Rapid photoinduced electron transfer is demonstrated over a distance of greater than 40 angstroms between metallointercalators that are tethered to the 5' termini of a 15-base pair DNA duplex. An oligomeric assembly was synthesized in which the donor is Ru(phen)2dppz2+ (phen, phenanthroline, and dppz, dipyridophenazine) and the acceptor is Rh(phi)2phen3+ (phi, phenanthrenequinone diimine). These metal complexes are intercalated either one or two base steps in from the helix termini. Although the ruthenium-modified oligonucleotide hybridized to an unmodified complement luminesces intensely, the ruthenium-modified oligomer hybridized to the rhodium-modified oligomer shows no detectable luminescence. Time-resolved studies point to a lower limit of 10(9) per second for the quenching rate. No quenching was observed upon metallation of two complementary octamers by Ru(phen)3(2+) and Rh(phen)3(3+) under conditions where the phen complexes do not intercalate. The stacked aromatic heterocycles of the DNA duplex therefore serve as an efficient medium for coupling electron donors and acceptors over very long distances.

We report evidence for fast photoinduced electron transfer mediated by the DNA helix that requires metal complexes that are avid intercalators of DNA. Here the donor bis(phenanthroline)(dipyridophenazine)ruthenium (II) [Ru(phen)2dppz2+1 and acceptor bis(9,10-phenanthrenequinone diimine)(phenanthrofine)rhodium(IU) [Rh(phi)2phen3+] intercalate into DNA with Kb > 10' M-1. Luminescence quenching experiments in the presence of two different lengths of DNA yield upward-curving Stern-Volmer plots and the loss of luminescence intensity far exceeds the change in emission lifetimes. In the presence of a nonintercalative electron acceptor, Ru(NH3)3+, Ru(phen)2dppz2+ luminescence is quenched much less efficiently compared to that found for the intercalative Rh(phi)2phen3+ quencher and follows linear SternVolmer kinetics; steady-state and time-resolved Stern-Volmer plots are comparable in scale. These experiments are consistent with a model involving fast long-range electron transfer between intercalators through the DNA helix.Understanding electron transfer over long distances is essential to the characterization of fundamental redox processes such as oxidative phosphorylation and is surely critical to the design of artificial photosynthetic systems and electroactive sensors (1,2). Experiments in many laboratories have focused on measurements of electron transfer rates between metal centers over long distances in proteins' or protein pairs as a function of distance, driving force, and the intervening medium (3, 4). Although the notion of charge transfer in nucleic acids has been postulated for some time (5-7), only recently has DNA been examined as a medium for electron transfer reactions (8-11). Experiments with radiation-damaged DNA have suggested the importance of DNAmediated electron transfer with regard to nucleic acid-based disease. Studies of DNA under extreme conditions (77 K, neutron bombardment) have suggested that radical species can migrate up to 100 bp away from the initial lesion (12, 13). Pulse radiolysis experiments of the cytotoxin daunorubicin intercalated into DNA reveal that this electronic migration is comparable in rate to excess electron mobility in conducting polymers (14). This dissipation of charge may actually be a mechanism by which redox damage to DNA at localized sites may be avoided.We have previously found that the rate of electron transfer between transition metal complexes is enhanced in the presence of DNA. Cationic tris(phenanthroline)metal complexes were used as donor-acceptor pairs (8, 10) since these complexes had been shown to bind to DNA noncovalently (Kb 5 x 103 M-1) through primarily two modes: (i) intercalation and (ii) surface binding (15)(16)(17)(18). In these experiments, the donor was photoexcited Ru(L)3+, while the acceptors were M(L)3+, M = Rh(III), Co(III), or Cr(III) and L = 1,10-phenanthroline or 2,2'-bipyridine. How DNA might mediate these electron transfer processes was difficult to discern in part because of the rapid equilibration between binding modes a...

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