Molecular orbital theory of the hydrogen bond : XXXII. The effect of H and Li association on the A---T and G---C pairs

Ab initio molecular orbital calculations have been performed to determine the structures and stabilization energies of the A---T and G---C base pairs and their complexes with H⁺ and Li⁺, H⁺ and Li⁺ association stabilizes the A---T pair except for Li⁺ association at O₄ in thymine. Protonation of thymine stabilizes the A---T pair to a greater extent than protonation of adenine. The association of H⁺ and Li⁺ with guanine stabilizes the G---C pair, but protonation of cytosine destabilizes G---C. Changes in the structures of the hydrogen bonds in the A---T and G---C pairs reflect changes in hydrogen bond strengths.