Peroxynitrite does not decompose to singlet oxygen ((1)Delta O-g(2)) and nitroxyl (NO-)

According to Khan et al. [Khan, A. U., Kovacic, D., Kolbanovskiy, A., Desai , M., Frenkel, K. & Geacintov, N. E. (2000) Proc. Natl. Acad. Sci. USA 97, 2984-2989], peroxynitrite (ONOO-) decomposes after protonation to singlet o xygen ((1)Delta(g)O(2)) and singlet oxonitrate (nitroxyl, (NO-)-N-1) in hig h yield. They claimed to have observed nitrosyl hemoglobin from the reactio n of NO- with methemoglobin; however, contamination with hydrogen peroxide gave rise to ferryl hemoglobin, the spectrum of which was mistakenly assign ed to nitrosyl hemoglobin. We have carried out UV-visible and EPR experimen ts with methemoglobin and hydrogen peroxide-free peroxynitrite and find tha t no NO- is formed. With this peroxynitrite preparation, no light emission from singlet oxygen at 1270 nm is observed, nor is singlet oxygen chemicall y trapped: however, singlet oxygen was trapped when hydrogen peroxide was a lso present, as previously described [Di Mascio, P., Bechara, E. J. H., Med eiros, M. H. G., Briviba, K. & Sies, H. (1994) FEBS Lett 355. 287-289]. Qua ntum mechanical and thermodynamic calculations show that formation of the p ostulated intermediate, a cyclic form of peroxynitrous acid (trioxazetidine ), and the products (NO-)-N-1 and (1)Delta(g)O(2) requires Gibbs energies o f ca. +415 kJ.mol(-1) and ca. +180 kJ.mol(-1), respectively. Our results sh ow that the results of Khan et al. are best explained by interference from contaminating hydrogen peroxide left from the synthesis of peroxynitrite.