The binding of Mg++ ions to polyadenylate, polyuridylate, and their complexes

Abstract The binding of Mg ++ to polyadenylate (poly A), Polyuridylate(poly U), and their complexes, poly (A + U) and poly (A + 2U), was studied by means of a technique in which the dye eriochrome black T is used to measure the concentration of free Mg − . The apparent binding constant K X = Mg N /Mg ++ N , N = site for Mg ++ binding (the phosphate group of the nucleotide), was found to decrease rapidly as the extent of binding increased and, at low extents of binding, as the concentration of Na − increased in poly A, poly (A + U), and poly (A + 2U), and somewhat less so in poly U. K x is generally in the range 10 4 > K X > 10 2 . The cause of these dependences is apparently, primarily, the displacement of Na + by Mg ++ in poly U and poly (A + U) on the basis of the similarity of extents of displacement measured in this work and those measured potentiometrically. was calculated and was found to approach zero as the concentration of Na + increased. In poly U, poly (A + U), and poly (A + 2U) at low Δ H ′ v.H. > 0, about + 2 kcal/“mole.” In poly A, also at low salt, Δ H ′ v.H. ≈ −4 kcal/“mol” for the initial binding of Mg ++ , and increases to +2 kcal/“mol” at saturation. This enthalpic variation probably accounts for the anticooperativity in the binding of Mg ++ not ascribable to the displacement of Na ++ .