Synthesis, characterization and DNA-binding properties of rac-[Ru(5,6-dmp)₂(dppz)]²⁺ - enantiopreferential DNA binding and co-ligand promoted exciton coupling

Abstract

The new mixed ligand complex [Ru(5,6-dmp)₂(dppz)]Cl₂ [5,6-dmp = 5,6-dimethyl-1,10-phenanthroline, dppz = dipyrido[3,2-a:2',3'-c]phenazine] has been isolated and its DNA-binding properties studied by employing UV-visible (UV-Vis), steady-state and time-resolved emission and circular dichroism spectral methods, viscometry, thermal denaturation and cyclic/differential pulse voltammetric techniques. The complex acts as a 'molecular light-switch' on binding to DNA, but the enhancement in emission intensity is only 75% of that of the parent complex [Ru(phen)₂(dppz)]²⁺ (phen = 1,10-phenanthroline). The emission decay curves and quenching studies suggest two different DNA-binding modes both involving intercalation of the dppz ligand of [Ru(5,6-dmp)₂(dppz)]Cl₂. The characteristic red-shift of the induced CD signal, which is not observed for the phen analogue, arises from exciton coupling. The hydrophobicity and polarizability of 5,6-dmp co-ligand strongly favour the formation of a stable structural and electronic scaffold on the DNA surface for the unbound molecules to couple with the DNA-bound complexes facilitating spontaneous assembly of novel extended molecular aggregates using DNA as a helical nanotemplate. This observation is consistent with the shift in Ru(II)/Ru(III) redox potential to more positive values with a dramatic drop in peak current on binding of the 5,6-dmp complex to calf thymus (CT) DNA. Equilibrium dialysis experiments monitored by CD spectroscopy unambiquously reveal the preferential binding of the Δ-enantiomer to the right-handed calf thymus (CT) DNA. The 5,6-dmp complex exhibits preferential binding to [d(AT)₆]₂ over [d(GC)₆]₂ and the complex aggregates formed consist of six [Ru(5,6-dmp)₂(dppz)]²⁺ cations per base pair of [d(AT)₆]₂; however, only one [Ru(phen)₂(dppz)]²⁺ cation per base pair is involved in DNA binding.