Na+ and Mg2+ ion effect on the stability of a poly I • poly A • poly I triple helix

Differential UV spectroscopy was used to study the temperature dependence of the conformational equilibrium in aqueous poly I . poly A . poly I (A2I) solutions containing Na+ (0.1-2 M) and Mg2+ (10(-5) -0.005 M) ions. Over the whole range of the studied Na+ and Mg2+ concentrations, the heating-induced destruction of the triple A2I helix is actually the A2I --> A + I + I + I (3 --> 1) transition. The rise of the transition temperature with increasing Na+ and Mg2+ contents is well described by Manning's and the "ligand" theories, which makes it possible to estimate the linear charge density on the single-stranded poly I (xi = 1.9 +/- 0.1) and the Mg2+-A2I binding constant (K = 1250 M-1 for the zero degree of binding). An analytical expression has been obtained, which correlates the constants Mg2+ binding to three- and single-stranded polynucleotides (K-3 and K-1, respectively) and the linear charge density on them. There are only minor distinctions between the K-3 values for A2U and A2I because these polynucleotides have similar structures. The difference in the K-1 values is also slight as single-stranded poly U, poly I, and poly A have similar conformations.