Salt effects on the denaturation of DNA

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Abstract
When DNA's of differing GC:AT base ratios, e.g. synthetic poly dAT, T4 DNA, calf thymus DNA, E. coli DNA, and M. Iysodeiklicus DNA, are heat-denatured at, neutral pH in increasing concentrations of Na2SO4 or Cs2SO4 as supporting electrolytes, the variation of melting temperature with average base composition, dTm/dXG C, changes from 45°C (in 0.002M Na) to 11°C (in 4.5M Na) and from 42°C (in 0.002M Cs) to 3°C (in 4.5M Cs). The decrease of dTm/dXG C is a monotonic function of decreasing water activity in the salt, solutions. We interpret this decreased composition dependence of the thermal stability of the various DNA's as being due to a destabilization of the GC base pairs relative to the AT base pairs by the concentrated salt media. A simple quantitative treatment shows that k = sG C/sA T decreases from a value of 4.14 (in 0.01.M Na) to 1.86 (in 3 M Na) and from 4.18 (in 0.01 M Cs) to 1.42 (in 3 M Cs). SA T is the equilibrium constant for the formation of a hydrogen-bonded AT base pair from a pair of unbonded bases at, the junction between a helical region and a denatured region and sG C is the like constant for the formation of a GC base pair. These results corroborate our previous findings of a strongly reduced composition dependence of the negative logarithm of the methylmercurie hydroxide concentration necessary to produce 50% denaturation when the helix–coil transition of DNA is studied in concentrated Cs2SO4 (ultracentrifugation) instead of in dilute Na2SO4 (ultraviolet spectrophotometry).