DNA-binding and cytotoxicity studies of ruthenium(II) mixed-ligand complexes containing two intercalative ligands

New Ru(II) polypyridyl complexes containing two intercalative ligands, [Ru(dppz)2phen]2+ (1) (dppz = dipyrido-[3,2-a:2′,3′-c]phenazine, phen = 1,10-phenanthroline) and [Ru(dppz)2dpq]2+ (2) (dpq = di-pyrido[3,2-f:2′,3′-h]quinoxaline) have been synthesized and characterized. The binding of both complexes with calf thymus DNA (CT-DNA) has been investigated by spectroscopic and viscosity measurement. Results indicate that complexes 1 and 2 bind to DNA through intercalation, and the DNA binding affinity of complex 2 is much stronger than that of complex 1. However, the equilibrium binding constants of both complexes with DNA are smaller than that of [Ru(phen)2(dppz)]2+ and [Ru(bpy)2(dppz)]2+ (bpy = 2,2´-bipyridine) with one intercalative ligand, which may be interpreted by the steric hindrance effect of the two intercalated ligand dppz. On the other hand, comparing complex 1 with complex 2, the former can serve as a molecular “light switch” for DNA, and this difference may be due to the different ancillary ligands in complexes 1 and 2. In addition, both complexes demonstrate different antitumor activities against selected tumor cell lines in vitro, and can make the cells apoptosis.