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The Effects of Aqueous Salt Solutions on the Activity Coefficients of Purine and Pyrimidine Bases and Their Relation to the Denaturation of Deoxyribonucleic Acid by Salts

From the ¹ From the Department of Medicine, Harvard Medical School, and the Massachusetts General Hospital, Boston, Massachusetts 02114

The activity coefficients of thymine, adenine, cytosine, deoxyadenosine, and adenosine have been determined in aqueous salt solutions from solubility measurements at 25.0°. The large differences in activity coefficients which are observed with different salts are mainly due to the anions, and the order of effectiveness of different salts on all of the bases is nearly the same. There is a good correlation between the effects of concentrated salt solutions on the activity coefficients of the bases and their effects on the denaturation of deoxyribonucleic acid. Salts which cause large decreases in the activity coefficients, or increase in the solubility, of the bases (the sodium salts of Cl₃CCOO^-, SCN^-, ClO₄^-, and I^-) are potent denaturants of DNA. Cations, including (CH₃)₄N⁺, have only small effects on the bases corresponding to their lack of effectiveness in denaturation. These results indicate that the denaturation of DNA by these concentrated salt solutions can be accounted for by their effects on the activity coefficients of the bases, and is attributable to the greater degree of exposure of the bases to solvent in denatured DNA than in the native form.

The mechanism of the activity coefficient effects is not known but they cannot be explained by salt effects on the activity of water, by electrostatic effects of the Debye-Kirk-wood type, by effects on the internal pressure of the solvent, or by effects on the cavity formation energy of Sinanoglu and Abdulnur. In addition there is no correlation between the effects of salts on the bases and their effects on the structure of water as reflected by the viscosity B coefficients and the partial molal entropy of ions and other parameters. It is suggested, but not proved, that anions which decrease the activity coefficient of the bases may do so by a direct interaction of anions with the polar groups in the bases.

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