Charge transfer in nucleic acids

A discussion is presented on the analysis of wave functions and energies obtained for fragments of B-DNA and Z-DNA. The fragments of B-DNA considered consist of two sugar, one phosphate, and one base unit (A, C, G or T); a fifth and larger fragment consists of three sugar, two phosphate, and two base (G and C) units. For Z-DNA one fragment equivalent in size to the largest one above was considered. The wave function computations (requiring a large basis set with about 1000 primitive Gaussian functions) have been obtained in the standard accuracy of SCF computations. The effects of the computed electronic charge transfer from the sugar units to the phosphates and to the bases are analyzed in the situation of Li+, Na+, and K+ interacting with B-DNA and Z-DNA. The charge transfer brings about a stabilization in the base positive ion interactions of about 10 kcal/mol for B-DNA and about 8 kcal/mol for Z-DNA. This example proves that reliable computations—even of qualitative nature—dealing with polynucleotides cannot ignore charge transfer. Problems related to different models for the determination of unit-to-unit interactions in nucleic acids are discussed, and an estimate of the relative stability of B-DNA and Z-DNA is presented. The results obtained for relative stabilities are given as preliminary, requiring additional testing, which at present is in progress.