Conformational structure variations in DNA: a PCILO study.
Perturbative configuration interaction using localized orbital (PCILO) computations have been carried out on conformational preference of d(GpC) and d(CpG) units of Z-II DNA. By keeping the sugar pucker fixed in the crystallographic conformation, PCILO energies have been calculated as a function of torsion angles alpha and zeta around two P-O ester bonds with preselected values of beta, gamma and epsilon. The results indicate that the d(GpC) unit is energetically more stable than the d(CpG) unit and prefers a g+ t conformation in (alpha-zeta) hyper-space observed in the crystal structure. The d(CpG) unit, on the other hand, shows intrinsic preference for g-g- conformation with other torsion angles close to DNA-B structure. The possibility of d(GpC) unit responsible for the stability of Z-helix has been discussed.