Characteristics of an oxidant formed during iron (II) autoxidation

When ethanol (100 mM) and the spin trapping agent DMPO were added to a solution of FeSO4 (0.1 mM) in phosphate buffer (40 mM), pH 7.4, the spectrum of the 1-hydroxyethyl radical spin adduct of DMPO could be detected by ESR spectroscopy. The ESR signal intensity of the 1-hydroxyethyl radical increased with higher concentrations of phosphate. Under these conditions, hydroxyl radical adducts could not be detected. However, if H2O2 (0.1 mM) was also added, the ESR spectra contained signals from both the hydroxyl and 1-hydroxyethyl radical adducts of DMPO. When ethanol was replaced with azide (100 mM) in these experiments, strong ESR signals from the azidyl radical adduct of DMPO were observed only in the Fenton system. The absence of hydroxyl radical adduct signals in the Fe(2+)-PO4 reaction could not be explained by instability of the hydroxyl radical adduct in the presence of Fe2+ or superoxide. Experiments with oxygen radical scavengers indicated that the oxidant formed during Fe2+ autoxidation was less reactive to benzoate, DMSO and tert-butanol than the hydroxyl radical. The available data indicate that the primary oxidant formed during Fe2+ autoxidation in phosphate buffer is not the hydroxyl radical, but may be an iron-oxygen complex such as a ferryl species.