SALT EFFECTS ON DENATURATION OF DNA

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). Copyright © 1969 John Wiley & Sons, Inc.