Joint interaction of ethidium bromide and methylene blue with DNA. The effect of ionic strength on binding thermodynamic parameters

Large amount of data of experimental and theoretical studies have shown that ethidium bromide (EtBr) and methylene blue (MB) may bind to nucleic acids via three modes: intercalation between two adjacent base pairs, insertion into the plane between neighboring bases in the same strand (semi-intercalation), and outside binding with negatively charged backbone phosphate groups. The aim of the given research is to examine the behavior of these two ligands at both separate and joint DNA binding. The obtained experimental data show that the effect of simultaneous binding of EtBr and MB on double-stranded DNA has a non-additive effect of separate binding. The analyses of the melting thermodynamic parameters of DNA complexes with two bound ligands suggest competitive mechanism of interaction.