Conformational studies of some 2':3'-cyclic mononucleotides in solution by different nuclear-magnetic-resonance methods.

The conformations of the 2':3'-cyclic mononucleotides of adenosine and cytidine in deuterium oxide has been studied at pH 2.3, using lanthanide ions as paramagnetic nuclear magnetic resonance (NMR) probes. It was not possible to find any single conformation for these molecules which accounts for the observed shift and relaxation data. This situation is in agreement with the interpretation of vicinal 1H-1H and 1H-31P coupling constants, which indicate that the ribofuranose and cyclic phosphate rings are in rapid equilibrium between different puckered forms. The interpretation of the lanthanide data in terms of an equilibrium between different conformations give average rotamer populations in good agreement with the coupling constant analysis. The conformations of these systems in aqueous solutions were found to be more flexible than in the solid state, where rigid planar ribofuranose rings have been observed. Adenosine 2':3'-monophosphate differs from cytidine 2':3'-monophosphate at the glycosidic link.