Complexes of silver (I) with nucleic acids

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The equilibrium properties of the Ag (I) complexes of DNA have been studied potentiometrically and spectrophotometrically at ionic strength of 0.1 M (NaC10[subscript 4]) and at pH's from 5.6 to 8.0. There are at least two different complexes which form at low silver ion concentrations. The strength of association of one complex (type I binding) is pH independent. The type I complexing reaction releases no protons from the DNA. A second complex (type II binding) is stronger at high pH. This complexing causes the release of approximately one proton per Ag (I) bound. DNA which is rich in GC binds Ag (I) more strongly than does AT rich DNA. However, at pH's between 5.6 and 8.0 more protons are released from AT-rich DNA's. It is proposed that the first complex (type I) involves only GC base pairs, with silver (I) adding to the sigma electrons of a ring nitrogen or interacting with the pi electrons. In the second complex (type II), Ag (I) replaces an N-1 (purine) to N-3 (pyrimidine) hydrogen bond, thus displacing one proton. The complexing behavior of various polynucleotides, nucleotides and substituted purines was studied, and the DNA complexes are discussed in the light of these results. A cesium sulfate equilibrium density gradient ultracentrifugation technique has been developed in which buoyant density differences between different DNA's are generated by the specific formation of their Ag (I) complexes. When Ag (I) is bound to the extent of one silver ion per DNA base pair, the buoyant density of either native or denatured DNA increases by about .17 g/cc. Competition experiments in which Ag (I) was added to a mixture of two different DNA's resulted in specific binding of the Ag (I) by one of the DNA species. At equilibrium in the ultracentrifuge two widely separated DNA bands were observed. The buoyant density difference between these bands corresponded to the difference in Ag (I) binding by the two species of DNA. A survey was done of various useful preparative separations based on this technique. One such preparative separation has been accomplished, and the separated components have been characterized.