Carbonate radical ion is the only observable intermediate in the reaction of peroxynitrite with CO2

The reaction of ONOO- with CO2 at alkaline pH was recently reported to form a transient absorption with a maximum at 640 nm and a half-life of ca. 4 m s at 10 degreesC [Meli et al. (1999) Helv. Chim. Acta 82, 722-725]. This tr ansient absorption was hardly affected by the presence of (NO)-N-., and the refore was attributed to the adduct ONOOC(O)O-. This conclusion contradicts all current experimental results as it suggests that the decomposition of this adduct via homolysis of the O-O bond into CO3.- and . NO2 is a minor p athway. In the present work the observations of Meli et al. will be shown t o be artifacts resulting from light coming from the UV region. When these e xperiments are carried out in the presence of appropriate cutoff filters, t he only observable intermediate formed in the reaction of ONOO- with CO2 at alkaline pH is the carbonate radical ion with a maximum at 600 nm. This tr ansient absorption is not observed in the presence of (NO)-N-. or ferrocyan ide. In the latter case ferricyanide is formed, and its yield was determine d to be 66 +/-2% of the initial concentration of peroxynitrite. The reactio n of ONOO- with 16 mM CO2 with and without ferrocyanide was also studied at pH 5.6-7.7 in the presence of 0.1 M phosphate, where both the initial pH a nd [CO2] remain constant. Under these conditions the rate constant of the d ecay of peroxynitrite was found to be identical to that of the formation of ferricyanide, indicating that ONOOC(O)(-) does not accumulate. These resul ts confirm our earlier observations, i.e., the reaction of peroxynitrite wi th excess CO2 takes place via the formation of about 33% CO3.- and (NO2)-N- . radicals in the bulk of the solution.