Binding of divalent copper and calcium ions to poly I

The effect of Cu2+ ions on the ultraviolet differential ( UVD ) spectra of single-stranded poly I was studied and the coordination (delta epsilon b) and conformation (delta epsilon c) components of the spectra calculated. The comparison of delta epsilon b and the UVD spectrum of protonated IMP leads to the conclusion that N(7) of inosine-5'-monophosphate (IMP) is a coordinating site for Ca2+ and Cu2+ ions on the polymer bases. The binding of Ca2+ and Cu2+ ions causes differently directed displacements of the four absorption bands of poly I, which are observed in the wavenumber range (50-34) X 10(3) cm-1. The calculation of concentration dependencies for the association constants (K") of Ca2+ and Cu2+ ions binding to poly I bases shows that the binding is cooperative. The K" values for the poly I + Ca2+ complex are two orders of magnitude lower than those for the poly I + Cu2+ complex. At low ion concentrations, binding to the poly I phosphates predominates and increases the degree of the polynucleotide helicity. At higher concentrations the spectra are mainly affected by the ion binding to bases, which results in melting of the helical parts of poly I. At Ca2+ concentrations exceeding 10(-3) M light-scattering aggregates are formed. The degree of monomer order in them is close to that observed in multistranded helices of poly I.