A bstractThe pH-induced helix-coil transition of DNA and its com plexes w ith EtB r is carried out at acidic pH in a wide interval of change of concentration ratio of EtBr/DNA. The binding isotherm s of EtBr on double and single-stranded DNA at pH = 7.0 and pH = 3.0 (t = 25 o C) are obtained by absorption and fluorim etric m ethods. Binding constants (K) and num ber of bases (n), corresponding to one binding site were determ ined. Non fluorescent "strong" com plex w ith ds-DNA at pH = 7.0 and t = 25 o C as well as "strong" and "weak" complexes with ss-DNA at pH = 3.0 and t = 25 o C are revealed.

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The binding of Hoechst 33258 with DNA at various ionic strengths of solution and different ligand concentrations has been investigated. Existence of more than one type of interactions of Hoechst 33258 with DNA has been revealed, which were very sensitive to the ionic strength. Hoechst 33258 doesn't show specificity to AT sequences of DNA at low ionic strength. High affinity binding mode becomes obvious at high ionic strength. The values of binding constants and binding site sizes for revealed strong and weak interactions have been determined.

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The helix-coil transition of DNA-ethidium bromide complexes in an interval of ionic strength of 2.0 x 10 −3 M ≤ µ Na+ ≤ 2.0 × 10 −2 M has been investigated. It has been revealed that at the certain high ligand-DNA ratios (r b ) the transition interval of the complex -(∆T) becomes equal to that of DNA itself (∆T 0 ). It has been shown that the values of r b at which δ∆T=∆T-∆T 0 =0 depends on ionic strength of a solution. Further increasing of ligand concentration leads to its conversion from stabilizer into the destabilizer of the double-stranded DNA.