The dichotomy in the DNA-binding behaviour of ruthenium(II) complexes bearing benzoxazole and benzothiazole groups

The substituted tris(bipyridine)ruthenium(II) complexes {[Ru(bpy)2(4,4′-bbob)]2+ and [Ru(bpy)2(5,5′-bbob)]2+ [where bpy = 2,2′-bipyridine and bbob = bis(benzoxazol-2-yl)-2,2′-bipyridine] have been prepared and compared to the previously studied complex [Ru(bpy)2(4,4′-bbtb)]2+ [where bbtb = bis(benzothiazol-2-yl)-2,2′-bipyridine]. From the UV/VIS titration studies, Δ-[Ru(bpy)2(4,4′-bbob)]2+ displays a stronger association than the Λ-isomer with calf-thymus DNA (ct-DNA). For [Ru(bpy)2(5,5′-bbob)]2+, there appears to be minimal interaction with ct-DNA. The results of fluorescence titration studies suggest that [Ru(bpy)2(4,4′-bbob)]2+ gives an increase in emission intensity with increasing ct-DNA concentrations, with an enantiopreference for the Δ isomer, confirmed by membrane dialysis studies. The fluorescent intercalation displacement studies revealed that [Ru(bpy)2(4,4′-bbob)]2+ and [Ru(bpy)2(5,5′-bbob)]2+ display a preference for more open DNA structures such as bulge and hairpin sequences. While Λ-[Ru(bpy)2(4,4′-bbtb)]2+ has shown the most significant affinity for all the oligonucleotides sequences screened in previous studies, it is the Δ isomer of the comparable benzoxazole ruthenium(II) complex (Δ-[Ru(bpy)2(4,4′-bbob)]2+) that preferentially binds to DNA.