Influence of the polypyridyl (pp) ligand size on the DNA binding properties, cytotoxicity and cellular uptake of organoruthenium(II) complexes of the type [(η6-C6Me6)Ru(L)(pp)]n+ [L = Cl, n=1; L = (NH2)2CS, n=2]

The DNA binding of polypyridyl (pp) (eta(6)-hexamethylbenzene)ruthenium(II) complexes of the type [(eta (6)-C6Me6)-RuCl(pp)](CF3SO3) (pp=phen, tap, dpq, dppz, dppn) 1-5 and [(eta(6)-C6Me6)Ru{(NH2)(2)CS}(pp)](CF3SO3)(2) (pp=dpq, dppz, dppn) 6-8 has been studied by UV/Vis spectroscopy, circular dichroism and viscosity measurements. Complexes 3-5, 7 and 8 are potent cytotoxic agents towards the human cancer cell lines MCF-7 and HT-29. Stable intercalative binding into CT DNA is indicated for the dpq and dppz complexes by large increases Delta T-m of 12-25 degrees C in the DNA thermal denaturation temperature for r=[complex]/[DNA]=0.1. Large viscosity increases for DNA in the presence of 3 and 4 are also in accordance with this binding mode as are the pronounced hypochromic UV/Vis shifts for the pi-pi(*) transitions of the dppz ligands of 4 and 7 in the range 360-400 nm. A small Delta T-m value of 2 degrees C and effectively unchanged viscosity suggest that Ru-N (nucleobase) coordinative binding is thermodynamically preferred for the larger polypyridyl ligand of 5 under equilibrium conditions, as is also the case for the small ligands phen and tap in complexes 1 and 2. CD spectra indicate that the B DNA conformation is essentially retained for interaction with the chloro complexes 1-5 but that very significant distortions occur for the thiourea complexes 6-8. The in vitro cytotoxicities of the chloro complexes 3-5 are dependent on the size of the polypyridyl ligand with IC50 values increasing in the order dppn