Biopolymers and cell. 2009; 25 (3): 218 - 225

Phase diagram of Ni²⁺ ions complexes with polyU×polyA×polyU

V. A. Sorokin, V. A. Valeev, E. L. Usenko

Aim. To investigate Ni²⁺ ion effect on the conformational equilibrium of the three-stranded polynucleotide polyU×polyA×polyU and to ascertain thermodynamic parameters of the metal complex formation. Methods. The differential UV spectroscopy and thermal denaturation. Results. Dependences of conformational transition (T_m) of poly U×polyA×polyU (A2U) on Ni²⁺ ion con- centration (up to 0.001 M) under conditions close to physiological ones (0.1 M Na⁺, pH 7) were obtained. At [Ni²⁺] < 3×10^–4 M two branches are observed in the phase diagram, corresponding to A2U → polyA×polyU (AU) + polyU (3→2) and AU→polyA + polyU (2→1) transitions. Only A2U→polyU + polyA + polyU (3→1) transition is realized at higher Ni²⁺ concentrations and upon A2U heating. Effective binding constants are determined for Ni²⁺ ions with AU (850 M^–1) and A2U (1300 M^–1) as well as 3→2 transition enthalpy (DH3→2 = 4±1 kcal/mol×triplet). Conclusions. By the equilibrium binding theory the thermodynamic nature of (T_m)2→3 different behavior in the phase diagram of AU in the presence of Mg²⁺ and Ni²⁺ ions was determined. A larger difference of the magnesium affinity to A2U and AU as compared with that to AU and poly A results in (T_m)2→3 decrease whereas the opposite ratio of Ni²⁺ ion binding constants induces its increasing.

Keywords: polynucleotides, metal ions, conformational transitions.