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

Usings luminescence spectroscopy and single photon counting, photoinduced e lectron transfer (ET) reactions between photoexcited [M(phen)(2)dppz(2+) (p hen = 1,10-phenanthroline, dppz = dipyridophenazine, M = Ru or Os) and the electron accepters Rh(phi)(2)bpy(3+) (phi = 9,10-phenanthrenequinone diimin e, bpy = 2,2'-bipyridine) and Ru(NH3)(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 absorpt ion spectroscopy. The comparison of ET reactions of the isostructural donor s Os and Ru with an intercalated acceptor, Rh(phi)(2)bpy(3+), and an extern ally bound acceptor, Ru(NH3)(6)(3+), helps to elucidate which factors are i mportant for electron transfer between DNA-bound intercalators. Ru(phen)(2) dppz(2+) and Os(phen)(2)dppz(2+) show nearly identical quenching by Rh(phi) (2)bpy(3+) for a given DNA polymer, with an efficient quenching process tha t occurs on a time scale much faster than the excited state lifetime. We fi nd that Rh(phi)(2)bpy(3+) and Ru(NH3)(6)(3+) show opposite trends for quenc hing of DNA-bound M(phen)(2)dppz(2+) Quenching by intercalated Rh(phi)(2)bp y(3+) is most efficient in AT-only DNA polymers and less efficient in GC-on ly polymers, whereas for groove-bound Ru(NH3)(6)(3+), the reverse is observ ed. The intrinsic excited state lifetime of Ru(phen)(2)dppz(2+) bound to DN A and the luminescence quenching efficiency by Ru(NH3)(6)(3+) provide indic ators of the solvent accessibility of the DNA-bound dppz donor. On this bas is, we attribute the difference in ET reactivity among the various DNA poly mers to differences in how well M(phen)(2)dppz(2+) stacks in DNA. (C) 2000 Elsevier Science S.A. All rights reserved.