15N CIDNP investigations of the peroxynitric acid nitration of L-tyrosine and of related compounds.
Peroxynitric acid (O2NOOH) nitrates L-tyrosine and related compounds at pH 2-5. During reaction with O2(15)NOOH 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 O2(15)NOOH 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, O2(15)NOOH reacts with glutathione and cysteine without the appearance of 15N CIDNP, indicating a direct oxidation without participation of free radicals.
Organische Chemie, Universität Dortmund, Otto-Hahn-Strasse 6, D-44221 Dortmund, Germany. lehnig@chemie.uni-dortmund.de
Manfred
M
Lehnig
Michael
M
Kirsch
D001205:Ascorbic Acid / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D005978:Glutathione / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D009566:Nitrates / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D009587:Nitrogen Isotopes; D010636:Phenols / Q000737:chemistry; D012997:Solvents; D013696:Temperature; D014443:Tyrosine / Q000737:chemistry*
D009566:Nitrates; D009587:Nitrogen Isotopes; D010636:Phenols; D012997:Solvents; C039642:peroxynitric acid; C106880:4-fluorophenol; D014443:Tyrosine; D005978:Glutathione; D003545:Cysteine; D009584:Nitrogen; D001205:Ascorbic Acid