Effects of Na+ and Mg++ ions on the helix–coil transition of DNA

Abstract The effects of monovalent (Na + ) and divalent (Mg ++ ) cations on the temperature and breadth of the helix–coil transition of phage DNA have been investigated. The experimental results confirm the findings of Dove and Davidson J. Mol. Biol. 5 , 467–478 (1962) for the limiting cases of zero divalent ion concentration and saturating levels of divalent ion, and extend their findings to the intermediate region of Mg ++ concentrations. A theory for the dependence of transition temperature on the ion concentrations is developed, utilizing the approach of Wyman Adv. Protein Chem. 19 , 223–286 (1964), modified to account for electrostatic nonideality of the polyelectrolytes. The theory is in agreement with Manning's treatment of the experiments of Dove and Davidson Biopolymers 11 , 937–949, 951–955 (1972) and is in fair agreement with experimental data over the entire range of ion concentrations. Further investigation of the structure and ion‐binding properties of the denatured form will be required before a quantitative comparison between theory and experiment can be performed.