Dependence of the melting temperature of DNA on salt concentration

Data on the decrease of the DNA melting temperature T_m with the salt concentration M are reported and discussed. The electrostatic free energy change in the helix–coil transition, ΔF_e is related to the potential, ψ, which represents the electrostatic repulsion between the phosphate charges; ψ is calculated as a function of M and of the distances between the charges of the two strands. The Debye‐Hückel approximation is shown to overestimate ψ. It is suggested that the high local concentration of the counterions in the immediate vicinity of the fixed charges screen these charges from interacting with other fixed charges, to the extent that the system behaves as if the fixed ions carry a reduced charge. The notion of a reduced charge represents in a single parameter the deviation of the Debye‐Hückel approximation from the true potential. A plot of T_m versus ΔF_e gives a straight line as predicted. ΔH₀ is calculated from the slope and found to be consistent with experimentally determined values. Our calculations support the hypothesis that the change of T_m with salt concentration is due to changes in the screened interactions between the fixed phosphate charges. In analyzing the results of these calculations, we are able on the one hand to indicate some of the limitations of the theoretical model and, on the other hand, draw some conclusions about the order of magnitude of the nonelectrostatic interaction energy of formation of the double helix.