicals. Oxygen, as well as cations, anions, and trace metals are present in all cellular fluids. These various substances may inhibit or destabilize various enzymes. 

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