Radical Mechanisms of the Decomposition of Peroxynitrite and the Peroxynitrite–CO2 Adduct and of Reactions with l-Tyrosine and Related Compounds as Studied by 15N Chemically Induced Dynamic Nuclear Polarization

Enhanced absorption is observed in the 15N NMR spectra of 15NO−3 during decomposition of peroxynitrite and the peroxynitrite–CO2 adduct at pH 5.25, indicating the formation of 15NO−3 in radical pairs [15NO•2, HO•] and [15NO•2, CO•−3]. During the reaction of peroxynitrite and the peroxynitrite–CO2 adduct with l-tyrosine, the 15N NMR signal of the nitration product 3-nitrotyrosine exhibits emission showing a radical pathway of its formation. The nuclear polarization is built up in radical pairs [15NO•2, tyr•] generated by free radical encounters of nitrogen dioxide and tyrosinyl radicals. The 15N NMR signal of 15NO−2 formed during reaction of peroxynitrite with l-tyrosine appears in emission. It is concluded that tyrosinyl radicals are generated by reaction of nitrogen dioxide with l-tyrosine. In contrast to this, 15NO−2 does not show 15N chemically induced dynamic nuclear polarization (CIDNP) during reaction of the peroxynitrite–CO2 adduct with l-tyrosine, indicating a different reaction mechanism, which is assumed to be a hydrogen transfer between CO•−3 and l-tyrosine. Emission is also observed in the 15N NMR signals of 2-nitro-4-fluorophenol, 3-nitro-4-hydroxyphenylacetic acid, 2-nitrophenol, and 4-nitrophenol during reaction of 4-fluorophenol, 4-hydroxyphenylacetic acid, and phenol with peroxynitrite and the peroxynitrite–CO2 adduct. 3-Nitro-4-hydroxyphenylacetic acid is also observed in emission during reaction of phenylacetic acid with peroxynitrite, but is not formed with the peroxynitrite–CO2 adduct. The magnitude of the 15N CIDNP effect during reaction of peroxynitrite with 4-fluorophenol and of the peroxynitrite–CO2 adduct with 4-fluorophenol and phenol is determined. It excludes the occurrence of nonradical reactions. Only weak emission signals are observed during the reaction of peroxynitrite with phenol in 15NO−2, 2-nitrophenol, and 4-nitrophenol. 2-Nitrophenol is only formed in traces, and 4-nitrophenol is only formed in higher yields. The latter might be generated in part via a nonradical pathway.