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(A21) solutions containing Na + (0.1-2 M) and Mg 2+ (10 -5 -0.005 M) ions. Over the whole range of the studied Na + and Mg 2+ concentrations, the heating-induced destruction of the triple A2I helix is actually the A2I → A + I + I (3 → 1) transition. The rise of the transition temperature with increasing Na + and Mg 2+ 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 (ξ = I 9 ± 0.1) and the Mg 2+ -A2I binding constant (K = 1 250 M -1 for the zero degree of binding). An analytical expression has been obtained, which correlates the constants of Mg 2+ binding to three- and single-stranded polynueleotides (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 1, and poly A have similar conformations.