CO3•−, THE RADICAL THAT CONNECTS PEROXYNITRITE AND FENTON CHEMISTRY

<p>Oxidative biochemistry centered about 35 years ago on the one-electron reduction of H<sub>2</sub>O<sub>2</sub> by Fe<sup>2+</sup>, the Fenton reaction, to yield HO<sup>·</sup> and a Fe(III)-complex. The discovery that NO<sup>·</sup> is formed <i>in vivo</i> and that it reacts with O<sub>2</sub><sup>·</sup><sup>−</sup> at a diffusion-controlled rate led to ONOO<sup>−</sup> as an additional oxidant. The rate constant of the Fenton reaction is 53 M<sup>−1</sup>s<sup>−1</sup> up to about pH 4, but above it the rate constant increases linearly with pH.  This acceleration of the Fenton reaction led to the hypothesis that above pH 5 formation of FeO<sup>2+</sup> predominates.  Thermodynamically, this species is comparable to HO<sup>·</sup> as an oxidant.  HCO<sub>3</sub><sup>−</sup> accelerates the reaction even more, and convincing evidence has been presented that the complex of Fe<sup>2+</sup> with CO<sub>3</sub><sup>2−</sup> reacts with H<sub>2</sub>O<sub>2</sub> to form CO<sub>3</sub><sup>·</sup><sup>−</sup> and a Fe(III)-complex, conceivably <i>via</i> FeO<sup>2+</sup> as an intermediate. The rapid reaction of ONOO<sup>−</sup> with CO<sub>2</sub> (<i>k</i> > 10<sup>7</sup> M<sup>−1</sup>s<sup>−1</sup>) leads to ONOOCO<sub>2</sub><sup>−</sup> that, depending on the CO<sub>2</sub> concentration, yields varying amounts of NO<sub>2</sub><sup>·</sup> and CO<sub>3</sub><sup>·</sup><sup>−</sup>.  These two oxidizing radicals together nitrate aromatic residues. Compared to 35 years ago, oxidative biochemistry is no longer concerned with the indiscriminate oxidations and additions of HO<sup>·</sup>, but with the more selective reactions of CO<sub>3</sub><sup>·</sup><sup>−</sup> and NO<sub>2</sub><sup>·</sup>.</p>