Peroxynitrite (sometimes called peroxonitrite)'s the anion with the formula ONOO−. It's an unstable "valence isomer" of nitrate, NO3−, which has the same formula but a different structure. Although peroxynitrous acid's highly reactive, its conjugate base peroxynitrite's stable in basic solution. It's prepared by the reaction of hydrogen peroxide with nitrite: » H2O2 + NO2− → ONOO− + H2O Peroxynitrite's an oxidant and nitrating agent. Because of its oxidizing propert… (More on Peroxynitrite)