Electronic effect of different positions of the –NO2 group on the DNA-intercalator of chiral complexes [Ru(bpy)2L]2+ (L = o-npip, m-npip and p-npip)

Abstract

New chiral Ru(II) complexes with intercalators L (L = o-npip, m-npip and p-npip) containing –NO₂ at different positions on the phenyl ring were synthesized and characterized by elemental analysis, ¹H NMR, ESI-MS and CD spectra. The DNA binding properties of these complexes have been investigated with UV-Vis, emission spectra, CD spectra and viscosity measurements. A subtle but detectable difference was observed in the interaction of these isomers with CT-DNA. Absorption spectroscopy experiments indicated that each of these complexes can interact with the DNA. The DNA-binding of the Δ-isomer is stronger than that of Λ-isomer. DNA-viscosity experiments provided evidence that both Δ- and Λ-[Ru(bpy)₂(o-npip)](PF₆)₂ bind to DNA with partial intercalation, and both Δ- and Λ-[Ru(bpy)₂(p-npip)](PF₆)₂ fully intercalate with DNA. However, Δ- and Λ- [Ru(bpy)₂(m-npip)](PF₆)₂ bind to DNA through different modes, i.e., the Δ isomer by intercalation and Λ isomer by partial intercalation. Under irradiation with UV light, Ru(II) complexes showed different efficiency of cleaving DNA. The most interesting feature is that neither 1 (Δ-1 and Λ-1) nor 3 (Δ-3 and Λ-3) emit luminescence either alone in aqueous solution or in the presence of DNA, whereas both Δ-2 and Λ-2 emit luminescence under the same conditions. In addition, theoretical calculations for these three isomer complexes have been carried out applying the density functional theory (DFT) method at the level of the B3LYP/LanL2DZ basis set, and the calculated results can reasonably explain the obtained experimental trends in the DNA-binding affinities or binding constants (K_b) and some spectral properties of the complexes.