The dramatically different DNA-binding properties of the two isomeric forms of a photochromic spiropyran have been demonstrated, enabling photoswitched DNA binding. The closed, UV-absorbing form shows no signs of interaction with DNA. Upon UV exposure the spiropyran is isomerized to the open form that binds to DNA by intercalation. The process is fully reversible as the corresponding dissociation process is induced by visible light.

Despite the extensive interest in structurally explaining the photophysics of DNA-bound [Ru(phen)(2)dppz](2+) and [Ru(bpy)(2)dppz](2+), the origin of the two distinct emission lifetimes of the pure enantiomers when intercalated into DNA has remained elusive. In this report, we have combined a photophysical characterization with a detailed isothermal titration calorimetry study to investigate the binding of the pure Δ and Λ enantiomers of both complexes with [poly(dAdT)](2). We find that a binding model with two different binding geometries, proposed to be symmetric and canted intercalation from the minor groove, as recently reported in high-resolution X-ray structures, is required to appropriately explain the data. By assigning the long emission lifetime to the canted binding geometry, we can simultaneously fit both calorimetric data and the binding-density-dependent changes in the relative abundance of the two emission lifetimes using the same binding model. We find that all complex-complex interactions are slightly unfavorable for Δ-[Ru(bpy)(2)dppz](2+), whereas interactions involving a complex canted away from a neighbor are favorable for the other three complexes. We also conclude that Δ-[Ru(bpy)(2)dppz](2+) preferably binds isolated, Δ-[Ru(phen)(2)dppz](2+) preferably binds as duplets of canted complexes, and that all complexes are reluctant to form longer consecutive sequences than triplets. We propose that this is due to an interplay of repulsive complex-complex and attractive complex-DNA interactions modulated by allosteric DNA conformation changes that are largely affected by the nature of the ancillary ligands.

A spiropyran photoswitch is activated using UV light and protons from a form that shows no interaction with DNA to a form that binds to DNA by intercalation. This scheme is interpreted as a biologically relevant logic AND gate with potential applications as a dually controlled anticancer drug.

The binuclear ruthenium complex [μ-bidppz(phen)(4)Ru(2)](4+) has been extensively studied since the discovery of its unusual threading intercalation interaction with DNA, a binding mode with extremely slow binding and dissociation kinetics. The complex has been shown to be selective towards long stretches of alternating AT base pairs, which makes it interesting, for example, as a model compound for anti-malaria drugs due to the high AT content of the genome of the malaria parasite P. falciparum. We have investigated the effect of bridging ligand structure on threading intercalation ability and found that length and rigidity as well as the size of the intercalated ring system are all factors that affect the rate and selectivity of the threading intercalation. In particular, we discovered a new DNA-threading compound, [μ-dppzip(phen)(4)Ru(2)](4+), which appears to be just at the border of being capable of threading intercalation and displays even greater selectivity for AT-DNA than the parent compound, [μ-bidppz(phen)(4)Ru(2)](4+).

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.