Direct evidence of singlet molecular oxygen generation from peroxynitrate, a decomposition product of peroxynitrite

The decomposition of peroxynitrite to nitrite and dioxygen at neutral pH follows complex kinetics, compared to its isomerization to nitrate at low pH. Decomposition may involve radicals or proceed by way of the classical peracid decomposition mechanism. Peroxynitrite (ONOOH/ONOO-) decomposition has been proposed to involve formation of peroxynitrate (O2NOOH/O2NOO-) at neutral pH (D. Gupta, B. Harish, R. Kissner and W. H. Koppenol, Dalton Trans., 2009, DOI: 10.1039/b905535e, see accompanying paper in this issue). Peroxynitrate is unstable and decomposes to nitrite and dioxygen. This study aimed to investigate whether O2NOO- formed upon ONOOH/ONOO- decomposition generates singlet molecular oxygen [O-2 ((1)Delta(g))]. As unequivocally revealed by the measurement of monomol light emission in the near infrared region at 1270 nm and by chemical trapping experiments, the decomposition of ONOO- or O2NOOH at neutral to alkaline pH generates O-2 ((1)Delta(g)) at a yield of ca. 1% and 2-10%, respectively. Characteristic light emission, corresponding to O-2 ((1)Delta(g)) monomolecular decay was observed for ONOO- and for O2NOOH prepared by reaction of H2O2 with NO2BF4 and of H2O2 with NO2- in HClO4. The generation of O-2 ((1)Delta(g)) from ONOO- increased in a concentration-dependent manner in the range of 0.1-2.5 mM and was dependent on pH, giving a sigmoid pro. le with an apparent pK(a) around pD 8.1 (pH 7.7). Taken together, our results clearly identify the generation of O-2 ((1)Delta(g)) from peroxynitrate [(ONOO-)-N-2 -> NO2- + O-2 ((1)Delta(g))] generated from peroxynitrite and also from the reactions of H2O2 with either NO2BF4 or NO2- in acidic media.