Enantioselective light switch effect of Δ- and Λ-[Ru(phenanthroline) dipyrido[3,2-a:2', 3'-c]phenazine] bound to G-quadruplex DNA.

The interaction of Δ- and Λ-[Ru(phen)DPPZ] (DPPZ = dipyrido[3,2-a:2', 3'-c]phenazine, phen = phenanthroline) with a G-quadruplex formed from 5'-GTGTGTGTG'(15-mer) was investigated. The well-known enhancement of luminescence intensity (the 'light-switch' effect) was observed for the [Ru(phen)DPPZ] complexes upon formation of an adduct with the G-quadruplex. The emission intensity of the G-quadruplex-bound Λ-isomer was 3-fold larger than that of the Δ-isomer when bound to the G-quadruplex, which is opposite of the result observed in the case of double stranded DNA (dsDNA); the light switch effect is larger for the dsDNA-bound Δ-isomer. In the job plot of the G-quadruplex with Δ- and Λ-[Ru(phen)DPPZ], a major inflection point for the two isomers was observed at x ≈ .65, which suggests a binding stoichiometry of 2:1 for both enantiomers. When the G base at the 8th position was replaced with 6-methyl isoxanthopterin (6MI), a fluorescent guanine analog, the excited energy of 6-MI transferred to bound Δ- or Λ-[Ru(phen)DPPZ], which suggests that at least a part of both Ru(II) enantiomers is close to or in contact with the diagonal loop of the G-quadruplex. A luminescence quenching experiment using [Fe(CN)] for the G-quadruplex-bound Ru(II) complex revealed downward bending curves for both enantiomers in the Stern-Volmer plot, which suggests the presence of Ru(II) complexes that are both accessible and inaccessible to the quencher and may be related to the 2:1 binding stoichiometry.