Cadmium ion effects on phase equilibrium in double-stranded poly(rA) poly(rU)

Abstract

Differential UV spectroscopy was used to study heating and Cd²⁺ ion induced conformational transitions in double-stranded poly(rA) poly(rU) (AU) and its components (single-stranded poly(rA) and poly(rU)) in buffer solutions (pH 6.5) containing 0.03M Na⁺. It was revealed that with the rise of the cadmium ion concentration single-stranded polynucleotides undergo a number of conformational transitions, the character of which is conditioned with the dominant type of the metal complex. The phase diagram of poly(rA)-poly(rU) measured in solution with cadmium ions at 0.03M Na⁺ is compared with diagrams for AU+Cd²⁺ at 0.1M Na⁺ and AU+Mg²⁺ at 0.01M Na⁺. It was shown that at low metal concentrations the helix-coil transition in AU is of one-phase cooperative character. When some critical concentration [Mt_cr] is reached, the 2—>1 transition is replaced with the disproportional 2AU—>A2U+poly(rA) (2—>3) transition after which the disruption of the triple helix (3—>1) takes place. The [Mt_cr] value depends on the sodium ion content in solution: the more the Na⁺ concentration, the higher concentration of divalent ions corresponds to the triple point. A series of Na⁺ concentrations ([Na⁺]= 0.01, 0.03 and 0.1 М) presets such [Mt_cr] positions as 4•10^-5, 6•10^-5 and 10^-4М that represent a linear dependence [Mt_cr] = f([Na⁺]) in the double logarithmic scale.The cadmium binding to poly(rA) bases results in the narrowing of the existence region of triple helices (in comparison with that of Mg²⁺ ions). The Cd²⁺ ion interaction with O4 of uracil causes the further destabilization of the triple helix.