15N chemically induced dynamic nuclear polarization during reaction of N-acetyl-L-tyrosine with the nitrating systems nitrite/hydrogen peroxide/horseradish peroxidase and nitrite/hypochloric acid.

During reaction of N-acetyl-l-tyrosine with the nitrating system (15)NO(-)(2)/H(2)O(2)/HRP at pH 7.5, emission is observed in the (15)N NMR signals of 3-nitro-N-acetyl-l-tyrosine and the 1-nitrocyclohexa-2,5-dien-4-one derivative of N-acetyl-l-tyrosine. The occurrence of (15)N CIDNP proves the formation of radical pairs. The nitration products are formed by recombination of (15)NO(*)(2) and N-acetyl-l-tyrosinyl radicals Tyrac(*). The (15)N NMR signals of (15)NO(-)(2) and of (15)NO(-)(3) show enhanced absorption. The (15)N CIDNP effects are built up in radical pairs [(15)NO(*)(2), Tyrac(*)](F) formed by diffusive encounters of the radicals. Nuclear polarization is transferred into (15)NO(-)(2) and (15)NO(-)(3) by disproportionation reactions of (15)NO(*)(2) and by H abstraction of (15)NO(*)(2) from N-acetyl-l-tyrosine. If N-acetyl-l-tyrosine is not present, (15)NO(-)(3) does not show (15)N CIDNP, indicating that peroxynitrite is not formed as an intermediate during the reaction. Peroxynitrate is not formed either, which is indicated by the absence of (15)N CIDNP during reaction or a (15)N NMR signal after reaction. Emission also appears in nitrated tyrosine residues of bovine serum albumin during reaction of albumin with the (15)NO(-)(2)/H(2)O(2)/HRP system. Analogous but smaller effects are observed during reaction of the nitrating system (15)NO(-)(2)/HOCl with N-acetyl-l-tyrosine at pH 7.5, proving the radical character of this reaction, too. The (15)N CIDNP effects are weak because of the low yield of nitration products.