Gibbs energy of formation of peroxynitrate-order restored

In a recent publication [Nauser et al. (2001) Chem. Res. Toxicol. 14, 248-350], the authors estimated a value of 14 +/- 3 kcal/mol for the standard Gibbs energy of formation of ONOO- and argued that the experimental value of 16.6 kcal/mol [Merenyi, G., and Lind, J. (1998) Chem. Res. Toxicol. 11, 243-246] is in error. The lower value would suggest that the yield of free radicals during decomposition of ONOOH into nitrate is negligibly low, i.e., less than 0.5%, though within the large error limit given, the radical yield might vary between 0.003% and ca. 80%. The experimental value of 16.6 +/- 0.4 kcal/mol was based on the determination of the rate constant of the forward reaction in the equilibrium ONOO- reversible arrow (NO)-N-. and O-2(.-) by use of C(NO2)(4), an efficient scavenger of O-2(.-) which yields C(NO2)(3)(-). Nauser et al. reported that addition of.NO has no significant effect on the rate of formation of C(N02)3-, and therefore the formation of C(No-2)(3-) is due to a process other then reduction of C(NO2)(4) by O-2 (.-) In addition, they argued that Cu(II) nitrilotriacetate enhances the rate of peroxynitrite decomposition at pH 9.3 without reduction of Cu(II). In the present paper, we show that the formation of C(N02)3- due to the presence peroxynitrite is completely blocked upon addition of . NO, Furthermore, the acceleration of the rate of peroxynitrite decomposition at pH 9 in the presence of catalytic concentrations of SOD ([ONOO-]/[SOD] > 30) results in the same rate constant as that obtained in the presence of C(NO2)4. These results can only be rationalized by assuming that ONOO- homolyses into (NO)-N-. and O-2(.-) With k = 0.02 S-1 at 25 degreesC. Thus, the critical experiments suggested by Nauser et al. fully support the currently accepted thermodynamics as well as the mode of decomposition of the ONOOH/ONOO- system.