15N Chemically Induced Dynamic Nuclear Polarization (15N-CIDNP) Investigations of the Peroxynitrite Decay and Nitration of N-Acetyl-L-tyrosine

During the decay of (15N)peroxynitrite (O15NOO−) in the presence of N‐acetyl‐L‐tyrosine (Tyrac) in neutral solution and at 268 K, the 15N‐NMR signals of 15NO$\rm{_2^ - }$ and 15NO$\rm{_3^ - }$ show emission (E) and enhanced absorption (A) as it has already been observed by Butler and co‐workers in the presence of L‐tyrosine (Tyr). The effects are built up in radical pairs [CO$\rm{_3^{\bullet - } }$, 15NO$\rm{_2^\bullet }$]S formed by OO bond scission of the (15N)peroxynitriteCO2 adduct (O15NOOCO$\rm{_2^ - }$). In the absence of Tyrac and Tyr, the peroxynitrite decay rate is enhanced, and 15N‐CIDNP does not occur. This is explained by a chain reaction during the peroxynitrite decay involving N2O3 and radicals NO. and NO$\rm{_2^\bullet }$. The interpretation is supported by 15N‐CIDNP observed with (15N)peroxynitrite generated in situ during reaction of H2O2 with N‐acetyl‐N‐(15N)nitroso‐dl‐tryptophan ((15N)NANT) at 298 K and pH 7.5. In the presence of Na15NO2 at pH 7.5 and in acidic solution, 15N‐CIDNP appears in the nitration products of Tyrac, 1‐(15N)nitro‐N‐acetyl‐L‐tyrosine (1‐15NO2‐Tyrac) and 3‐(15N)nitro‐N‐acetyl‐L‐tyrosine (3‐15NO2‐Tyrac). The effects are built up in radical pairs [Tyrac., 15NO$\rm{_2^\bullet }$]F formed by encounters of independently generated radicals Tyrac. and 15NO$\rm{_2^\bullet }$. Quantitative 15N‐CIDNP studies show that nitrogen dioxide dependent reactions are the main if not the only pathways for yielding both nitrate and nitrated products.