Fast photoinduced electron transfer through DNA intercalation.

Abstract

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...

Charge transfer and transport in DNA

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Photoelectrochemical Detection of Oxidative DNA Damage Induced by Fenton Reaction with Low Concentration and DNA-Associated Fe2+

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A New Mathematical Model for Calculating the Electronic Coupling of a B-DNA Molecule

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