Phase equilibrium in poly(rA) • poly(rU) complexes with Cd2+ and Mg2+, ions, studied by ultraviolet, infrared, and vibrational circular dichroism spectroscopy

Ultraviolet (UV) and infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy were used to study conformational transitions in the double-stranded poly(rA). poly(rU) and its components-single-stranded poly(rA) and poly(rU) in buffer solution (pH 6.5) with 0.1M Na+ and different Mg2+ and Cd2+ (10(-6) to 10(-2)M) concentrations. Transitions were induced by elevated temperature that changed from 10 up to 96 degrees C. IR absorption and VCD spectra in the base-stretching region were obtained for duplex, triplex, and single-stranded forms of poly(rA) (.) poly(rU) at [Mg2+],[Cd2+]/[P] = 0.3. For single-stranded polynucleotides, the kind of conformational transition (ordering -> disordering -> compaction, aggregation) is conditioned by the dominating type of Me2+ polymer complex that in turn depends on the ion concentration range. The phase diagram obtained for poly(rA) (.) poly(rU) has a triple point ([Cd2+] 10(-4)M) at which the helix-coil (2 -> 1) transition is replaced with a disproportion transition 2AU -> A2U + poly(rA) (2 -> 3) and the subsequent destruction of the triple helix (3 -> 1). The 2 I transitions occur in the narrow temperature interval of 2 degrees-5 degrees. Unlike 2 -> 1 and 3 -> 1 melting, the disproportion 2 3 transition is a slightly cooperative one and observed over a wide temperature range. At [Me2+] similar to 10(-3)M, the temperature interval of A2U stability is not less than 20 degrees C. In the case of Ca2+, it increases with the rise of ion concentration due to the decrease of T-m(2 -> 3). The T-m(3)-> 1 value is practically unchanged up to [Cd2+]similar to 10(-3)M. Differences between diagrams for Mg2+ and Cd2+ result from the various kinds of ion binding to poly(rA) (.) poly(rU) and poly(rA). (c) 2005 Wiley Periodicals, Inc.