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 Mg(2+) 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 Mg(2+)-polynucleotide complexes in ranges of temperatures 20-96 degrees C and concentrations (10(-5)-10(-2)) M Mg(2+). Three of them have a 'critical' point at which the type of the conformational transition changes. The value of the 'critical' concentration ([Mg(t)(2+)](cr)=(4.5+/-1.0) x 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 Ni(2+) ions too. The phase diagram of the (A2U+Mg(2+)) complex with 0.01 M Na(+) has no 'critical' point: temperatures of (3-->2) and (2-->1) transitions increase in the whole Mg(2+) range. In (AU+Mg(2+)) 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.