The interactions of Mg2+ and Ni2+ with several oligo-adenylic acids: Equilibria and kinetics

Abstract

Equilibria and kinetics of the binding of Mg 2+ and Ni 2+ to the oligo-adenylic acids ( Ap) 2 A, ( Ap) 7 A, and ( Ap) 11 A have been investigated at 25°C, ionic strength I ≈ 0.5 M. The equilibria were studied by means of UV spectrophotometric titrations. The observed changes in absorbancy are consistent with those expected for a 1:1 biding stoichiometry. Only for Ni 2+ + ( Ap) 11 A the spectral data are fitted somewhat better with the assumption of a stepwise binding of two Ni 2+ ions to one ligand molecule. The kinetics of the binding processes were studied by applying the temperature-jump relaxation technique. Measurable relaxation effects could not be detected for the systems Mg 2+ + L (L = ligand), apparently indicating that the Mg 2+ is bound predominantly by electrostatic forces (i.e., outer-sphere) to these oligonucleotides under our conditions. By contrast, relaxation effects in the ms range were observed with Ni 2+ + L. The concentration dependence of the time constants can be described by a two-step mechanism in which the first step is assigned to the fast outer-sphere association, with outer-sphere stability constants K 0 equal to those found for the binding of Mg 2+. Again, in the case of Ni 2+ + ( Ap) 11 A the kinetic data can be described better by a mechanism which involves the stepwise binding of 2 Ni 2+ ions. However, only one of the two relaxation times expected for such a process is observed (the longer one). The results of this study are discussed together with the data for several closely related systems.