Interactions des ions métalliques avec le DNA. III. Stabilité et configuration des complexes Ag–DNA

The thermal stabilization and the configurational changes of DNA were studied during the binding process of silver ion. Melting curves of silver–DNA complexes were analyzed according to the method of Felsenfeld. An increase of T_m occurred during the formation of the first complex with a preferential stabilization of G–C pairs. In the case of DNA from H. influenzae, the transforming power was maintained after heating in presence of silver at a temperature where AT pairs were dissociated but G-C pairs were not. Shape modifications of the molecule were followed by light scattering, both with native and ultrasonic degraded DNA. In the case of native DNA, the binding of positive ions modified the electrostatic potential and increased the flexibility of the molecule. In the case of rod shaped fragments of DNA, the progressive formation of a kind of polyampholyte was accompanied by aggregation phenomena. The formation of the second complex, with proton release, induced changes of secondary structure and possibly a tilt of the plane of the base.