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

The mechanism of decomposition of peroxynitrite (OONO^-) in aqueous sodium phosphate buffer solution at neutral pH was investigated. The OONO^- was synthesized by directly reacting nitric oxide with superoxide anion at pH 13. The hypothesis was explored that OONO^-, after protonation at pH 7.0 to HOONO, decomposes into ¹O₂ 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 ¹O₂ and NO^- in high yields was observed. The ¹O₂ generated was trapped as the transannular peroxide (DPAO₂) of 9,10-diphenylanthracene (DPA) dissolved in carbon tetrachloride. The nitroxyl anion (NO^-) formed from HNO (pKa 4.5) was trapped as nitrosylhemoglobin (HbNO) in an aqueous methemoglobin (MetHb) solution. In the presence of 25 mM sodium bicarbonate, which is known to accelerate the rate of decomposition of OONO^-, the amount of singlet oxygen trapped was reduced by a factor of ≃2 whereas the yield of trapping of NO^- by methemoglobin remained unaffected. Because NO₃/^- is known to be the ultimate decomposition product of OONO^-, these results suggest that the nitrate anion is not formed by a direct isomerization of OONO^-, but by an indirect route originating from NO^-.