Enantioselective luminescence quenching of DNA light-switch [Ru(phen) 2dppz]2+ by electron transfer to structural homologue [Ru(phendione)2dppz]2+

Journal article, 2005

The quenching of the luminescence of [Ru(phen)2dppz] 2+ by structural homologue [Ru(phendione)2dppz] 2+, when both complexes are bound to DNA, has been studied for all four combinations of Δ and Λ enantiomers. Flow linear dichroism spectroscopy (LD) indicates similar binding geometries for all the four compounds, with the dppz ligand fully intercalated between the DNA base pairs. A difference in the LD spectrum observed for the lowest-energy MLCT transition suggests that a transition, potentially related to the final localization of the excited electron to the dppz ligand in [Ru(phen)2dppz]2+, is overlaid by an orthogonally polarized transition in [Ru(phendione) 2dppz]2+. This would be consistent with a low-lying LUMO of the phendione moiety of [Ru(phendione)2dppz]2+ that can accept the excited electron from [Ru(phen)2dppz]2+, thereby quenching the emission of the latter. The lifetime of excited A-[Ru(phen)2dppz]2+ is decreased moderately, from 664 to 427 ns, when bound simultaneously with the phendione complex to DNA. The 108 ns lifetime of opposite enantiomer, A-[Ru(phen)2dppz]2+, is only shortened to 94 ns. These results are consistent with an average rate constant for electron transfer of approximately 1·106 s -1 between the phenanthroline- and phendione-ruthenium complexes. At binding ratios close to saturation of DNA, the total emission of the two enantiomers is lowered equally much, but for the A enantiomer, this is not paralleled by a decrease in luminescence lifetime. A binding isotherm simulation based on a generalized McGhee - von Hippel approach shows that the Δ enantiomer binds approximately 3 times stronger to DNA both for [Ru(phendione)2dppz]2+ and [Ru(phen)2dppz] 2+. This explains the similar decrease in total emission, without the parallel decrease in lifetime for the A enantiomer. The simulation also does not indicate any significant binding cooperativity, in contrast to the case when Δ-[Rh(phi)2bipy]3+ is used as quencher. The very slow electron transfer from [Ru-(phen)2dppz]2+ to [Ru(phendione)2dppz]2+, compared to the case when [Rh(phi)2phen]3+ is the acceptor, can be explained by a much smaller driving free-energy difference.