EPR SPIN-TRAPPING STUDY ON THE OXIDIZING SPECIES FORMED IN THE REACTION OF THE FERROUS ION WITH HYDROGEN-PEROXIDE

Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping reagent for HO., we compared the ratio of rate constants for the reaction of HO. with HO. scavengers (k) to those for the reaction with DMPO (k(DMPO)) in a photolysis or a Fenton (Fe(II)-H2O2) system. Assuming that the k/k(DMPO) ratio measures the extent to which HO. is free in solution relative to 100% in a photolysis system, we concluded that HO. formed in the Fenton reaction is not totally free in solution. The extent to which it is not free, but bound in some kind of complex, depended upon the type of chelator used and increased in the order Fe(II)ADP < Fe(II)-phosphate = Fe(II)EDTA < Fe(II)DETAPAC. There was a remarkable difference in the mode of the Fenton reaction between Fe(II)DETAPAC and Fe(II)EDTA, particularly at high Fe(II) concentrations (0.1 mM). An ethanol-oxidizing species other than HO., presumably the ferryl ion, was detected in the Fe(II)EDTA reaction but not in the Fe(II)DETAPAC reaction. The major oxidizing species in the Fe(II)EDTA-H2O2 reaction changed from the ferryl ion to HO. as the H2O2 concentration was increased, while it was invariably HO. alone in the Fe(II)DETAPAC-H2O2 reaction. Benzoate and tert-butyl alcohol, known as typical HO. scavengers, were shown to react not only with HO. but also with the ferryl ion in the Fe(II)EDTA reaction. Similar scavenging effects were observed with histidine, formate, and mannitol.