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

Anomalous H-1 NMR spectra have been obtained for two related carcinoge n-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 di fferences exist between subspectra for all corresponding protons of th e nucleoside moiety, whereas all such chemical shift differences are s mall for the protons of the benzo[a]pyrene moiety. Thus, at intermedia te rates of exchange, the nucleoside resonances are broadened beyond r ecognition, 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 o f attachment of the carcinogen to the nucleic acid base. The possibili ty of an absence of resonances from a nucleoside moiety can complicate structure elucidation of unknown carcinogen-nucleoside adducts and re lated compounds, especially in trace analysis.