Dioxygen toxic radicals from

The mechanism of reduction of dioxygen has been partially clarified, mainly thanks to the low temperature trapping technique designed by Chance et al. [129], Although the precise mechanism is still not understood, it seems probable that dioxygen is reduced to water in two concerted two-electron steps (Fig. 3.3) in which certain intermediates have been identified (see Refs. 8, 92, 97-100, 129-133). One important feature of this particular mechanism is that it affords a switch from one-electron transfer reactions (of cytochromes c, a and Cu ) to effective two-electron steps in the reduction of Oj. The latter is necessary for thermodynamic and kinetic reasons and to effectively prevent release of toxic oxygen radicals from the active site (see Ref. 99). 

---