Thermodynamics and kinetics of the interaction of copper (II) ions with native DNA

Based on equilibrium binding studies, as well as on kinetic investigations, two types of interactions of Cu2+ ions with native DNA at low ionic strength could be characterized, namely, a nondenaturing and a denaturing complex formation. During a fast nondenaturing complex formation at low relative ligand concentrations and at low temperatures, different binding sites at the DNA bases become occupied by the metal ions. This type of interaction includes chelate formation of Cu2+ ions with atoms N(7) of purine bases and the oxygens of the corresponding phosphate groups, chelation between atoms N(7) and O of C(6) of the guanine bases, as well as the formation of specific intestrand crosslink complexes at adjacent G°C pairs of the sequence dGpC. CD spectra of the resulting nondenatured complex (DNA–Cu2+)nat may be interpreted in terms of a conformational change of DNA from the B‐form to a C‐like form on ligand binding. A slow cooperative denaturing complex formation occurs at increased copper concentrations and/or at increased temperatures. The uv absorption and CD spectra of the resulting complex, (DNA–Cu2+)denat, indicate DNA denaturation during this type of interaction. Such a conclusion is confirmed by microcalorimetric measurements, which show that the reaction consumes nearly the same amount of heat as acid denaturation of DNA.