Influence of the orientation of hydroxyl groups on the puckering modes of furanoid rings

Abstract

The influence of the orientations of HO-2 and HO-3 on the puckering of the ring in (2 S,3 R)-tetrahydrofuran-2,3-diol ( 1) and (3 R)-tetrahydrofuran-3-ol ( 2) has been investigated by consistent force-field methods. In the E or W regions of the pseudorotation cycle, there are no local energy minima. The local minima in the N and S regions are affected by different orientations of the hydroxyl groups, with up to 12.2 kJ.mol −1 for 1 and 6.9 kJ.mol −1 for 2. If statistical weights of different rotamers are used to calculate N/S equilibrium constants, both conformational states are equally populated. However, if the orientations of the hydroxyl groups are restricted to simulate behaviour of these molecular fragments in DNA double-helices, the equilibrium shifts towards N. The results indicate that, in the vapor state, a unique one-dimensional N-S energy-barrier does not exist and that there is a spectrum of barriers depending on the orientation of the hydroxyl groups.