STUDIES ON THE INTERACTIONS OF CO2+ AND ZN2+ WITH THE POLYNUCLEOTIDES POLY(A) AND POLY(C)

UV-Spectrophotometric techniques have been used to study the binding of Co2+ and Zn2+ to the single-stranded polynucleotides poly(A) and poly(C). The spectrophotometric data can be described by a simple binding equilibrium, i.e. by a 2-state model. The apparent binding constants which have been evaluated with poly(A), K(Co-poly)(A) = 4.5 . 10(3) M-1 and K(Zn-poly)(A) = 2.9 . 10(3) M-1 (25-degrees-C, ionic strength I = 0.1 M, pH 6.5 - 7.0), are compared to those for Ni2+ and Mg2+ with poly(A), determined earlier. It is concluded that Co2+, Zn2+ and Ni2+ form inner-sphere complexes with poly(A), in contrast to Mg2+. Kinetic studies confirmed the inner-sphere binding mode for Co2+ (and Ni2+). In the case of Zn2+, however, the inner-sphere substitution reactions are apparently too fast to be distinguished from the helix-coil transitions of the poly-nucleotide. A second method for equilibrium measurements, based on using murexide as an indicator of the concentration of free Co2+, gave for K(Co-poly)(A) a value of 5.5 . 10(3) M-1 at I = 0.1 M. At I = 0.01 M, the binding constants K(Ni-poly)(A) and K(Co-poly)(A) are about 30-fold higher than at 0.1 M, according to the uv-data. With poly(C), the stability constants of all 4 metal ions are nearly the same, about 650 +/- 100) M-1 at I = 0.1 M. The available evidence suggests that these metal ions are bound to poly(C) mainly by electrostatic interactions, with little or no inner-sphere coordination.