Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators

The pH of the solution along with chelation and consequently coordination of iron regulate its reactivity. In this study we confirmed that, in general, the rate of Fe(II) autoxidation increases as the pH of the solution is increased, but chelators that provide oxygen ligands for the iron can override the affect of pH. Additionally, the stoichiometry of the Fe(II) autoxidation reaction varied from 2:1 to 4:1, dependent upon the rate of Fe(II) autoxidation, which is dependent upon the chelator. No partially reduced oxygen species were detected during the autoxidation of Fe(II) by ESR using DMPO as the spin trap. However, upon the addition of ethanol to the assay, the DMPO:hydroxyethyl radical adduct was detected. Additionally, the hydroxylation of terephthalic acid by various iron-chelator complexes during the autoxidation of Fe(II) was assessed by fluorometric techniques. The oxidant formed during the autoxidation of EDTA:Fe(II) was shown to have different reactivity than the hydroxyl radical, suggesting that some type of hypervalent iron complex was formed. Ferrous iron was shown to be able to directly reduce some quinones without the reduction of oxygen. In conclusion, this study demonstrates the complexity of iron chemistry, especially the chelation of iron and its subsequent reactivity.