Divalent metal ion effect on helix–coil transition of high molecular weight DNA in neutral and alkaline solutions

Effect of Mg2+, Ca2+, Ni2+ and Cd2+ ions on parameters of DNA helix–coil transition in sodium cacodylate (pH 6.5), Tris (pH 8.5) and sodium tetraborate (pH 9.0) buffers have been studied by differential UV–visible spectroscopy and by thermal denaturation. Anomalous behavior of the melting temperature Tm and the melting interval ΔT in the presence of MgCl2 was observed in Tris, but not in cacodylate or tetraborate buffers. Changes in the buffer type and pH did not influence Tm and ΔT dependence on Ca2+ and Cd2+ concentrations. Decrease of the Tm and ΔT of DNA in the presence of Ni2+ and Cd2+ was caused by preferential ion interaction with N7 of guanine. This type of interaction was also found for Mg2+ in Tris buffer. The anomalous decrease in the Tm and ΔT values was connected to formation of complexes between metal ions and Tris molecules. Transition of DNA single-stranded regions into a compact form with the effective radius of the particles of 300 ± 100 Å was induced by Mg2+ ions in Tris buffer.