Another step toward understanding the binding mode of other derivatives of [Ru(phen)₂L] complexes

Linear dichroism (LD), UV-Vis and fluorescence spectroscopy were used to investigate the interaction of the Δ- and Λ-enantiomers of the two complexes of [Ru(phen)₂ 2-phenylimidazophenanthroline]⁺² (referred to as B-complex) and [Ru(phen)₂ 2-(1-pyrenyl)limidazophenanthroline]⁺² (named as P-complex), with ct-DNA. Absorption titration experiments of the enantiomers showed significant hypochromism with red shift indicating a strong perturbation of the DNA on the intra-ligand (IL) and the metal-to-ligand charge transfer (MLCT) transitions of the complexes, and fluorescence titration curves indicate strong binding. LD spectra for the B-enantiomers bound to DNA were qualitatively similar to spectra for the well-studied intercalator [Ru(phen)₂dppz]⁺², indicating intercalation of the phenyl-imidazophenanthroline part of the complex. However, for Λ-P, the LD-spectrum resembled Δ-B, and Δ-P resembled Λ-B, indicating a mirror image relationship of the transition moment directions relative to the helix axis when going from B to P. For interpretation of the data, and clarifying the nature of the binding mode of the P-complexes, a model was proposed based on the crystal structure previously presented by the Cardin group. The reversed LD is suggested to be due to partial intercalation of the pyrene moiety, pushing the Ru(phen)₂-moiety out from the opposite groove and allowing it to rotate anticlockwise around the pyrene-imidazole bond.Communicated by Ramaswamy H. Sarma.