DNA binding and bending by dinuclear complexes comprising ruthenium polypyridyl centres linked by a bis(pyridylimine) ligand

The interaction of enantiomerically pure dinuclear complexes of the form [Ru2(L-L)4L1]4+ (where L-L = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) and L1 = bis(pyridylimine) ligand ((C5H4N)CN(C6H4))2CH2)) with ct-DNA have been investigated by absorbance, circular dichroism, fluorescence displacement assays, thermal analysis, linear dichroism and gel electrophoresis. The complexes all bind more strongly to DNA than ethidium bromide, stabilise DNA and have a significant bending effect on DNA. The data for Δ,Δ-[Ru2(bpy)4L1]4+ are consistent with it binding to DNA outside the grooves wrapping the DNA about it. By way of contrast the other complexes are groove-binders. The phen complexes provide a chemically and enantiomerically stable alternative to the DNA-coiling di-iron triple-helical cylinder previously studied. In contrast to the di-iron helicates, the phen complexes show DNA sequence effects with Δ,Δ-[Ru2(phen)4L1]4+ binding preferentially to GC and Λ,Λ-[Ru2(phen)4L1]4+ to AT.