[Effect of double helix conformation transitions on DNA hypochromism]

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.