Pressure Dependence of Peroxynitrite Reactions. Support for a Radical Mechanism

Activation volumes (ΔV^⧧) have been determined for several reactions of peroxynitrite using the stopped-flow technique. Spontaneous decomposition of ONOOH to NO₃^- in 0.15 M phosphate, pH 4.5, gave ΔV^⧧ = 6.0 ± 0.7 and 14 ± 1.0 cm³ mol^-1 in the presence of 53 μM and 5 mM nitrite ion, respectively. One-electron oxidations of Mo(CN)₈^4- and Fe(CN)₆^4-, which are first order in peroxynitrite and zero order in metal complex, gave ΔV^⧧ = 10 ± 1 and 11 ± 1 cm³ mol^-1, respectively, at pH 7.2. The limiting yields of oxidized metal complex were found to decrease from 61 to 30% of the initially added peroxynitrite for Mo(CN)₈^3- and from 78 to 47% for Fe(CN)₆^3- when the pressure was increased from 0.1 to 140 MPa. The bimolecular reaction between CO₂ and ONOO^- was determined by monitoring the oxidation of Fe(CN)₆^4- by peroxynitrite in bicarbonate-containing 0.15 M phosphate, pH 7.2, for which ΔV^⧧ = −22 ± 4 cm³ mol^-1. The Fe(CN)₆^3- yield decreased by ∼20% upon increasing the pressure from atmospheric to 80 MPa. Oxidation of Ni(cyclam)²⁺ by peroxynitrite, which is first order in each reactant, was characterized by ΔV^⧧ = −7.1 ± 2 cm³ mol^-1, and the thermal activation parameters ΔH^⧧ = 4.2 ± 0.1 kcal mol^-1 and ΔS^⧧ = −24 ± 1 cal mol^-1 K^-1 in 0.15 M phosphate, pH 7.2. These results are discussed within the context of the radical cage hypothesis for peroxynitrite reactivity.