Thermodynamic Investigations of [(phen)(2)Ru(tatpp)Ru(phen)(2)](4+) Interactions with B-DNA

摘要

While the antitumor activity of P4+ is relatively well understood, the binding mechanism and thermodynamics for formation of (P4+ center dot DNA) complexes remain in question. The thermodynamic parameters (K-a, Delta G, Delta H, and -T Delta S) for formation of DNA complexes of the ruthenium dimer, [(phen)2Ru(tatpp)Ru(phen)2](4+) (abbreviated as P4+), where phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3 Delta,2 Delta-1:2 Delta,3 Delta-n]-pentacene, were determined using isothermal titration calorimetry. Calorimetric and spectroscopic titration experiments were performed in which P4+ was added to three duplex DNAs of different lengths. We determined that P4+ binds to duplex DNA at 298 K with modest affinity (K-a approximate to 3.8 x 10 (5) M-1, Delta G approximate to -7.6 kcal/mol), that the enthalpy change is unfavorable (Delta H approximate to +2.1 kcal/mol), and that complex formation is driven by a large favorable change in entropy (-T Delta S approximate to -9.7 kcal/mol). These thermodynamic values were found to be approximately independent of the length of the DNA, and the stoichiometry of the (P4+ center dot DNA) complexes was determined to be 1 P4+/2 DNA bp, at least for the two shorter DNAs. On the basis of the thermodynamic parameters, and the binding stoichiometry (verified in ESI-MS experiments), we conclude that P4+ is intercalating between two adjacent DNA base pairs and that the neighbor sites on either side of the bound ligand are excluded from binding additional P4+.