Co-Operative Conformational Properties of Nucleosides, Nucleotides and Nucleotidyl Units in Solution

Abstract

Nuclear magnetic resonance spectroscopy has made an important contribution to an understanding of the conformations and interactions of nucleic acids derivatives in solution (Davies 1978a). Methods are available for determining detailed descriptions of sugar ring conformations (Altona and Sundaralingam 1972, 1973) and the conformational properties of four out of the six bonds of the sugar-phosphate backbone utilising ¹H-¹H, ¹H-^{3 1}P or ^{1 3}C-^{3 1}P vicinal coupling constants (Davies, 1978a). Many nmr methods have been used to determine glycosidic bond conformations in a qualitative manner. No method provides an unequivocal description of the syn and anti conformational ranges or the equilibrium between them but methods based on relaxation measurements {n.O.e (Guéron et al, 1973) and T₁ measurements (Lüdemann et al, 1975; Chachaty et al, 1976)} and ³ J(¹³C, H1′) observations (Lemieux, 1973; Davies, 1976) offer the possibility of quantitative determinations of glycosidic bond conformations. Each single bond is taken to be flexible and appropriate nmr parameters are analysed in terms of the time-averaged equilibrium of contributions from a number of conformers for each bond. The available conformational regions are defined, as far as possible, by those observed in the solid state by X-ray crystallography (Sundaralingam, 1973; Jack et al, 1976).