Equilibrium binding studies and viscosity experiments are described that characterize the interaction of Δ-and Δ-[Ru(o-phen)3]2+ with calf thymus DNA. The mode of binding of these compounds to DNA is a matter of controversy. Both isomers of [Ru(o-phen)3]2+ were found to bind but weakly to DNA, with binding constants of 4.9 (±0.3) X 104 M−1 and 2.8 (±0.2) X 104 M−1 determined for the Δ and Δ isomers, respectively, at 20 °C in a solution containing 5 mM Tris-HCl (pH 7.1) and 10 mM NaCl. We determined that the quantity δ log K/δ log [Na+] equals 1.37 and 1.24 for the Δ and Δ isomers, respectively. Application of polyelectrolyte theory allows us to use these values to show quantitatively that both the Δ and Δ isomers are essentially electrostatically bound to DNA. Viscosity experiments show that binding the Δ isomer does not alter the relative viscosity of DNA to any appreciable extent, while binding of the Δ isomer decreases the relative viscosity of DNA. From these viscosity results, we conclude that neither isomer of [Ru(o-phen)3]2+ binds to DNA by classical intercalation.
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