Salt effects on the denaturation of DNA. V. Preferential interactions of native and denatured calf thymus DNA in Na2SO4 solutions of varying ionic strength

Precision density measurements were performed at 25°C on Na‐DNA‐Na2SO4 mixtures in the presence of either 0.005 m cacodylic acid buffer (pH 6.8) or in the presence of 0.1 m NaOH (pH 12.3). From measurements executed under equilibrium dialysis conditions, the so‐called “density increments” (∂ρ/∂c2)μ0 for native (pH 6.8), heat‐denatured (pH 6.8), and alkali‐denatured (pH 12.6) Na‐DNA were evaluated as a function of Na2SO4 concentration. (∂ρ/∂c2)μ0 for native DNA was found to decrease almost linearly with ionic strength I1/2 of the supporting electrolyte. The density increment for Na‐DNA at pH 12.6, on the other hand, increases in more or less linear fashion with I1/2. (∂ρ/∂c2)μ0 for heat‐denatured DNA at pH 6.8 is not affected very much by increasing salt strength. From density measurements performed on the Na‐DNA–Na2SO4 mixtures at fixed concentrations of diffusible components, the partial specific volumes ν2° of native (pH 6.8), heat‐denatured (pH 6.8), and alkali‐denatured (pH 12.6) Na‐DNA were determined as a function of Na2SO4 concentration. All ν2° values, irrespective of the secondary structure of the DNA, increase with increasing salt concentration although the increase for heat denatured DNA (pH 6.8) is barely noticeable. ν2° of both native and heat‐denatured DNA (pH 6.8) extrapolates to a value of 0.50o ml/g at vanishing salt concentration; ν2° of DNA in 0.1 m NaOH, on the other hand, assumes the value 0.2o ml/g. Distribution coefficients of diffusible components, expressed in terms of preferential water and salt interaction, were evaluated from the (∂ρ/∂c2)μ0 data, solvent densities, and partial specific volumes of all solution components. All interaction parameters depend strongly on salt concentration and on the conformation of DNA. From data collected and from information available in the literature it is concluded that Na2SO4, for instance, displaces water of hydration from native DNA much more readily with increasing salt concentration than does NaCl. The solvation properties of the denatured forms of Na‐DNA are rather complex but appear to be in harmony with whatever information can be gathered from the literature.