Fast photoinduced electron transfer through DNA intercalation

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+] and acceptor bis(9,10-phenanthrenequinone diimine)(phenanthroline)rhodium(III) [Rh(phi)2phen3+] intercalate into DNA with Kb > 10(6) 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+(6), Ru(phen)2dppz2+ luminescence is quenched much less efficiently compared to that found for the intercalative Rh(phi)2phen3+ quencher and follows linear Stern-Volmer 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.