Mg2+ ion effect on conformational equilibrium of poly A . 2 poly U and poly A poly U in aqueous solutions

Differential UV spectroscopy and thermal denaturation were used to study the Mg²⁺ ion effect on the conformational equilibrium in poly A · 2 poly U (A2U) and poly A · poly U (AU) solutions at low (0.01 M Na⁺) and high (0.1 M Na⁺) ionic strengths. Four complete phase diagrams were obtained for Mg²⁺–polynucleotide complexes in ranges of temperatures 20–96 °C and concentrations (10⁻⁵–10⁻²) M Mg²⁺. Three of them have a ‘critical’ point at which the type of the conformational transition changes. The value of the ‘critical’ concentration ([Mg_t²⁺]_cr=(4.5±1.0)×10⁻⁵ M) is nearly independent of the initial conformation of polynucleotides (AU, A2U) and of Na⁺ contents in the solution. Such a value is observed for Ni²⁺ ions too. The phase diagram of the (A2U+Mg²⁺) complex with 0.01 M Na⁺ has no ‘critical’ point: temperatures of (3→2) and (2→1) transitions increase in the whole Mg²⁺ range. In (AU+Mg²⁺) phase diagram at 0.01 M Na⁺ the temperature interval in which triple helices are formed and destroyed is several times larger than at 0.1 M Na⁺. Using the ligand theory, a qualitative thermodynamic analysis of the phase diagrams was performed.