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 [Mtcr] 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 [Mtcr] 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 [Mtcr] positions as 4•10-5, 6•10-5 and 10-4М that represent a linear dependence [Mtcr] = 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.
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