The effects of structural variations of thiophene-containing Ru(II) complexes on the acid–base and DNA binding properties

Abstract

A phenylthiophenyl-bearing Ru(II) complex of [Ru(bpy)2(Hbptip)](PF6)2 {bpy = 2,2′-bipyridine, Hbptip = 2-(4-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesized and characterized by elemental analysis, 1H NMR spectroscopy, and electrospray ionization mass spectrometry. The ground- and excited-state acid–base properties of the complex were studied by UV–visible absorption and photoluminescence spectrophotometric pH titrations and the negative logarithm values of the ground-state acid ionization constants were derived to be pK a1 = 1.31 ± 0.09 and pK a2 = 5.71 ± 0.11 with the pK a2 associated deprotonation/protonation process occurring over 3 pK a units more acidic than thiophenyl-free parent complex of [Ru(bpy)2(Hpip)]2+ {Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}. The calf thymus DNA-binding properties of [Ru(bpy)2(Hbptip)]2+ in Tris–HCl buffer (pH 7.1 and 50 mM NaCl) were investigated by DNA viscosities and density functional theoretical calculations as well as UV–visible and emission spectroscopy techniques of UV–visible and luminescence titrations, steady-state emission quenching by [Fe(CN)6]4−, DNA competitive binding with ethidium bromide, DNA melting experiments, and reverse salt effects. The complex was evidenced to bind to the DNA intercalatively with binding affinity being greater than those for previously reported analogs of [Ru(bpy)2(Hip)]2+, [Ru(bpy)2(Htip)]2+, and [Ru(bpy)2(Haptip)]2+ {Hip = 1H-imidazo[4,5-f][1,10]phenanthroline, Htip = 2-thiophenimidazo[4,5-f][1,10]phenanthroline, Haptip = 2-(5-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline}.