The Generation of Hydroxyl Radicals in the Reaction of Molecular Oxygen with Polyphosphate Complexes of Ferrous Ion

The reaction of Fe2+ with molecular oxygen (autoxidation) was investigated in 20 mM phosphate buffer (pH 7.4) at 37°C using a fluorescent ^{\bullet}{\rm OH}$ probe, coumarin-3-carboxylic acid. The autoxidation of unchelated Fe2+ produces OH radicals. Polyphosphatic chelators (pyrophosphate and tri- and tetrapoly phosphate) enhanced the generation of radicals. This effect was explained by an alteration of the reaction mechanism. The two-electron reduction of the oxygen molecule and the generation of hydrogen peroxide intermediates are the major reactions during Fe2+ autoxidation. The polyphosphatic complexes of ferrous ion reduce molecular oxygen and reactive oxygen intermediates by a one-electron mechanism. The chelation of ferrous ion increases the generation of the superoxide radical and production of ^{\bullet}{\rm OH}$ during ferrous ion autoxidation and in the Fenton reaction. The results consider the ferrous ion-polyphosphate system as convenient model for the generation of hydroxyl radical in biological systems.