Binding of divalent metal ions to synthetic double-stranded polyribonucleotides

Abstract

The equilibria for the binding of Mg²⁺, Ni²⁺ and Co²⁺ to the synthetic double-stranded polyribonucleotides poly(A)·poly(U), poly(I)·poly(C) and poly(G)·poly(C) have been investigated by UV spectrophotometry and (in part) by a metal-ion indicator technique. At ionic strength 0.1 mol dm^–3 the apparent binding constants of Mg²⁺ are close to 2 × 10³ dm³ mol^–1 in all cases; those for Ni²⁺ and Co²⁺ are 4–40 fold higher, depending on the type of purine base involved. The binding constants to poly(A)·poly(U) are similar to those for the extensively stacked single-stranded poly(A). Kinetic studies indicated that Mg²⁺ is bound mainly by electrostatic interactions (i.e. outer sphere) to the polynucleotides, whereas in the case of Ni²⁺ inner-sphere co-ordination (site binding) also occurs. The inner-sphere substitution at Ni²⁺(aq) is reflected by reaction effects in the millisecond range, detected by stopped-flow techniques. The kinetic data can be rationalized only by assuming (at least) two successive inner-sphere binding steps, presumably due to co-ordination to N⁷ of the purine bases and to a phosphate oxygen atom.