Electric dichroism and sedimentation velocity studies of DNA-Hg(II) and DNA-Ag(I) complexes

The cause of the induced CD bands of DNA-Hg2+ and DNA-Ag+ complexes has been interpreted as a condensed psi state (Walter, A. and Luck, G. (1977) Nucl. Acids Res. 4, 539-550). Electric dichroism and sedimentation velocity experiments indicate that there is no tertiary structure formation of DNA-heavy-metal-ion complexes to support a psi state. The increase of the sedimentation coefficient of DNA-heavy-metal-ion complexes can be accounted for by an increase in molecular weight and a decrease in the partial specific volume upon the binding of Hg2+ and Ag+. Electric dichroism measurements show that the new optical bands produced by the binding of Hg2+ to DNA and the mode I binding of Ag+ to DNA result in transition moments polarized in the plane of the bases. Binding mode II of the DNA-Ag+ complex at 265 nm gives an optical perturbation where the transition moment has an out-of-plane contribution. The CD studies reported in the following paper give further insight into the binding sites and binding modes of these two metal ions.