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)84- and Fe(CN)64-, 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)83- and from 78 to 47% for Fe(CN)63- 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)64- by peroxynitrite in bicarbonate-containing 0.15 M phosphate, pH 7.2, for which ΔV = −22 ± 4 cm3 mol-1. The Fe(CN)63- 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.