DNA binding of .DELTA.- and .LAMBDA.-[Ru(phen)2DPPZ]2+

Linear dichroism (LD) spectroscopy and steady-state as well as time-resolved luminescence spectroscopy have been used to investigate the interaction of the DELTA and LAMBDA enantiomers of Ru(phen)2DPPZ2+ (phen = 1,10-phenanthroline; DPPZ = dipyrido[3,2-a:2',3'-c]phenazine) with DNA. The pure enantiomers, which were difficult to separate by traditional resolving methods, were synthesized via a chiral precursor. Changes in luminescence, isotropic absorption and excited state lifetimes upon binding, and the LD observed in flow-oriented DNA systems provide detailed information about the DNA binding of the enantiomers. Flow LD shows that both enantiomers bind to DNA in a well-defined manner with an orientation of the dipyridophenazine chromophore consistent with intercalation of this moiety between base-pairs. Both enantiomers are found to show luminescence in the presence of DNA to which they bind very strongly (K almost-equal-to 10(8) M-1); however, the relative luminescence quantum yield of the bound DELTA enantiomer is 6-10 times larger than that of the bound LAMBDA enantiomer. Furthermore, for each enantiomer two distinct excited state lifetimes are found in varying proportions depending on the binding ratio. The large difference in luminescence quantum yield between the enantiomers is interpreted in terms of slightly different intercalation geometries of the dipyridophenazine ligand, resulting in different protections from quenching by solvent water and diastereomeric differences in the interactions between enantiomers bound in contigue on DNA.