The interaction of mixed-ligand square-planar complexes with calf thymus DNA.

The interactions among the complexes [M(bpy)(en)]-(ClO4)2, M(biq)(en)2(M = Pt or Pd; bpy = 2,2'-bipyridine, biq = 2,2'-biquinoline and en = ethylenediamine), [Pd(4,4'R2bpy)(en)] (ClO4)2(R = CH3 or C6H5), and [Pd(bpy)(R, R' N(CH2)2NR",R"')] (ClO4)2(R = R' = R" = R'" = H; R = CH3, R' = R" = R"' = H; ; R = CH3, R' = H, R" = CH3, R'" = H; R= C2H5, R' = R" = R"' = H; ; R = C2H5, R' = H, R" = C2H5, R'" = H) and calf thymus DNA have been studied via absorption and denaturation experiments. The processes give rise to: i) large bathochromic shifts and substantial hypochromicity of the absorption bands of the complexes, and ii) an increase in the DNA melting temperature larger than that observed for the known intercalator, ethidium bromide. Binding constants, K, have been determined spectrophotometrically at 25 degrees C, pH 7, and various ionic strengths, using the McGhee-von Hippel approach. Plots of log K vs. log [Na+] are linear, but the magnitudes of the slopes are always lower than expected on the basis of the formal +2 charge of the complexes. K values are larger for those substrates containing a greater number of aromatic rings; the presence of methyl substituents in the substances does not alter the binding avidity if substitution is at NH2 groups of ethylenediamine, while the binding strength is enhanced when substitution is at bpy rings. Homologous palladium (II) and platinum (II) complexes show very similar interaction patterns with DNA.