[Calculations of stability of A and B forms of dA6.dT6 and dG6.dC6 duplexes by molecular mechanics method in an aqueous solution and approximation of the statistical model. Analysis of unusual stability of the dA6.dT6 B form].

The statistical model of the environment for estimating the influence of the aqueous electrolyte solution on the macromolecule has been developed. The energy of the water-fasteners between macromolecule atoms has been calculated. The calculations of A and B form dA6.dT6 and dG6.dC6 duplexes have been made. The unusual stability of B form in dA6.dT6 and their unstability in dG6.dC6 have been analysed. B form duplexes have optimal energy of Van-der-Waals interactions, but a more strained ribose-phosphate backbone, the latter being the reason of its high conformational lability. The aqueous solution stabilizes B form duplexes. The main types of sequence-dependent B form stabilizing interactions are: electrostatic interactions between phosphate groups and bases, the effect of the solvent molecular structure and energy of water fasteners between nucleic bases and ribose-phosphate backbone.