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-CaCl 2 and DNA-MgCl 2 at various pH values and ionic strengths at 25 °C. Discontinuities occur in the conductance curves at mole ratios of Ca 2+ and Mg 2+ 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 Ca 2+ or Mg 2+/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, CaCl 2, and MgCl 2. For dialysed DNA at 26 ° C and 260 nm the decrease of absorbance with increased salt concentration was halted for MgCl 2 and CaCl 2 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 ( 1 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.