Salt effects on the denaturation of DNA

✍ Scribed by Dieter W. Gruenwedel; Chi-Hsia Hsu

• Publisher: Wiley (John Wiley & Sons)
• Year: 1969
• Tongue: English
• Weight: 816 KB
• Volume: 7
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1969.360070412

✦ Synopsis

When DNA's of differing GC:AT base ratios, e.g. synthetic poly dAT, T4 DNA, calf thymus DNA, E. coli DNA, and M. lysodeikticus DNA, are heat-denatured a t neutral pH in increasing concentrations of N&SO4 or CsZSO4 as supporting electrolytes, the variation of melting temperature with average base composition, dT,/dXGc, changes from 45°C (in 0.002M Na) to ll°C (in 4.5M Na) and from 42°C (in 0.002M Cs) to 3OC (in 4.5M Cs). The decrease of dT,/dXoc 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 = SGC/SAT decreases from a value of 4.14 (in 0.01M Na) to 1.86 (in 3M Na) and from 4.18 (in 0.01M Cs) to 1.42 (in 3M Cs). SAT 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 S G 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 methylmercuric hydroxide concentration necessary to produce 50% denaturation when the heli-coil transition of DNA is studied in concentrated CSZSOI (ultracentrifugation) instead of in dilute N*SOk (ultraviolet spectrophotometry).