Enantiomeric ruthenium(II) complexes binding to DNA: binding modes and enantioselectivity

A series of enantiomerically pure polypyridyl ruthenium(II) complexes, Delt a- and Lambda-[Ru(bpy)(2) (HPIP)](PF6)(2) (Delta-1 and Lambda-1; bpy = 2,2' -bipyridine, HPIP = 2-(2-hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline), Delta- and Lambda-[Ru(bpy)(2)(HNAIP)](PF6)(2) (Delta-2 and Lambda-2; HNAIP = 2-(2-hydroxy-1-naphthyl)imidazo[4,5-f][1,10]phenanthroline), Delta- and Lambda-[Ru(bpy)(2) (HNOIP)](PF6)(2) (Delta-3 and Lambda-3; HNOIP = 2-(2-hyd roxy-5-nitrophenyl)imidazo [4,5-f][1,10]phenanthroline), and Delta- and Lam bda-[Ru(bpy)(2)(DPPZ)](PF6)(2) (Delta-4 and Lambda-4; DPPZ = dipyridophenaz ine), have been synthesized. Binding behavior of these chiral complexes to calf thymus DNA (CT-DNA) has been investigated by electronic absorption, st eady-state emission, and circular dichroism spectroscopies, as well as by v iscosity measurements and equilibrium dialysis binding studies. Several poi nts came from the results. (1) The DNA-binding properties were distinctly d ifferent for the [Ru(bpy)(2)L](2+) (L = HPIP, HNAIP, HNOIP) series of ruthe nium(II) complexes, which indicates that the photophysical behavior of the complexes on binding to DNA can be modulated through ligand design. (2) Dif ferent binding rates of individual enantiomers of complexes 1 and 4 to DNA were observed through dialysis experiments. The Lambda enantiomer bound mor e rapidly than the Delta enantiomer and their different intercalative bindi ng geometries were suggested to be responsible. (3) Both Delta-2 and Lambda -2 bound weakly to CT-DNA; Delta-2 may bind through a partial intercalation mode, whereas Lambda-2 may bind in the DNA groove. (4) There was no notice able enantioselectivity for complexes 1, 3, and 4 on binding to CT-DNA. Bot h of their enantiomers can intercalate into DNA base pairs. It is noted tha t Delta-3 and Lambda-3 exhibited almost identical spectral changes on addit ion of CT-DNA, and a similar binding manner of the isomers to the double he lix was proposed.