Specificity in the interaction between some chiral Co(III) complex ions and DNA in aqueous solution

Abstract

The binding of some chiral Co(III) complex ions by native DNA in aqueous solution at pH 7.2 has been studied. Equilibrium dialysis and “phase-separation” data show that the affinity for the complex counter-ions used by the polyelectrolyte in 0.01 M NaCl follows the order (–)₅₄₆[Co(en)₃]³⁺=(+)₅₄₆[Co(en)₃]³⁺ > (–)₅₄₆[Co(en)₂L-phe]²⁺>(–)₅₄₆[Co(en)₂L-leu]²⁺. Melting temperature measurements on DNA-complex ions systems indicate that forces other than electrostatic play an important role in the association process investigated. Evidence of a specific site binding is presented in the case of tris(ethylenediamine)cobalt(III). Directional modes of ion pairing are indicated by the changes in the visible c.d. spectra of [Co(en)₃]³⁺ ions in DNA solutions, which are the larger the higher is the percent of bound complex. The peculiar changes in the circular dichroism spectra of DNA in the presence of [Co(en)₃]³⁺ ions in the region of 240–280 nm strongly support the hypothesis of a perturbation in the geometry of the deoxyribonucleic acid secondary structure which accompanies the ion-association complex. Implications of the different structural characteristics of the other complex ions studied on the binding process with DNA are also discussed.