[Experimental and theoretical studies of the formation of DNA complexes with biologically active ligands containing chromophore groups, depending on the ionic strength of the solution].

The binding of two ligands--dyes pyronin G (PG) and methylene blue (MB)--to DNA depending on the Na+ concentration was studied spectrophotometrically in the visible and UV ranges. The theoretical model taking into account the binding of a dye to DNA by means of aggregation and intercalation on two independent lattices was used for computation of the complexation parameters. The results of optimization of spectrophotometric concentration dependences show that the ligand occupies two phosphates in the process of aggregation on a negatively charged matrix. In is shown that the position of isobestic points can serve as an additional criterion for choosing the best values of the cooperativity parameters, absorption spectra of ligands in the aggregated state, and other parameters of the complex formation. The obtained intercalation constants show a linear dependence of 1gK(int) vs 1g C0Na for both PG and MB. These equations were used in a modified version of the computer program which allows optimization of the absorption spectra for DNA-ligand mixtures in a wide range of Na+ concentrations simultaneously, and estimation of the constant for Na+ binding to DNA phosphate (K1 < 10).