Carbon-13 NMR in conformational analysis of nucleic acid fragments. 3. The magnitude of torsional angle epsilon in d(TpA) from CCOP and HCOP NMR coupling constants.

Carbon-13 NMR spectra of the deoxyribonucleotide d(TpA), 3',5'-cyclic AMP and 3',5'-cyclic dAMP were measured. It is shown that the different substitution of C2' in deoxyribonucleotides versus ribonucleotides does not affect the vicinal C2'-C3'-O3'-P coupling to a measurable extent. Therefore, the same set of Karplus parameters may be used for the C2'-C3-O3'-P couplings in ribonucleotides and in deoxyribonucleotides. Vicinal carbon-phosphorus and proton-phosphorus coupling constants are used to calculate the magnitude of the torsion angle epsilon (C4'-C3'-O3'-P), which amounts to 195(0) in the trans conformer and to 261(0) in the gauche(-) conformer.