The Solution Conformational Preferences of the Sugar and Sugar Phosphate Constituents of RNA and DNA

The analysis of the proton NMR spectra of stereospecifically deuterated (S)-tetrahydrofuranmethanol, methyl β-D-ribofuranoside, methyl β-D-2-deoxyribofuranoside, and the 5-phosphates of the sugar acetals leads to several important and previously unrecognized observations about the determinants of the solution conformations of nucleosides and nucleotides: (1) in the unsubstituted sugars and their phosphates, the preferred furanoside ring conformation is the 3'-endo pucker and not the 2'-endo pucker observed in most nucleosides and nucleotides; (2) in the unsubstituted sugars and their phosphates, the gauche-gauche rotamer about the C4-C5 bond does not predominate as is found in the nucleosides and nucleotides but rather the two rotamers in which the vicinal C-0 bonds are gauche to one another (gauche-gauche and gauche-trans) are about equally favored; (3) the observed predominant occurrence of the gauche-gauche rotamer about the C₄-C_5' bond in nucleosides and nucleotides is presumably the result of an interaction between the substituent on C_5' and the heterocyclic base, which is made more favorable by a shift in the furanoside ring conformation; and (4) the rotameric distribution about the C₅-O₅ bonds in the sugar acetal phosphates is the same as that observed in nucleosides and nucleotides. These observations suggest that the nature of the furanoside ring conformation and the rotameric distributions about the C₄-C_5' and C₅O_5' bonds may not be as intimately interdependent as previously believed.