[Spectroscopic study of the effect of binding of bivalent manganese ions on the conformation of polyriboadenylic acid].

UV-differential spectra and CD spectra of poly(A) were studied in the presence of manganese ions at 10(-3) and 0.1 M Na+ in solution. It was shown that at at both low and high ionic strengths Mn2+ ions in low concentrations form bindings with phosphates of the polymer, increasing the degree of its helicity. At higher concentrations Mn2+ ions start to bind with poly(A) bases leading to a conformational helix-random coil transition. With manganese contents exceeding 10(-3) M, aggregates are formed, in which the mutual orientation of bases is characterized by a high degree of order. The calculation of concentration dependences of Mn2+ - poly(A) base binding constants shows that the process is cooperative. As the ionic strength increases from 10(-3) to 0.1 M Na+, the constant of the poly(A) - Mn2+ complex formation decreases by about an order of magnitude.