15N CIDNP investigations of the peroxynitric acid nitration of l-tyrosine and of related compounds

Abstract

Peroxynitric acid (O₂NOOH) nitrates L-tyrosine and related compounds at pH 2–5. 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˙, PhO˙]^F or [¹⁵NO₂˙, ArH˙⁺]^F formed by diffusive encounters of ¹⁵NO₂˙ with phenoxyl-type radicals PhO˙ or with aromatic radical cations ArH˙⁺. 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⁻ also appears in emission. This is explained by singlet–triplet transitions in radical pairs [¹⁵NO₂˙, ¹⁵NO₃˙]^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⁻ 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.