The effect of pH on the conversion of superoxide to hydroxyl free radicals

The conversion of superoxide (O₂ ^{{minus sign, dot below}}) to the hydroxyl (HO.) free radical by superoxidedriven Fenton reactions was measured by the formation of hydroxylated derivatives from benzoate. Among a range of catalysts required for the conversion, the Fe³⁺EDTA complex was the most effective. The effect of superoxide dismutase and catalase indicated that O₂ ^{{minus sign, dot below}} and H₂O₂ were essential reactants, while the formation of authentic HO. was confirmed by the inhibiting capacities of formate, t-butanol, and mannitol. The conversion of O₂ ^{{minus sign, dot below}} to HO. was tested over a broad pH range, and was found to be highest at pH 4.8 whether Fe³⁺EDTA or free Fe³⁺ were used as the catalysts. When Fe³⁺EDTA was used at the optimum pH, every HO. produced required 3.7 O₂ ^{{minus sign, dot below}} radicals, close to the theoretical limit of one HO. from every three O₂ ^{{minus sign, dot below}} radicals generated.