The effects of aqueous neutral‐salt solutions on the melting temperatures of deoxyribonucleic acids

Abstract The helical stability of a variety of DNA samples, ranging in base composition from 0 to 72 mole‐% GC, has been studied by heat denaturation at neutral pH in increasing concentrations of LiCl, NaCl, KCl, CsCl, Li 2 SO 4 , and K 2 SO 4 . The variation of melting temperature with average base composition, dT m / dX GC , was found to decrease drastically in the concentrated salt media, e.g., from 41°C in 0.006 M LiCl to 29°C in 3.2 M LiCl, and from 39°C in 0.003 M Li 2 SO 4 to 18°C in 1.6 M Li 2 SO 4 . At the same time, the thermal transition is much more cooperative in the concentrated salt solutions than at low ionic strength. Indeed, at limiting salt concentrations, the transition breadth seems to reach a minimum value irrespective of the compositional heterogeneity of the DNA samples. Attempts to correlate the observed decrease of dT m / dX GC with predicted changes in the enthalpy of melting, deduced from a simple theoretical treatment, experimental data on the binding of counterions and water to DNA, and experimental data on thermal denaturation, were unsuccessful. However, the strongly reduced composition dependence of the melting temperature can be understood in terms of a destabilizing effect of the concentrated salt media on GC‐base pairs. It is suggested, though not proven, that the destabilization involves the displacement of water molecules from the DNA helix.