Peroxynitric acid (O2NOOH) nitrates l-tyrosine and related compounds at pH 2â 5. During reaction with O2¹⁵NOOH in the probe of a ¹⁵N NMR spectrometer, the NMR signals of the nitration products of l-tyrosine, N-acetyl-l-tyrosine, 4-fluorophenol and 4-methoxyphenylacetic acid appear in emission indicating a nitration via free radicals. Nuclear polarizations are built up in radical pairs [¹⁵NO2.rad;, PhO.rad;]^F or [¹⁵NO2.rad;, ArH.rad;⁺]^F formed by diffusive encounters of ¹⁵NO2.rad; with phenoxyl-type radicals PhO.rad; or with aromatic radical cations ArH.rad;⁺. Quantitative ¹⁵N CIDNP investigations with N-acetyl-l-tyrosine and 4-fluorophenol show that the radical-dependent nitration is the only reaction pathway. During the nitration reaction, the ¹⁵N NMR signal of ¹⁵NO3^â also appears in emission. This is explained by singletâ triplet transitions in radical pairs [¹⁵NO2.rad;, ¹⁵NO3.rad;]^S generated by electron transfer between O2¹⁵NOOH and H¹⁵NO2 formed as a reaction intermediate. During reaction of peroxynitric acid with ascorbic acid, ¹⁵N CIDNP is again observed in the ¹⁵N NMR signal of ¹⁵NO3^â showing that ascorbic acid is oxidized by free radicals. In contrast to this, O2¹⁵NOOH reacts with glutathione and cysteine without the appearance of ¹⁵N CIDNP, indicating a direct oxidation without participation of free radicals.