| 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. |
