146 The binding mechanisms of methylene blue with DNA

The melting of DNA and its complexes with methylene blue (MB) at .002 and .02 M Na⁺ ionic strengths of the solution in the concentration ratio changing interval 0 < r ≤ .33, where r = [ligand]/[DNA], has been carried out. It was revealed that the change of the melting interval width (δΔT) of the complexes depending on ligand concentration at .02 M Na⁺ increases at r ≤ .1 and gets to the saturation at r > .1, while at .002 M Na⁺, this parameter acquires a bell-like shape depending on ligand concentration. The change of melting temperature T_m increases with ligand concentration enhancement at the mentioned ionic strengths of the solution, moreover at .002 M ionic strength of the solution the melting temperature is significantly higher than at .02 M Na⁺ (Arakelyan et al., 2014; Vardevanyan et al., 2013). Comparison of the obtained data with analogous ones obtained for DNA–EtBr complexes permits to conclude that at low ionic strengths of the solution MB binds to DNA by intercalation, semi-intercalation as well as electrostatic mechanisms. At the same time at .02 M ionic strength of the solution, MB binds to DNA only by semi-intercalation and electrostatic modes. It was also shown that at .02 M ionic strength of the solution the melting interval width change of MB complexes with DNA enhances depending on the increase of GC-sequence content in its. Therefore the obtained data indicate that MB as EtBr may bind to DNA by different mechanisms. Furthermore if in the case of EtBr the intercalation takes place at not only low (.002 M) but also relatively high ionic strengths of the solution (>.002 M), in the case of MB at .02 M ionic strength of the solution the intercalation mode is not performed. These data also reveal the preference of ligand molecules to GC-sequences at intercalation or semi-intercalation.