NMR-INVISIBLE NUCLEOSIDES IN ADDUCTS FORMED FROM CARCINOGENIC NITROBENZO[A]PYRENES

Anomalous H-1 NMR spectra have been obtained for two related carcinogen-nucleoside adducts in which a 1- or 3-aminobenzo [a] pyrene ring is bound from its C-6 position to the N-2 position of 2'-deoxyguanosine. NOESY and COSY measurements at low temperature in methanol-d4 revealed chemical exchange between at least two forms. Large chemical shift differences exist between subspectra for all corresponding protons of the nucleoside moiety, whereas all such chemical shift differences are small for the protons of the benzo[a]pyrene moiety. Thus, at intermediate rates of exchange, the nucleoside resonances are broadened beyond recognition, whereas the aminobenzo[a]pyrene resonances appear normal. This is linked with the large anisotropic ring-current field from the carcinogen ring system, and restricted internal rotation at the site of attachment of the carcinogen to the nucleic acid base. The possibility of an absence of resonances from a nucleoside moiety can complicate structure elucidation of unknown carcinogen-nucleoside adducts and related compounds, especially in trace analysis.