title: Phase equilibrium in poly(rA).poly(rU) complexes with Cd2+ and Mg2+ ions, studied by ultraviolet, infrared, and vibrational circular dichroism spectroscopy.
abstract: 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+] approximately 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 --> 1 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+] approximately 10(-3) M, the temperature interval of A2U stability is not less than 20 degrees C. In the case of Cd2+, 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+] approximately 10(-3)M. Differences between diagrams for Mg(2+) and Cd2+ result from the various kinds of ion binding to poly(rA).poly-(rU) and poly(rA).
authors.affiliation: B. I. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Science of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine.
authors.forename: Yurii
authors.initials: Y
authors.lastname: Blagoi
authors.forename: Galina
authors.initials: G
authors.lastname: Gladchenko
authors.forename: Laurence A
authors.initials: LA
authors.lastname: Nafie
authors.forename: Teresa B
authors.initials: TB
authors.lastname: Freedman
authors.forename: Victor
authors.initials: V
authors.lastname: Sorokin
authors.forename: Vladimir
authors.initials: V
authors.lastname: Valeev
authors.forename: Yanan
authors.initials: Y
authors.lastname: He
mesh_terms: D002104:Cadmium / Q000737:chemistry*; D002413:Cations, Divalent / Q000737:chemistry; D002942:Circular Dichroism; D008274:Magnesium / Q000737:chemistry*; D009690:Nucleic Acid Conformation; D011063:Poly A-U / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
chemical_list: D002413:Cations, Divalent; D002104:Cadmium; D011063:Poly A-U; D008274:Magnesium