Electron transfer between metal complexes bound to DNA: variations in sequence, donor, and metal binding mode

Using luminescence spectroscopy and single photon counting, photoinduced electron transfer (ET) reactions between photoexcited [M(phen)_2dppz ^(2+) (phen = 1,10-phenanthroline, dppz=dipyridophenazine, M=Ru or Os) and the electron acceptors Rh(phi)_2bpy^(3+) (phi=9,10-phenanthrenequinone diimine, bpy=2,2′-bipyridine) and Ru(NH_3)_6^(3+) were studied as a function of DNA sequence in long DNA polymers. In addition, the thermal back reactions between M(III) and reduced acceptor were also followed by transient absorption spectroscopy. The comparison of ET reactions of the isostructural donors Os and Ru with an intercalated acceptor, Rh(phi)_2bpy^(3+), and an externally bound acceptor, Ru(NH_3)_6^(3+), helps to elucidate which factors are important for electron transfer between DNA-bound intercalators. Ru(phen)_2dppz^(2+) and Os(phen)_2dppz^(2+) show nearly identical quenching by Rh(phi)_2bpy^(3+) for a given DNA polymer, with an efficient quenching process that occurs on a time scale much faster than the excited state lifetime. We find that Rh(phi)_2bpy^(3+) and Ru(NH_3)_6^(3+) show opposite trends for quenching of DNA-bound M(phen)_2dppz^(2+). Quenching by intercalated Rh(phi)_2bpy^(3+) is most efficient in AT-only DNA polymers and less efficient in GC-only polymers, whereas for groove-bound Ru(NH_3)_6^(3+), the reverse is observed. The intrinsic excited state lifetime of Ru(phen)_2dppz^(2+) bound to DNA and the luminescence quenching efficiency by Ru(NH_3)_6^(3+) provide indicators of the solvent accessibility of the DNA-bound dppz donor. On this basis, we attribute the difference in ET reactivity among the various DNA polymers to differences in how well M(phen)_2dppz^(2+) stacks in DNA.