Silver(I) complexes with DNA and RNA studied by Fourier transform infraredspectroscopy and capillary electrophoresis

Ag(I) is a strong nucleic acids binder and forms several complexes with DNA such as types I, II, and Ill. However, the details of the binding mode of silver(I) in the Ag-polynucleotides remains unknown. Therefore, it was of i nterest to examine the binding of Ag(I) with calf-thymus DNA and bakers yea st RNA in aqueous solutions at pH 7.1-6.6 with constant concentration of DN A or RNA and various concentrations of Ag(I). Fourier transform infrared sp ectroscopy and capillary electrophoresis were used to analyze the Ag(I) bin ding mode, the binding constant, and the polynucleotides' structural change s in the Ag-DNA and Ag-RNA complexes. The spectroscopic results showed that in the type I complex formed with DNA, Ag(I) binds to guanine N7 at low ca tion concentration (r = 1/80) and adenine N7 site at higher concentrations (r = 1/20 to 1/10), but not to the backbone phosphate group. At r = 1/2, ty pe II complexes formed with DNA in which Ag(I) binds to the G-C and A-T bas e pairs. On the other hand, Ag(I) binds to the guanine N7 atom but not to t he adenine and the backbone phosphate group in the Ag-RNA complexes. Althou gh a minor alteration of the sugar-phosphate geometry was observed, DNA rem ained in the B-family structure, whereas RNA retained its A conformation. S catchard analysis following capillary electrophoresis showed two binding si tes for the Ag-DNA complexes with K-1 = 8.3 X 10(4) M-1 for the guanine and K-2 = 1.5 X 10(4) M-1 for the adenine bases. On the other hand, Ag-RNA add ucts showed one binding site with K = 1.5 X 10(5) M-1 for the guanine bases .