Cation effects on the denaturation of DNA

The midpoint of the thermal denaturation curve of a DNA sample increases linearly with the logarithm of the ionic strength for 0·0003 M μ M ; the broadening effect is interpreted as indicating that the transition is less cooperative at low μ.

Abstract

The midpoint (Tm) of the thermal denaturation curve of a DNA sample increases linearly with the logarithm of the ionic strength for 0·0003 M < μ < 0·5 M. This ionic strength dependence does not depend on the base ratio of the DNA sample. The transition breadth increases with decreasing ionic strength. The broadening effect is interpreted as indicating that the transition is less cooperative at low μ. At μ = 3 × 10−4M, the divalent ions Mg2+ and Co2+ are bound almost stoicheiometrically by DNA and Tm increases by 35 to 45°C. The transition breadth is greater for 1/2 equivalent of divalent ion per mole of DNA phosphate than for zero or one equivalent; this is attributed to stronger ion binding by native than by denatured DNA. Silver ion at a ratio of 0·2 Ag+ per base increases Tm by about 40°C and broadens the transition. The pH values for denaturation by acid and base for several DNAs are reported.