Fibre X-ray and conformational study of the binding of metal ions on DNA

Results obtained from X-ray diffraction as well as from conformational analysis of Ag-DNA fibres are presented. For small percentages of Ag+ bound and high humidity, the B-DNA form is maintained. As the percentage of Ag+ is increased, the helical parameters of the B-DNA are modified. These modifications are directly related to the percentage of G-C bases. The periodicity of the DNA fibres are perturbed as Ag+ is mainly bound to G-C pairs and, thus, only the equatorial diffracted intensities can be compared to values calculated from molecular models. It is shown, by this way, that the first binding site is located on N7 of G. A second site is situated between N3 and N1 of the G-C pair, at the place of a hydrogen bond. A molecular model of the Ag-DNA complex is proposed and shown to be in agreement with experimental data. Results obtained allow to get some information on the binding of other ions such as Cu2+ and Hg2+ which give very little modification of the fibre X-ray patterns.