Oligonuclear polypyridylruthenium(II) complexes incorporating flexible polar and non-polar bridges: synthesis, DNA-binding and cytotoxicity

Abstract

The paper reports the synthesis and characterisation of a series of flexible di-bidentate bridging ligands in which two 4-methyl-2,2'-bipyridine groups are linked at the 4'-position by polymethylene (bbn), linear polyether (bbOn) or linear alkylamine (bbNn) chains of varying length (n). The enantiomers (ΔΔ/ΛΛ) of the rac forms of the ruthenium(II) dinuclear complexes incorporating these ligands – i.e. [{Ru(phen)2}2(μ-BL)]4+ (phen = 1,10-phenanthroline; BL = bbn, bbOn or bbNn) – have been isolated by reaction of Δ- or Λ-[Ru(phen)2(py)2]2+ (py = pyridine) with the respective bridging ligands. Mononuclear species - in which only one of the bidentate moieties of the bridging ligand is coordinated – have also been isolated, as well as trinuclear and tetranuclear species involving the bb7 bridge. Fluorescence displacement studies of the DNA-binding of the dinuclear complexes containing the bbOn and bbNn bridges generally revealed a lower affinity than their bbn analogues for an oligonucleotide containing a single bulge site; the mononuclear complexes showed a lower affinity - and the trinuclear and tetranuclear complexes a higher affinity – than the dinuclear species, revealing an interesting interplay of lipophilicity, electrostatics and size in the complex/nucleic acid interaction. Cytotoxicity studies of these complexes against a murine leukaemia cell line revealed that the presence of the polyether or polyamine links in the chain lowered the cytotoxicity compared with their polymethylene analogues, and that the bb7-bridged trinuclear and tetranuclear complexes showed considerably enhanced cytotoxicity compared with the dinuclear Rubb7 analogue.