Salt effects on the denaturation of DNA. III. A calorimetric investigation of the transition enthalpy of calf thymus DNA in Na 2SO 4 solutions of varying ionic strength

Abstract

The enthalpy of the helix-coil transition of calf thymus DNA has been determined with the help of an adiabatic differential scanning calorimeter at pH 6.8 in Na 2SO 4 solutions of increasing ionic strength. The transition enthalpy ΔH was measured by monitoring the additional electrical heat needed to maintain adiabatic conditions in the calorimeter when the DNA was undergoing the heat-induced conformational transition. ΔH was found to vary linearly, within a certain range of ionic strength, with the logarithm of the mean molal activity a ± of the supporting electrolyte. For example, the value of the transition enthalpy increases in a linear fashion from ΔH = 7.80 kcal/mole base pairs in 2.5 mM Na 2SO 4 plus 5 mM sodium cacodylate buffer, pH 6.8 (−log a ± ( T m) = 2.08; ( T m) signifies that the activity is valid at the transition temperature T m) to ΔH = 9.59 kcal/mole base pairs in 0.280 M Na 2SO 4 plus 5 mM sodium cacodylate, pH 6.8 (−log a ±( T m) = 0.88). As the salt concentration is increased above this value, ΔH levels off and finally decreases slightly. A maximum value for ΔH, ΔH = 9.70 kcal/mole base pairs, was obtained in 0.498 M Na 2SO 4 plus 5 mM sodium cacodylate, pH 6.8 (−log a ± ( T m) = 0.71). The entropy ΔS of the helix-coil transition of calf thymus DNA changes with salt concentration in a pattern similar to that exhibited by ΔH. The plot, however, appears to be somewhat sigmoidal in shape. Indications are that the dependence of both ΔH and ΔS on salt activity is rather complex in nature. The possibility that it is in reality a temperature effect is recognized also. The melting temperatures ( T m) obtained via calorimetry and conventional ultraviolet spectrophotometry are in excellent agreement with each other. dT m d(−log a ±(T m ) = −23° where the slope of the plot T m versus (−log a ±( T m) ) is constant. From the variation of T m with (−log a ±( T m )) it can be calcula that at low ionic strength the helix-coil transition is accompanied by the release of 0.34 mole counterions per mole DNA base pairs. The gradual change in the slope dT m d(−log a ±(T m ) ) above 0.280 M Na 2SO 4 can be understood at the quantitative level if, for instance, changes in the water activity of the solvent are taken into account also. Certain peculiarities in the shape of the calorimetric heat denaturation curves of calf thymus DNA have been noted. They are discussed in this work.