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

Δ

- and Λ-[Ru(bpy)₂(HPIP)](PF₆)₂ (Δ-1 and Λ-1; bpy =2,2′-bipyridine, HPIP=2-(2-hydroxyphenyl)imidazo[4,5-f][1,10]phenan-throline), Δ- and Λ-[Ru(bpy)₂(HNAIP)](PF₆)₂ (Δ-2 and Λ-2; HNAIP=2-(2-hydroxy-1-naphthyl)imidazo[4,5-f][1,10]phenanthroline), Δ- and Λ-[Ru(bpy)₂ (HNOIP)](PF₆)₂ (Δ-3 and Λ-3; HNOIP=2-(2-hydroxy-5-nitrophenyl)imidazo[4,5-f][1,10]phenanthroline), and Δ- and Λ-[Ru(bpy)₂(DPPZ)](PF₆)₂ (Δ-4 and Λ-4; DPPZ=dipyridophenazine), have been synthesized. Binding behavior of these chiral complexes to calf thymus DNA (CT-DNA) has been investigated by electronic absorption, steady-state emission, and circular dichroism spectroscopies, as well as by viscosity measurements and equilibrium dialysis binding studies. Several points came from the results. (1) The DNA-binding properties were distinctly different for the [Ru(bpy)₂L]²⁺ (L=HPIP, HNAIP, HNOIP) series of ruthenium(II) complexes, which indicates that the photophysical behavior of the complexes on binding to DNA can be modulated through ligand design. (2) Different binding rates of individual enantiomers of complexes 1 and 4 to DNA were observed through dialysis experiments. The Λ enantiomer bound more rapidly than the Δ enantiomer and their different intercalative binding geometries were suggested to be responsible. (3) Both Δ-2 and Λ-2 bound weakly to CT-DNA; Δ-2 may bind through a partial intercalation mode, whereas Λ-2 may bind in the DNA groove. (4) There was no noticeable enantioselectivity for complexes 1, 3, and 4 on binding to CT-DNA. Both of their enantiomers can intercalate into DNA base pairs. It is noted that Δ-3 and Λ-3 exhibited almost identical spectral changes on addition of CT-DNA, and a similar binding manner of the isomers to the double helix was proposed.