The interaction of calcium and magnesium ions with deoxyribonucleic acid.

Conductance and equilibrium dialysis studies are reported for the aqueous systems (native calf thymus) DNA-CaCl₂ and DNA-MgCl₂ at various pH values and ionic strengths at 25 °C. Discontinuities occur in the conductance curves at mole ratios of Ca²⁺ and Mg²⁺ to nucleate phosphorus of 0.125, 0.30, and 0.50. The dialysis results show the formation of complexes of stoichiometry 0.50 and 1.00 mol Ca²⁺ or Mg²⁺/mol nucleate phosphorus (2:1 and 1:1 complexes), the latter only in neutral or alkaline solutions, in agreement with the conductance discontinuity at 0.50. The other discontinuities may be due to preferential binding in the formation of the 2:1 complex. Binding constants for the 2:1 complexes are evaluated. Absorption-temperature profiles have been determined for “native” and dialysed DNA in the presence of NaCl, CaCl₂, and MgCl₂. For dialysed DNA at 26 ° C and 260 nm the decrease of absorbance with increased salt concentration was halted for MgCl₂ and CaCl₂ at a concentration corresponding to the formation of the 2:1 complex. The absorbance of “native” DNA did not decrease. T_m and the reciprocal of the hypochromic rise ($\text{1}\text{h}$) increased linearly with log (salt concn). Values of T_m were the same at 230, 260, and 280 nm, but h was greater at 230 and 280 than at 260 nm, which may be due to the existence of alternating blocks of (A + T) and (G ? C) pairs. The entropy of transition was in the order Ca > Mg ? Na.