Salt effects on the denaturation of DNA. IV. A calorimetric study of the helix-coil conversion of the alternating copolymer poly[d(A-T)

Abstract

The enthalpy ΔH, entropy ΔS, and the temperature T m of the conformational transition of poly[d(A-T)] from the ordered to the randomly oriented state have been determined at pH 6.8 with the help of an adiabatic differential scanning calorimeter in Na 2SO 4 solutions of increasing ionic strength. Spectrophotometric denaturation experiments supplemented the calorimetric measurements. All thermodynamic parameters were found to vary strongly with salt concentration: both ΔH and T m increase linearly with the logarithm of the mean molal activity a ± of Na 2SO 4. However, whereas the dependence of T m on salt activity remains linear over the entire salt concentration range employed ΔH decreases abruptly in the most concentrated Na 2SO 4 solutions. The entropy of melting changes with salt concentration in a pattern similar to that displayed by ΔH. The data on ΔH as well as data derived from the maximum slopes of the calorimetric heat denaturation curves were used to calculate the cooperative length L h , the stacking free energy ϵ, and the cooperativity parameter σ of poly[d(A-T)] as a function of ionic strength. L h decreases with increasing salt concentration whereas σ increases. ϵ assumes more positive values with increasing salt molality. These changes then are in agreement with the generally held belief that an increase in salt concentration leads to an increase in the “loop” content of the copolymer. Suggestions have been offered to explain the observed sudden decrease of ΔH in concentrated Na 2SO 4.