BINDING OF DIVALENT METAL-IONS TO SYNTHETIC DOUBLE-STRANDED POLYRIBONUCLEOTIDES

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 x 10(3) 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.