Conformational analysis of the 2′‐deoxyribofuranose ring from proton–proton coupling constants: Analysis of a nucleoside‐carcinogen adduct formed from 2‐acetylaminofluorene utilizing a three‐state model

The conformation of the sugar moiety of 8‐(N‐fluoren‐2‐ylamino)‐2′‐deoxyguanosine in solution has been examined as a function of temperature by 1H‐nmr spectroscopy. Analysis of coupling constants shows that lowering the temperature to −50°C in methanol shifts the conformational equilibrium of the sugar ring resulting in a C2′‐endo conformation at a mole fraction of 0.97. The computed phase angle of pseudorotation and amplitude of pucker are 154° and 36°, respectively, with very little discrepancy between the five calculated coupling constants and coupling constants extrapolated from the temperature profiles. A computer program has been written enabling a three‐state best‐fit analysis. The three‐state analysis indicates an equilibrium between C2′‐endo, C3′‐endo, and 04′‐endo conformations. In aqueous solution, the computed mole fraction of the 04′‐endo form is 0.18 at 30°C. The conformation associated with the sugar ring and the C4′C5′ bond is compared to that of 2′‐deoxyguanosine.