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 )](ClO4)(2) (M = Pt or Pd; bpy = 2,2'-bipyridine, biq = 2,2'-biquinoli ne and en = enthylenediamine), [Pd(4,4'R(2)bpy)(en)](ClO4)(2)(R = CH3 or C6H5), and [Pd(bpy)(R(2), R'N(CH2)(2)NR '',Rtriple prime)] (ClO4)(2 )(R = R' = R '' = Rtriple prime = H; R = C-H3, R' = R '' = Rtriple pri me = H; ; R = CH3, R' = H, R '' = CH3, Rtriple prime = H; R = C2H5, R' = R '' = Rtriple prime = H; ; R = C2H5, R' = H, R '' = C2H5, Rtriple prime = H) and calf thymus DNA have been studied led via absorption an d denaturation experiments. The processes give rise to: i) large batho chromic shifts and substantial hypochromicity of the absorption bands of the complexes, and ii) an increase in the DNA melting temperature l arger 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 magn itudes of the slopes are always lower than expected on the basis of th e formal +2 charge of the complexes. K values are larger for those sub strates containing a greater number of aromatic rings; the presence of methyl substituents in the substances does not alter the binding avid ity if substitution is at NH2 groups of ethylenediamine, while the bin ding strength is enhanced when substitution is at bpy rings. Homologou s palladium(II) and platinum(II) complexes show very similar interacti on patterns with DNA. (C) 1997 Elsevier Science Inc.