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

The interaction of Δ- and Λ-[Ru(phen)2DPPZ]2+ (DPPZ = dipyrido[3,2-a:2′, 3′-c]phenazine, phen = phenanthroline) with a G-quadruplex formed from 5′-G2T2G2TGTG2T2G2–3′(15-mer) was investigated. The well-known enhancement of luminescence intensity (the ‘light-switch’ effect) was observed for the [Ru(phen)2DPPZ]2+ 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)2DPPZ]2+, 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)2DPPZ]2+, 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)6]4- 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.