Reactivity of hypervalent iron with biological compounds

Although it is well established that the ferryl (Fe==O2+) and perferryl (Fe==O3+) oxidation states of iron have a beneficial role in enzymatic oxidation and hydroxylation processes, as small complexes that are suspected to be formed in Fenton reactions, they may have deleterious effects on living cells. I discuss the basic chemistry of small hypervalent iron complexes. Their reactivity with biological compounds is illustrated by considering ascorbic acid and essential amino acids. A method is proposed for the determination of the ratio of [.OH]/[ferryl] in Fenton-type reactions.