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

The interaction of - and -[Ru(phen)(2)DPPZ](2+) (DPPZ=dipyrido[3,2-a:2, 3-c]phenazine, phen=phenanthroline) with a G-quadruplex formed from 5-G(2)T(2)G(2)TGTG(2)T(2)G(2-3)(15-mer) was investigated. The well-known enhancement of luminescence intensity (the light-switch' effect) was observed for the [Ru(phen)(2)DPPZ](2+) complexes upon formation of an adduct with the G-quadruplex. The emission intensity of the G-quadruplex-bound -isomer was 3-fold larger than that of the -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 -isomer. In the job plot of the G-quadruplex with - and -[Ru(phen)(2)DPPZ](2+), a major inflection point for the two isomers was observed at x approximate to .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 - or -[Ru(phen)(2)DPPZ](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.