[Structural and energetic properties of the four configurations of the A.T and G.C DNA base pairs].

Using the methods of non-empirical quantum chemistry at the MP2/6-311++G(2df,pd)// B3LYP/6-311++G(d,p) level of theory it was established for the first time, that Hoogsteen, reverse Hoogsteen, Watson-Crick and reverse Watson-Crick configurations of the A.T and G.C DNA base pairs are isoelectronic and isomorphic structures with similar dynamic properties. Based on these results, non-ionisation mechanism of the Hoogsteen <"breathing" of the G*.C* DNA base pair, namely transformation of the tautomerised (Lowdin's) G-C base pair with Watson-Crick geometry into the Hoogsteen electroneutral G*.C* H base pair stabilized by the three O6H...N4, N3H...N7 and C8H...02 H-bonds, was postulated. It is suggested that such scenario activates only in those cases, when DNA is not located in aqueous solution, but works together with proteins and cytosine protonation at the N3 atom is precluded.