Interaction of metallic cations with DNA VI. Specific binding of Mg⁺⁺ and Mn⁺⁺

The binding of Mg 2+ and Mn 2+ by DNA by a divalent cation specific electrode and by ultracentrifugation. Both techniques give similar results for the stoichiometry of the reaction. An oscillating densiemete allowed us to detect small changes of volume accompanying the binding. The reaction was also followed by circular dichroism measurements. Interpretation of the results is only possible if one assumes an electrostate site‐binding of Mg 2+ to phosphate group, and a chelation Mn 2+ between the phosphate group and the N 7 of the guanine. Physical modifications accompanying these two types of binding are discused and compared to the role of these cations in some biological systems involving DNA.