A Dynamic Correlation between Ribose Conformation and Glycosyl Torsion Angle of Dissolved Xanthosine Studied by Continuous‐Wave‐Mode and Pulsed Nuclear‐Magnetic‐Resonance Methods

The solution conformation of xanthosine in liquid N2H3 has been determined by nuclear magnetic resonance. The correlation times for rotational diffusion were derived from 13C relaxation measurements. A Karplus analysis of the high‐resolution spectra of the ribose moiety yields the conformation of the sugar from the H–H coupling constants. The mole fraction of the sugar in the N conformation is 0.4 at room temperature and increases slightly as the temperature decreases. From nuclear Overhauser enhancement studies, and T1 measurements of the various protons it is deduced that the N ribose is correlated with an anti conformation of the base (Y∼ 210°) while the S form of the sugar is coupled to a glycosyl torsion angle in the syn range (Y ∼ 50°–90°).