Lehnig Manfred, Kirsch Michael
Organische Chemie, Universität Dortmund, Otto-Hahn-Strasse 6, D-44221 Dortmund, Germany.
Org Biomol Chem. 2006 Feb 21;4(4):721-9. doi: 10.1039/b515856g. Epub 2006 Jan 20.
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.
过氧亚硝酸（O2NOOH）在pH值为2至5的条件下使L-酪氨酸及相关化合物发生硝化反应。在15N核磁共振光谱仪的探头中与O2(15)NOOH反应时，L-酪氨酸、N-乙酰-L-酪氨酸、4-氟苯酚和4-甲氧基苯乙酸硝化产物的核磁共振信号以发射形式出现，表明是通过自由基进行硝化。核极化在由15NO2与苯氧基型自由基PhO或芳族自由基阳离子ArH*+扩散相遇形成的自由基对[15NO2*，PhO*]F或[15NO2*，ArH*+]F中形成。对N-乙酰-L-酪氨酸和4-氟苯酚进行的定量15N化学诱导动态核极化研究表明，自由基依赖性硝化是唯一的反应途径。在硝化反应过程中，15NO3-的15N核磁共振信号也以发射形式出现。这是由O2(15)NOOH与作为反应中间体形成的H15NO2之间的电子转移产生的自由基对[15NO2*，15NO3*]S中的单重态-三重态跃迁所解释的。在过氧亚硝酸与抗坏血酸反应过程中，在15NO3-的15N核磁共振信号中再次观察到15N化学诱导动态核极化，表明抗坏血酸被自由基氧化。与此相反，O2(15)NOOH与谷胱甘肽和半胱氨酸反应时没有出现15N化学诱导动态核极化，表明是直接氧化，没有自由基参与。
