On the complexing of deoxyribonucleic acid by silver(I)

Abstract

The complexes of silver(I) with DNA have been studied spectrophotometrically and potentiometrically at an ionic strength of 0.1 M (NaClO4) and pH ∼ 5.6. Let rb be the moles of Ag+ bound per DNA base. There is one complex for 0 < rb < 0.2, with a characteristic spectrum and a high binding affinity for Ag+. There is a second complex for 0.2 < rb < 0.5 with a different spectrum and somewhat weaker binding. Still higher complexes form at higher concentrations of Ag+, but we have not studied these extensively. The Ag-DNA complexes have about the same high intrinsic viscosity as the uncomplexed DNA, indicating that the helical structure is not disrupted. There is no proton release from the DNA upon formation of the first complex. Upon formation of the second complex, there is some proton release which is greater for AT-rich DNA's. We propose that the first complex involves addition of Ag+ to the sigma electron pair of N-7 or N-3 of the purines in alternate base pairs. In the second complex, Ag+ is added to the remaining base pairs; for AT pairs, it replaces the N-1 (adenine) to N-3 (thymine) hydrogen bond; for GC pairs, it again adds to N-7 or N-3 of the guanosine. The complexing behavior of AT polymer is quite different from that of the natural DNA's.