Charge separation in a ruthenium-quencher conjugate bound to DNA

A novel tris heteroleptic dipyridophenazine complex of ruthenium(II), [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+, containing a covalently tethered ruthenium pentammine quencher coordinated through a bridging histidine has been synthesized and characterized spectroscopically and biochemically in a DNA environment and in organic solvent. Steady-state and time-resolved luminescence measurements indicate that the tethered Ru complex is quenched relative to the parent complexes [Ru(phen)(dppz)(bpy')]2+ and [Ru(phen)(dppz)(bpy'-his)]2+ in DNA and acetonitrile, consistent with intramolecular photoinduced electron transfer. Intercalated into guanine-containing DNA, [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+, upon excitation and intramolecular quenching, is capable of injecting charge into the duplex based upon the EPR detection of guanine radicals. DNA-mediated charge transport is also indicated using a kinetically fast cyclopropylamine-substituted base as an electron hole trap. Guanine damage is not observed, however, in measurements using the guanine radical as the kinetically slower hole trap, indicating that back electron-transfer reactions are competitive with guanine oxidation. Moreover, transient absorption measurements reveal a novel photophysical reaction pathway for [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+ in the presence of DNA that is competitive with the intramolecular flash-quench process. These results illustrate the remarkably rich redox chemistry that can occur within a bimolecular ruthenium complex intercalated in duplex DNA.