Studies of calcium ion binding to poly A

Ultraviolet differential spectra of poly A we studied in the presence of Ca2+ ions with 10(-3)M Na+ in the solution. At concentrations lower than 10(-3)M Ca2+, the ions bind to phosphate groups of the single helical polymer, thus increasing its degree of helicity. At higher concentrations, the ions start binding to the bases of poly A, producing aggregates whose effective radius, as found with an electric microscope, is not more than 10(2) A. These particles stack to form aggregates of an order-of-magnitude higher size. The mutual orientation of bases in the poly A aggregates is of a high degree of order. The calculation of concentration dependences of Ca2+-poly A binding constants shows that this process is cooperative.