[The effect of electrostatic interactions on the conformation of the sugar-phosphate backbone in DNA]. / Vliianie élektrostaticheskikh vzaimodeistvii na konformatsiiu sakharo-fosfatnogo ostova v DNK.

The influence of sugar ring flexibility in DNA on the mechanism of the B<-->A conformational transition is studied. The dipole moment of the deoxyribose as a function of its puckered states is calculated by the quantum-mechanical method using the MINDO/3 approximation. The interaction of the sugar dipole with the neighbour molecular groups in polynucleotide chain is estimated. The sugar dipole interaction witch phosphate groups and counterions is shown to be strong and capable to deform the pseudorotation potential of deoxyribose. The effective pseudorotation potential of sugar ring in the B- and A-helices is obtained. The results are used to explain the behaviour of Raman bands in the region of sugar-phosphate vibrations. The mechanism of the effect of electrostatic forces on the sugar-phosphate backbone conformation which is essential for the B<-->A and other structure transitions is offered.

[Bend of the furanose ring of the double-helical DNA]. / Izuchenie izgiba furanoznogo kol'tsa dvuspiral'noi DNK.

The interpretation of conformationally dependent double-helical DNA Raman bands, which stipulates their appearance with the furanose ring-puckering vibration is suggested. Quantum-mechanical calculations of the furanose ring-puckering vibration occurring by a pseudorotation pathway are carried out. Vibrational states eigen values and eigen functions are calculated for a double-well pseudorotation potential of the furanose. The obtained value of the pseudorotation barrier 4.2-3.6 kcal/mol proves to be in a good agreement with other theoretical and experimental estimates of the barrier.

[Mechanism of hydrogen exchange in DNA]. / O mekhanizme vodorodnogo obmena v DNK.

The hydrogen exchange mechanism of DNA bases imino- and amino protons is explained according to a suggested exchangeable state model. This state can be formed as a result of significant structural fluctuations which are not complete base pair openings. The exchangeable state model represents the hydrogen-bonded complex of two nucleotide bases and one water molecule made up dynamically. Quantum-mechanical calculations for the values of energy as the function of H-bond proton positions in the complex were carried out and the two-proton positions in the complex were carried out and the two-proton transfer is shown to be possible. The suggested model allows to give a noncontradictary interpretation of the hydrogen exchange data in double-helical polynucleotides.

[Effect of double helix conformation transitions on DNA hypochromism]. / O vliianii konformatsionnykh izmenenii dvoinoi spiralia na gipokhromism DNK.

The sensitivity of the first UV-absorption band hypochromism of DNA to changes in double helix form has been investigated theoretically. The hypochromism of DNA is studied within the perturbation theory and the nearest neighbour approximation. The heterogeneity of DNA is taken into account by introducing the nearest neighbour frequencies. The electronic characteristics of monomers (nucleotide bases) are calculated by the quantum mechanical PPP CI method. The hypochromism of A-, B-, C-, D- and T-forms of DNA is calculated. A slight dependence of the hypochromism on double helix conformation is obtained. This result is in agreement with experimental data. The reasons for the observed non-sensitivity of the DNA UV-absorption spectrum to conformational transitions within a two-stranded structure are explained. It is shown that the theory leads to an agreement with experiments because a large number of electronic transitions in nucleotide bases and the heterogeneity of DNA are taken into account.

[Theoretical investigation on the hypochromism of dinucleoside phosphates]. / Teoreticheskoe izuchenie gipokhromizma dinukeozidmonofosfatov

The hypochromism of stacked dimers of the nucleotide bases taken as models of the dinucleoside phosphates and dinucleotides was studied with the use of the configuration interaction and pertubation theory methods. General expression for the hypochromism of the polynucleotides is given in the first order perturbation theory with three different ways for approximation of the matrix elements of the perturbation operator. This expression was used for calculation of the dimer hypochromism in terms of theoretically calculated monomer characteristics. Dependence of the hypochromism on the dimer conformation was investigated. The results obtained so far demonstrate that it is important to take into account the electronic transitions in the vacuum UV region. This approach will enable one to elucidate the contribution of neighbouring bases into the DNA hypochromism.