[6] Detection of hydroxyl radicals by aromatic hydroxylation

The hydroxyl radical •OH is the most reactive species. The hydroxyl radical reacts at, or close to, a diffusion-controlled rate with all biological molecules, and its half-life in cells is estimated to be 10-9 sec. It is produced when water is exposed to ionizing radiation, fragmenting the oxygen-hydrogen covalent bond, leaving a single electron on hydrogen and one on oxygen. Most of the •OH generated in vivo, except during excessive exposure to ionizing radiation, probably comes from the metal ion-dependent breakdown of hydrogen peroxide (H2O2). Reactions of all these are important when considering the toxicological effects of metal poisoning, but only iron(II)- and copper(I)-dependent reactions could occur in vivo under normal conditions. The iron(II)-dependent formation of •OH is better known as the “Fenton reaction,” and it is far more complicated. There has been repeated controversy as to the formation of •OH in this reaction at physiological pH (7.4), because other iron-oxygen complexes are often postulated as alternatives.