Tris(phenanthroline)ruthenium(II) enantiomer interactions with DNA: mode and specificity of binding

Absorbance and fluorescence methods, circular dichroism, UV melting experiments, viscosity, and competition dialysis were used to study the interaction of delta and lambda tris(phenanthroline)ruthenium(II) with DNA. The results of these studies indicated that both isomers bind to DNA by a single mode. The two isomers differ, however, in their effect on the hydrodynamic properties of DNA as measured by viscosity and, therefore, probably differ in their individual binding modes. The optical properties of the fully bound compounds differ from those of the free, but the perturbations of their visible absorbance and fluorescence emission spectra are modest when compared to changes observed for other DNA binding compounds. Binding of both isomers to DNA was found to be weak (in comparison to proven intercalators), with binding constants on the order of 10(4) M-1 determined for their binding to calf thymus DNA. A small, positive enthalpy was found for the binding of each isomer to DNA, suggesting that binding is entropically driven. Both isomers increased the melting temperature of DNA, with little quantitative difference between the two. A modest base specificity was found for each isomer, with the delta isomer preferentially binding to GC base pairs, and the lambda isomer preferentially binding to AT base pairs. Competition dialysis was used to examine the preference of delta and lambda Ru for right-handed B DNA and left-handed Z DNA. Neither isomer exhibits significant selectivity for these radically different DNA secondary structures.