ESR spin trapping studies into the nature of the oxidizing species formed in the Fenton reaction: pitfalls associated with the use of 5,5-dimethyl-1-pyrroline-N-oxide in the detection of the hydroxyl radical.

Several investigators have challenged the widely held view that the hydroxyl radical is the primary oxidant formed in the reaction between the ferrous ion and hydrogen peroxide. In recent studies, using the ESR spin trapping technique. Yamazaki and Piette found that the stoichiometry of oxidant formation in the reaction between Fe2+ and H2O2 often shows a marked deviation from the expected value of 1:1 (I. Yamazaki and L. H. Piette (1990) J. Am. Chem. Soc. 113, 7588-7593). In order to account for these observations, it was suggested that additional oxidizing species are formed, such as the ferryl ion (FeO2+), particularly when iron is present at high concentration and chelated to EDTA. In this paper it is shown that secondary reactions, involving the redox cycling of iron and the oxidation of the hydroxyl radical adduct of the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) by iron, operate under the reaction conditions employed by Yamazaki and Piette. Consequently, the stoichiometry of oxidant formation can be rationalized without the need to envisage the formation of oxidizing species other than the hydroxyl radical. It is also demonstrated that the iron(III) complex of DETAPAC can react directly with DMPO to form the DMPO hydroxyl radical adduct (DMPO/OH) in the absence of hydrogen peroxide. Therefore, to avoid the formation of (DMPO/OH) as an artefact, it is suggested that DETAPAC should not be used as a reagent to inactivate containing adventitious iron in experiments using DMPO.