Ruthenium(II) complexes of redox-related, modified dipyridophenazine ligands: synthesis, characterization, and DNA interaction

Abstract

The synthesis, spectral characterization, and electrochemical properties of [Ru(phen)₂(qdppz)]²⁺, which incorporates a quinone-fused dipyridophenazine ligand (naphtho[2,3-α]dipyrido[3,2-h:2‘,3‘-f]phenazine-5,18-dione, qdppz), are described in detail. Chemical or electrochemical reduction of [Ru(phen)₂(qdppz)]²⁺ leads to the generation of [Ru(phen)₂(hqdppz)]²⁺a complex containing the hydroquinone form (hqdppz = 5,18-dihydroxynaphtho[2,3-a]dipyrido[3,2-h:2‘,3‘-f]phenazine) of qdppz. Absorption and viscometric titration, thermal denaturation, topoisomerase assay, and differential-pulse voltammetric studies reveal that [Ru(phen)₂(qdppz)]²⁺ is an avid binder of calf-thymus DNA due to a strong intercalation by the ruthenium-bound qdppz, while [Ru(phen)₂(hqdppz)]²⁺ binds to DNA less strongly than the parent "quinone"-containing complex. DNA-photocleavage efficiencies of these complexes also follow a similar trend in that the MLCT-excited state of [Ru(phen)₂(qdppz)]²⁺ is more effective than that of [Ru(phen)₂(hqdppz)]²⁺ in cleaving the supercoiled plasmid pBR 322 DNA (λ_exc = 440 ± 5 nm), as revealed by the results of agarose gel electrophoresis experiments. The photochemical behaviors of both the quinone- and hydroquinone-appended ruthenium(II) complexes in the presence of DNA not only provide valuable insights into their modes of binding with the duplex but also lead to detailed investigations of their luminescence properties in nonaqueous, aqueous, and aqueous micellar media. On the basis of the results obtained, (i) a photoinduced electron transfer from the MLCT state to the quinone acceptor in Ru(phen)₂(qdppz)]²⁺ and (ii) quenching of the excited states due to proton transfer from water to the dipyridophenazine ligand in both complexes are invoked to rationalize the apparent lack of emission of these redox-related complexes in the DNA medium.