ONOO Reactivity The Concept of Radical Ends

Homolytic fission of the 0—0 bond to give -OH and -N02 requires 20 kcal/mol (Koppenol et ai, 1992). Compelling evidence that the equilibrium between OHZ-NOi and ONOOH lies almost exclusively to the 

Quite simply, OH and NO2 react with each other so rapidly that the product, ONOO , cannot be prone to decompose via the same route. 

We have previously proposed that the hydroxyl radical-like reactivity of ONOO occurs via a vibrationally excited form of trans ONOOH (Crow et al., 1994). The reactive species formed is best described as a high-energy intermediate in which the ONO—OH bond is stretched but not broken prior to reaction with target molecules. Thus, no free -OH nor free NO2 is seen. 

The question of homolytic fission of ONOOH will no doubt continue to be debated among free radical chemists. From the standpoint of biologically relevant reaction products, the exact mechanism by which ONOOH produces products characteristic of hydroxy radical is less important. In fact, the hydroxyl radical-like reactivity of ONOOH may be of minor importance in terms of overall toxicity. 

Bearing in mind these caveats with regard to forming oxidizing radicals from homolytic fission of ONOOH, it can be instructive to think in terms of radical ends of ONOOH. ONOOH can be viewed as a molecule comprising two potent one-electron oxidants joined at the hips , that typically react in a concerted fashion. Thus, ONOOH is capable of two electron oxidations like nitration and hydroxylation, depending on the specific target and reaction conditions. For example, initial attack of the hydroxyl end of ONOOH on phenol could produce phenoxyl radical and 

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