The HO Radical
The HO radical is the most reactive species known in biology. HO * is isoelectronic with the fluorine atom and characterized by a midpoint potential of +2.33 V at pH 7 (for comparison, the normal reduction
potential of fluorine is +2.85 V). In cells, the extremely dangerous HO* radical can be formed by reduction of H2O2 via the Haber-Weiss reaction (Haber and Weiss, 1934) catalyzed by Fe2+ (Kehrer, 2000) (see Figure 55).
HO* radicals immediately attack proteins and lipids in the immediate environment of the site of production, thus giving rise to oxidative degradation (Halliwell, 2006). Cells cannot detoxify HO radicals and, therefore, a protection can only be achieved by suppression of H2O2 formation in the presence of Fe2+ using metal binding proteins like ferritins or metallothioneins (Hintze and Theil, 2006). On the other hand,
HO radicals can be produced in programmed cell death as part of defense mechanisms to pathogenic infections (Gechev et al., 2006).
It has to be mentioned that the HO radical is not the only possible product of the reaction between H2O2 and Fe2+. New calculations on the electronic structure and ab initio molecular dynamics simulations have shown that the formation of the ferri-oxo species [FeIV(O2-)(H2O)s]2+ is energetically favored by about 100 kJ/mol compared to the generation of the HO radical (Yamamoto et al., 2012). Therefore, in future mechanistic studies, the species [FeIV(O2)(H20)s]2+ should be taken into account for mechanistic considerations on the oxidative reactions of H2O2 in the presence of Fe2+.