  By the theory based on the equilibrium binding model, phase diagrams have
been calculated for the poly rA poly rU system in the presence of Mg2+, Ni2+,
Cd2+, with Na+ 0.1 M. Binding constants (KA, KU, KAU, KA2U) have been
determined, the best agreement with the early experimental results being
observed. Calculations of diagrams with Mg2+ are shown to permit obtaining
satisfactory results if steady mean constants of ion binding to polynucleotides
of all the mentioned structures are used. But in the cases with Ni2+ and Cd2+,
constants dependences on ion concentrations must be taken into account,
especially with high ion contents in the presence of which compaction of
molecules of single-stranded poly A emerges. It has been revealed that the main
factor being responsible for differences in diagrams for Ni2+ and Cd2+ is a
significant variation of their constants of binding to poly A. Temperature
dependences of conformational transitions obtained by UV spectroscopy for low
polymer concentrations are compared with those obtained by IR and VCD
spectroscopy at high contents of these molecules and, accordingly, metal ions
in solution. Differences and peculiarities conditioning irreversibility of the
earlier observed disproportional 2->3 transition are clarified.
