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

摘要

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.