[X-ray diffraction study of the binding of Cu2+ and Ag2+ cations to DNA]

An X-ray diffraction study has been undertaken on fibres of two different DNA compositions respectively corresponding to 42 and 72% of (G-C) base percentages. The fibres were prepared after binding of copper or silver ions to the DNA gels. With a relative humidity of 92%, the B-form has been observed for these two DNA molecules in the presence of copper ions or with a low percentage of silver ion binding. When the silver ion percentage is increased, a new form appears. From the comparison between the experimental and calculated intensities one can note that the proposed binding sites for copper involve few modifications in the X-ray patterns. For the silver, the calculated intensities show that the metal atom is near the double helix axis. For low percentages of silver the site between successive guanine bases cannot be distinguished with certainty by intensity analysis.