Interaction of rac-[Ru(5,6-dmp)₃]²⁺ with DNA:  Enantiospecific DNA Binding and Ligand-Promoted Exciton Coupling

The X-ray crystal structure of the complex rac-[Ru(5,6-dmp)₃]Cl₂ (5,6-dmp = 5,6-dimethyl-1,10-phenanthroline) reveals a distorted octahedral coordination geometry with the Ru−N bond distances shorter than in its phen analogue. Absorption spectral titrations with CT DNA reveal that rac-[Ru(5,6-dmp)₃]²⁺ interacts (K_b, (8.0 ± 0.2) × 10⁴ M^-1) much more strongly than its phen analogue. The emission intensity of the 5,6-dmp complex is dramatically enhanced on binding to DNA, which is higher than that of the phen analogue. Also, interestingly, time-resolved emission measurements on the DNA-bound complex shows biexponential decay of the excited states with the lifetimes of short- and long-lived components being higher than those for the phen analogue. The CD spectral studies of rac-[Ru(5,6-dmp)₃]²⁺ bound to CT DNA provide a definite and elegant evidence for the enantiospecific interaction of the complex with B-form DNA. Competitive DNA binding studies using rac-[Ru(phen)₃]²⁺ provide support for the strong binding of the complex with DNA. The Δ-enantiomer of rac-[Ru(5,6-dmp)₃]²⁺ binds specifically to the right-handed B-form of poly d(GC)₁₂ at lower ionic strength (0.05 M NaCl), and the Λ-enantiomer binds specifically to the left-handed Z-form of poly d(GC)₁₂ generated by treating the B-form with 5 M NaCl. The strong electronic coupling of the DNA-bound complex with the unbound complex facilitates the change in its enantiospecificity upon changing the conformation of DNA. The ¹H NMR spectra of rac-[Ru(5,6-dmp)₃]²⁺ bound to poly d(GC)₁₂ reveal that the complex closely interacts most possibly in the major grooves of DNA. Electrochemical studies using ITO electrode show that the 5,6-dmp complex stabilizes CT DNA from electrocatalytic oxidation of its guanine base more than the phen analogue does.