Reaction of CO₂ with ONOO^-: One Molecule of CO₂ Is Not Enough

With CO₂ present in excess, ONOO^- reacts to form an adduct in solution and in the solid state, most likely ONOOCO₂^-. In solution, the adduct appears within 2 ms and absorbs at 300 with an extinction coefficient, which is either 50% or 100% (preferred) of that of ONOO^-, 1.70 × 10³ M^-1 cm^-1, and at 685 nm with an extinction coefficient of 85 M^-1 cm^-1. When solid [(CH₃)₄N][ONOO] is treated with CO₂, these two maxima are red-shifted by 30-50 nm. The equilibrium constant for adduct formation in solution is (4.5 ± 0.5) × 10³ M. The adduct reacts further with another CO₂ at a rate of (2.6 ± 0.8) × 10⁴ M^-1 s^-1 and produces 2 CO₂ and NO₃^-. Thermochemical calculations show that ΟΝΟΟCO₂^- is a strong two-electron oxidizing agent, E°(ONOOCO₂^-, H⁺/NO₂^-, HCO₃^-) = +1.28 V at pH 7 and an even stronger one-electron oxidizing agent E°'(ONOOCO₂^-, H⁺/NO₂^•, HCO₃^-) = +1.51 V at pH 7. The extent of homolysis, that is formation of NO₂^• and CO₃^•-, is small, slightly less than 1% relative to ONOO^- at the physiological concentration of CO₂ of 1.3 mM in plasma. Thus, ONOOCO₂^- is more relevant than CO₃^•- under in vivo conditions.