DNA sequence and ancillary ligand modulate the biexponential emission decay of intercalated [Ru(L)2dppz]2+...

The bi-exponential emission decay of [Ru(L)(2)dppz](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 (τ(i)) and pre-exponential factors (α(i)) depend on all three parameters. With [poly(dA-dT)](2), the variation of α(i) with [Nu]/[Ru] has little dependence on L for Λ-[Ru(L)(2)dppz](2+) but a substantial dependence for Δ-[Ru(L)(2)dppz](2+). With [poly(dG-dC)](2), by contrast, the Λ-enantiomer α(i) 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)(2) dppz](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 α(i) with binding ratio. [Ru(phen)(2)dppz](2+) emission is quenched by guanosine in DMF, which may further rationalise the shorter lifetimes observed with guanine-rich DNA.