title: Salt effects on the denaturation of DNA.
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. lysodeikticus DNA, are heat-denatured at neutral pH in increasing concentrations of N(a)(2)SO(4) or C(s)(2)SO(4) as supporting electrolytes,the variation of melting temperature with average base composition, dT(m)/dX(G)(C), changes from 45°C (in 0.002M Na) to ll°C (in 4.5M Na) and from 42°C (in 0.002M Cs) to 3°C(in 4.5M Cs). The decrease of dT(m)/dX(G)(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 = 8GC/SAT decreases from a value of 4.14 (in 0.01MN(a)) 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 SGC 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 helix-coil transition of DNA is studied in concentrated Cs(s)SO(4)(ultracentrifugation) instead of in dilute N(a)(2)SO(4) (ultraviolet spectrophotometry).
authors.affiliation: Group in Biophysiman.d Department of Food Science and Technology, University of California,Davis, California 96616, USA
authors.forename: D W
authors.initials: DW
authors.lastname: Gruenwedel
authors.forename: C H
authors.initials: CH
authors.lastname: Hsu
mesh_terms: D000818:Animals; D017122:Bacteriophage T4 / Q000235:genetics; D001482:Base Composition; D002417:Cattle; D002586:Cesium / Q000494:pharmacology; D004247:DNA / Q000737:chemistry / Q000187:drug effects*; D004269:DNA, Bacterial / Q000737:chemistry / Q000187:drug effects*; D004279:DNA, Viral / Q000737:chemistry / Q000187:drug effects*; D004926:Escherichia coli / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D008837:Micrococcus / Q000235:genetics; D009691:Nucleic Acid Denaturation / Q000187:drug effects; D012492:Salts / Q000494:pharmacology*; D013431:Sulfates / Q000494:pharmacology; D013696:Temperature; D044366:Transition Temperature; D014867:Water / Q000737:chemistry
chemical_list: D004269:DNA, Bacterial; D004279:DNA, Viral; D012492:Salts; D013431:Sulfates; D014867:Water; C012036:sodium sulfate; D002586:Cesium; C025407:cesium sulfate; D004247:DNA; C427203:calf thymus DNA