Flexibility of 3',5' deoxyribonucleoside diphosphates.

In 3',5' deoxyribonucleoside diphosphates, in addition to the nature of the base and the sugar puckering, there are six single bond rotations. However, from the analysis of crystal structure data on the constituents of nucleic acids, only three rotational angles, that are about glycosyl bond, about C4'-C5' and about C3'-O3' bonds, are flexible. For a given sugar puckering and a base, potential energy calculations using non-bonded, electrostatic and torsional functions were carried out by varying the three torsion angles. The energies are represented as isopotential energy surfaces. Since the availability of the real-time color graphics, it is possible to analyse these isopotential energy surfaces. The calculations were carried out for C3' exo and C3' endo puckerings for deoxyribose and also for four bases. These calculations throw more light not only on the allowed regions for the three rotational angles but also on the relationships among them. The dependence of base and the puckering of the sugar on these rotational angles and thereby the flexibility of the 3',5' deoxyribonucleoside diphosphates is discussed. From our calculations, it is now possible to follow minimum energy path for interconversion among various conformers.