Determination of the backbone torsion angle epsilon in nucleic acids.

The multiplet structure of cross peaks in double-quantum-filtered COSY NMR spectra is analysed for those resonances that include passive heteronuclear couplings. Interestingly, the cross peak involving the sugar-ring protons H2' and H3' in nucleic acids display an E. COSY-type appearance exclusively when the backbone torsion angle epsilon (C4'-C3'-O3'-P) adopts a gauche(-) conformation. This observation allows an unambiguous analysis of the conformation around epsilon, without the knowledge of 3JCP coupling constants.