Radical driven Fenton reactions--evidence from paraquat radical studies for production of tetravalent iron in the presence and absence of ethylenediaminetetraacetic acid.

Micromolar concentrations of nonchelated ferrous sulfate catalyze a reaction between H2O2 and radiolytically generated paraquat radicals, causing the concurrent oxidation of deoxyribose to thiobarbituric acid reactive products. The oxidation yield per paraquat radical increases with increasing concentration of deoxyribose, and decreases as the instantaneous or steady-state concentration of paraquat radicals is increased, thus explaining previous anomalies in which oxidation was not observed at high paraquat radical concentrations. The process is not mediated by OH. (which gives different products) but is attributed to an oxidizing intermediate resulting from the two electron oxidation of Fe2+ to a peroxo complex, or a derivative of tetravalent iron. Similar but less pronounced concentration dependences occur in the corresponding oxidation of formate or of deoxyribose catalyzed by Fe(EDTA), where at pH 7.3 90% of the pathway is attributed to one electron oxidation of the Fe2+(EDTA) by H2O2, producing OH., while two electron oxidation accounts for the remaining 10%.