Oxidations of iron(II)/(III) by hydrogen peroxide: from aquo to enzyme

Abstract The mechanism of reaction of hexaquo iron(II) with hydrogen peroxide has been unresolved for 70 years. Most scientists, perhaps by default, have accepted the free radical chain mechanism of Barb et al. (Trans. Faraday Soc. 47 (1957) 462). However an earlier proposal involved formation of the ferryl ion, FeO2+ (J. Am. Chem. Soc. 54 (1932) 2124). Recent work has favored a mechanism involving FeO2+ and FeOFe5+ species. Similarly there are differences of opinion on the mechanism of reaction of iron(III), both hexaquo and chelated, with hydrogen peroxide. These differences have fostered a recent burst of activity, with claims on one hand that hydroxyl radicals play a key role, and on the other, that there is a non-free radical mechanism. In contrast, the mechanism of reaction of the heme-containing peroxidase and catalase enzymes with hydrogen peroxide, orders of magnitude faster than reactions of iron(II)/(III), now appears to be well established. In this paper I attempt, as objectively as possible, to delineate the proposed mechanisms, discuss their possible physiological relevance, and summarize the current state of knowledge.