Effect of Zn2+ and temperature on the conformational equilibrium of single-stranded polyA in neutral solutions

Effect of Zn2+ ions on the conformation of polyA in cacodilic buffer at pH 7 was investigated by differential UV spectroscopy (DUV) and by thermal denaturation. The shapes of the DUV spectra and melting curves suggest a transition of polyA into a more ordered “metallized”, possibly double-helical conformation at Zn2+ concentrations above 3 × 10−5 M. A phase diagram of polyA complexes with Zn2+ was constructed for the temperature range from 20 °C to 95 °C and Zn2+ concentrations between 10−5 M and 5 × 10−4 M. It was found that the transition of a single strand into the “metallized” form is possible only if the length of the disordered single-stranded region becomes larger than a certain critical value, ranging between 98% and 78% as the metal concentration increases from 3 × 10−5 to 5 × 10−4 M.