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 15NO−2/H2O2/HRP at pH 7.5, emission is observed in the 15N 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 15N CIDNP proves the formation of radical pairs. The nitration products are formed by recombination of 15NO•2 and N-acetyl-l-tyrosinyl radicals Tyrac•. The 15N NMR signals of 15NO−2 and of 15NO−3 show enhanced absorption. The 15N CIDNP effects are built up in radical pairs [15NO•2, Tyrac•]F formed by diffusive encounters of the radicals. Nuclear polarization is transferred into 15NO−2 and 15NO−3 by disproportionation reactions of 15NO•2 and by H abstraction of 15NO•2 from N-acetyl-l-tyrosine. If N-acetyl-l-tyrosine is not present, 15NO−3 does not show 15N 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 15N CIDNP during reaction or a 15N NMR signal after reaction. Emission also appears in nitrated tyrosine residues of bovine serum albumin during reaction of albumin with the 15NO−2/H2O2/HRP system. Analogous but smaller effects are observed during reaction of the nitrating system 15NO−2/HOCl with N-acetyl-l-tyrosine at pH 7.5, proving the radical character of this reaction, too. The 15N CIDNP effects are weak because of the low yield of nitration products.