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

Abstract

AbstractThe 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 KX = [MgN]/[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. Kx is generally in the range 104 > KX > 102. 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 ΔH′ v.H. > 0, about + 2 kcal/“mole.” In poly A, also at low salt, ΔH′ 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++.