[The nature of different influence of Cd2+ ions on the conformation of three-stranded polyU-polyA-polyU and...

The methods of UV (DUV) spectroscopy and thermal denaturation were used to study the effect of Cd2+ ions on the conformational equilibrium of three-stranded (A21, A2U) and single-stranded (poly U, poly A and poly I) polynucleotides in aqueous solutions containing 0.1 M Na+ (pH 7). An analysis of the form and intensities of DUV-spectra of poly A, poly I and A2I revealed the presence of two types of complexes: interaction with N7 of purines, resulting in the formation of macrochelates and binding to N1 of poly A and poly I. Cd2+ ions do not bind to heteroatoms of A2U nitrogen bases, and, therefore, the conformation of its structure remains unchanged up to a concentration of Cd2+ 0.01 M. A "critical" concentration (1.5x10(-4) M) of Cd2+ ions exists above which A2I transits cooperatively into a new helical conformation, which has a lower thermostability. It is supposed that, during the formation of metallized A2I, Cd2+ ions form bridges between the adenine and hypoxanthine of its homopolynucleotide circuits, being arranged inside the triple helix.