Luminescence of ruthenium(II) polypyridyls: Evidence for intercalative binding to Z-DNA

Photophysical studies have been undertaken to characterize the binding interactions of enantiomers of Ru(phen)₃²⁺, Ru(DIP)₃²⁺, and racemic Ru(bpy)₂dppz²⁺ (where phen = 1,10-phenanthroline, DIP = 4,7-diphenylphenanthroline, and dppz = dipyridophenazine) with Z-form poly d(GC). Parallel enhancements in steady state luminescent intensity and a lengthening of luminescent lifetimes are seen for ruthenium enantiomers with Z-DNA as for B-DNA but with enantioselectivities reversed. Greater enhancements are seen for Δ-isomers with the right-handed helix but for Δ-isomers with the left-handed helix. Ru(bpy)₂dppz²⁺, an avid intercalator in B-DNA, displays no luminescence free in aqueous solution, but luminesces brightly bound to either B- or Z-poly d(GC). Stern-Volmer quenching studies also support the enantioselective preference in binding to B-DNA by Δ-isomers and a reversal with binding to Z-DNA preferentially by the Δ-isomers. Steady state polarization studies indicate a rigid association of the complexes with both B-and Z-DNA on the time-scale of their emission and again with symmetrical enantioselectivities for the left and right-handed helices. Given the well characterized intercalative association of the complexes with B-DNA, the parallel results seen here with Z-DNA point strongly to a comparable intercalative association with the Z-form helix. That molecules may interact with Z-DNA through intercalation has not been demonstrated previously and now requires consideration in describing the range of interactions of small molecules and proteins with Z-DNA.