Nitration and Nitrosation by Peroxynitrite:  Role of CO2 and Evidence for Common Intermediates

Peroxynitrite reacts with morpholine (MorH) to give both N-nitromorpholine (Mor-NO2) and N-nitrosomorpholine (Mor-NO). The pH profile of Mor-NO2 formation shows a bell-shaped curve with a maximum yield of 1.6 mol % (relative to peroxynitrite) centered around pH 9.0. Yields of Mor-NO, on the other hand, increase until the pH reaches 10.0 and then reach a plateau; the maximum yield of Mor-NO is 12 mol % relative to peroxynitrite. Both the nitration and nitrosation of MorH are catalyzed by low levels of CO2; however, excess CO2 dramatically reduces the yields of Mor-NO but not Mor-NO2, and the combined yields of Mor-NO and Mor-NO2 are about the same under conditions of high and low concentrations of CO2. These data indicate that both nitration and nitrosation by peroxynitrite are free radical processes. The morpholine radical (Mor•), formed from the reactions of carbonate and/or hydroxyl radicals with MorH, reacts with either •NO or •NO2 and serves as a common precursor for Mor-NO and Mor-NO2.