The decomposition of peroxynitrite to nitroxyl onion (NO-) and singlet oxygen in aqueous solution

The mechanism of decomposition of peroxynitrite (OONO-) in aqueous sodium p hosphate buffer solution at neutral pH was investigated. The OONO- was synt hesized by directly reacting nitric oxide with superoxide anion at pH 13. T he hypothesis was explored that OONO-, after protonation at pH 7.0 to HOONO , decomposes into O-1(2) and HNO according to a spin-conserved unimolecular mechanism. Small aliquots of the concentrated alkaline OONO- solution were added to a buffer solution (final pH 7.0-7.2), and the formation of O-1(2) and NO- in high yields was observed. The O-1(2) generated was trapped as t he transannular peroxide (DPAO(2)) of 9,10-diphenylanthracene (DPA) dissolv ed in carbon tetrachloride. The nitroxyl anion (NO-) formed from HNO (pKa 4 .5) was trapped as nitrosylhemoglobin (HbNO) in an aqueous methemoglobin (M etHb) solution. In the presence of 25 mM sodium bicarbonate, which is known to accelerate the rate of decomposition of OONO-, the amount of singlet ox ygen trapped was reduced by a factor of approximate to 2 whereas the yield of trapping of NO by methemoglobin remained unaffected. Because NO3- is kno wn to be the ultimate decomposition product of OONO-, these results suggest that the nitrate anion is not formed by a direct isomerization of OONO-, b ut by an indirect route originating from NO-.