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DNA Sequence and Ancillary Ligand Modulate the Biexponential Emission Decay of Intercalated [Ru(L)2dppz]2+ Enantiomers



The bi-exponential emission decay of [Ru(L)2dppz]2+ (L=N,N′-diimine ligand) bound to DNA has been studied as a function of polynucleotide sequence, enantiomer, and nature of L (phenanthroline vs. bipyridine). The lifetimes (τi) and pre-exponential factors (αi) depend on all three parameters. With [poly(dA-dT)]2, the variation of αi with [Nu]/[Ru] has little dependence on L for Λ-[Ru(L)2dppz]2+ but a substantial dependence for Δ-[Ru(L)2dppz]2+. With [poly(dG-dC)]2, by contrast, the Λ-enantiomer αi values depend strongly on the nature of L, whereas those of the Δ-enantiomer are relatively unaffected. DNA-bound linked dimers show similar photophysical behaviour. The lifetimes are identified with two geometries of minor-groove intercalated [Ru(L)2dppz]2+, resulting in differential water access to the phenazine nitrogen atoms. Interplay of cooperative and anti-cooperative binding resulting from complex–complex and complex–DNA interactions is responsible for the observed variations of αi with binding ratio. [Ru(phen)2dppz]2+ emission is quenched by guanosine in DMF, which may further rationalise the shorter lifetimes observed with guanine-rich DNA.