Fe. Evans et al., NMR-INVISIBLE NUCLEOSIDES IN ADDUCTS FORMED FROM CARCINOGENIC NITROBENZO[A]PYRENES, Magnetic resonance in chemistry, 31(10), 1993, pp. 931-936
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.