Magnesium ion effect on the helix‐coil transition of DNA

Abstract

Abstract The effect of magnesium ions on the parameters of the DNA helix‐coil transition has been studied for the concentration range 10 −6 –10 −1 M at the ionic strengths of 10 −3 M Na + . Special attention has been given to the region of low ion concentrations and to the effect of polyvalent metallic impurities present in DNA. It has been shown that binding with Mg ++ increases the DNA stability, the effect being observed mainly in the concentration range 10 −6 –10 −4 M . At[Mg ++ ]>10 −2 M the thermal stability of DNA starts to decrease. The melting range extends to concentrations ∼10 −5 M and then decreases to 7–8°C at the ion content of 10 −3 M . Asymmetry of the melting curves is observed at low ionic strengths ([Na + ] = 10 −3 M ) and [Mg ++ ] ⩽ 10 −5 M . The results, analyzed in terms of the statistical thermodynamic theory of double‐stranded homopolymers melting in the presence of ligands, suggest that the effects observed might be due to the ion redistribution from denatured to native DNA. An experimental DNA–Mg ++ phase diagram has been obtained which is in good agreement with the theory. It has been shown that thermal denaturation of the system may be an efficient method for determining the ion‐binding constants for both native and denatured DNA.