Molecular orbital calculations on the conformation of nucleic acids and their constituents: III. Backbone structure of di- and polynucleotides

Abstract

Molecular orbital calculations, using the all-valence electrons method PCILO (Perturbative Configuration Interaction using Localized Orbitals), carried out previously for the glycosyl torsion angle χ cn have now been extended to the principal torsion angles of the backbone structure of polynucleotides, Ψ, Ф, ω, ω′ and Ф′ (in the notations of Sundaralingam). Conformational energy maps have been constructed for the four combinations of two consecutive angles: (ω′-ω), (Ф′-ω′), (ω-Ф) and (Ф-Ψ) with appropriate values adopted for the torsion of the angles not involved in the particular map under construction. The results of the PCILO computations are compared with data available from X-ray crystallography of related compounds and with similar calculations carried out by the empirical methods and by a simple quantum mechanical procedure (Extended Hückel Theory).