Interpretation of cytidine proton chemical shifts and J coupling constants calculated by DFT

With a DFT approach [B3LYP/6-31G(d,p)] the geometry of the ribonucleoside cytidine was optimized and NMR parameters (proton chemical shifts and J couplings) were calculated. Calculations of proton chemical shifts with varied base orientations of a North sugar conformer revealed a through-space effect between the OH2′ proton and the carbonyl oxygen of the base. The 3JH1′C6 couplings were found to be similar for trans and gauche orientation of proton and carbon. This was rationalized by a NBO analysis of interaction energies between donor and acceptor natural bond orbitals. The different 3JH1′C6 coupling constants upon rotation of the base could be correlated to the charge delocalization along the coupling pathway which was independent of the ribose conformation. The 1JH6C6 and 1JH1′C1′ coupling constants seem to be less dominated by the hybridization of the corresponding carbon atoms.