Oxidative biochemistry centered about 35 years ago on the one-electron reduction of H₂O₂ by Fe²⁺, the Fenton reaction, to yield HO^· and a Fe(III)-complex. The discovery that NO^· is formed in vivo and that it reacts with O₂^·− at a diffusion-controlled rate led to ONOO⁻ as an additional oxidant. The rate constant of the Fenton reaction is 53 M⁻¹s⁻¹ 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 FeO²⁺ predominates.  Thermodynamically, this species is comparable to HO^· as an oxidant.  HCO₃⁻ accelerates the reaction even more, and convincing evidence has been presented that the complex of Fe²⁺ with CO₃²⁻ reacts with H₂O₂ to form CO₃^·− and a Fe(III)-complex, conceivably via FeO²⁺ as an intermediate. The rapid reaction of ONOO⁻ with CO₂ (k > 10⁷ M⁻¹s⁻¹) leads to ONOOCO₂⁻ that, depending on the CO₂ concentration, yields varying amounts of NO₂^· and CO₃^·−.  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^·, but with the more selective reactions of CO₃^·− and NO₂^·.

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