INTERACTION OF DELTA-[RU(PHEN)(2)DPPZ](2-[RU(PHEN)(2)DPPZ](2+) WITH DNA - A CALORIMETRIC AND EQUILIBRIUM BINDING STUDY() AND LAMBDA)

Fluorescence and absorption spectroscopy, isothermal titration calorim etry, and viscosity measurements have been used to characterize the in teraction of Delta and Lambda [Ru(phen)(2)DPPZ](2+) with calf thymus D NA. The method of continuous variations revealed two distinct binding stoichiometries for both Delta- and Lambda-DPPZ, corresponding to 0.7 and 3 mol of base pair/mol of ligand. Binding isotherms were obtained for the two enantiomers, both of which show strong binding to DNA, wit h K = 3.2 x 10(6) M(-1) bp and 1.7 x 10(6) M(-1) bp for the Delta and Lambda isomers, respectively, at 25 degrees C in solutions containing 50 mM NaCl. Titration calorimetry gave Delta H values of +0.3 kcal mol (-1) for Delta-DPPZ and +2.9 kcal mol(-1) for Lambda-DPPZ for their in teraction with DNA. These small positive enthalpies, which were confir med using thermal difference spectroscopy, indicated that the binding of these compounds to DNA is entropically driven. An enthalpy of +2.5 kcal mol(-1) was obtained for the binding of the parent compound, tris (phenanthroline)-Ru(II), to DNA. Titration of all three compounds into buffer gave a nonnegligible heat of dilution. The salt dependence of the binding constant was examined for both isomers. The slope SK = (de lta logK/delta log[Na+]) was found to be 1.9 and 2.1 for the Delta and Lambda isomers, respectively. By using polyelectrolyte theory to inte rpret the observed salt dependence of the equilibrium constant, it can be shown that there is a significant nonelectrostatic contribution to the binding constant. Relative viscosity experiments showed that both Delta- and Lambda-DPPZ increase the length of rod-like DNA, in a mann er consistent with binding by classical intercalation. Fluorescence en ergy transfer experiments provided additional evidence for the interca lation of Delta- and Lambda-[Ru(phen)(2)DPPZ](2+) into DNA.