Extreme Conformational Flexibilitv of the Furanose

Consistent force field caiculations have been used to calculate the variation of energy along the pseudorotationai paths of ribose and deoxyribose rings. This energy changes by only 10.5 kcalimol as the ring pucker changes from C3’-endo to C2’-endo. Therefore, the torsion angle 4’ of the sugar-phosphate backbone of nucleic acids is more flexible than a normal C-C single-bond torsion angle and cannot be frozen at a value corresponding to either of these puckers when other torsion angles are varied. An analytical expression for the variation of the ring energy and conformation as a function of $’ is given to enable this extreme flexibility to be inciuded in future calculations of nucieotide conformations. The finding that the furanose ring is so easily deformed has far-reaching implications about the structure of polynucleotides and the rigidity of DNA. The furanose ring occupies a central pos i t ion in the chemihave chosen to keep the furanose ring fixed at one or two cal s tructure of nucleic acids by l inking phosphate groups into conformations but have al lowed the single bond torsion angles a backbone and providing attachment points for the bases. Any in the backbone to vary cont inuously. Rigidly f ixing the conchanges of conformation of this ring couid have a profound formation of a five-membered ring would seem too limiting effect on the possible conformation of DNA and RNA molewhen one considers that cyciopentane can freely change the cules. Previous calculations on nucleotide conformationsl-8 ring pucker along a path known as pseudorotation.9-13 On the