| Abstract: | The interaction of Na+ with poly A, poly U, poly A·poly U, and Poly A·2 poly U has been investigated by means of potentiometry, by means of potentiometry, by means of a linked-function analysis of its effect on the binding of Mg++ ions, and of K+ by means of an analysis of its effect on the sedimentation coefficients of the polymers. The last method was found to be inapplicable. The results of the other two methods were found to be consistent, except in the case of poly A where the existence of base stacking, influenced by the binding of Mg++, significantly affects the linked-function analysis. The results are also consistent with the effects of the concentration of Na+ ions on the thermally induced conformational transitions of poly A·poly U and poly A·2 poly U, and with the extents of “binding” of Na+ to DNA measured by equilibrium and by transport methods. The interaction of Na+ with polynucleotides appears to be physically quite specific, although its thermodynamic basis is not clear. The extent of binding of Na+, Ψ, was found to be independent of the total Na+ concentration but a quadratic function of the extent of Mg++ binding, θ. In the absence of Mg++, Ψ = 0.35–0.38 for poly U, 0.40 for poly A, 0.59 for poly A·poly U, and 0.66 for poly A·2 poly U. |
