We report an atomic resolution X-ray crystal structure containing both enantiomers of rac-[Ru(phen)2dppz](2+) with the d(ATGCAT)2 DNA duplex (phen = phenanthroline; dppz = dipyridophenazine). The first example of any enantiomeric pair crystallized with a DNA duplex shows different orientations of the Λ and Δ binding sites, separated by a clearly defined structured water monolayer. Job plots show that the same species is present in solution. Each enantiomer is bound at a TG/CA step and shows intercalation from the minor groove. One water molecule is directly located on one phenazine N atom in the Δ-enantiomer only.

[Ru(phen)2(dppz)]2+ has been studied since the 1990s due to its ‘light-switch’ properties. It can be used as a luminescent DNA probe, with emission switched on through DNA binding. The luminescence observed is dependent on the solvent accessibility of the pyrazine nitrogen atoms, and therefore is sensitive to changes in both binding site of the cation and chromophore orientation. The compound is also chiral, and there are distinct differences between the enantiomers in terms of the emission behaviour when bound to a variety of DNA sequences. Whilst a number of binary DNA-complex X-ray crystal structures are available, most include the Λ enantiomer and there is very little structural information about binding of the Δ enantiomer. Here, we present the first X-ray crystal structure of a Δ enantiomer bound to well-matched DNA, in the absence of the other, Λ enantiomer. We show how the binding site observed here can be related to a more general pattern of motifs in the crystallographic literature and propose that the Δ enantiomer can bind with five different binding modes, offering a new hypothesis for the interpretation of solution data.

Polypyridyl ruthenium complexes have been intensively studied and possess photophysical properties which are both interesting and useful. They can act as probes for DNA, with a substantial enhancement in emission when bound, and can induce DNA damage upon photoirradiation. Therefore the synthesis and characterization of DNA binding of new complexes is an area of intense research activity. While the knowledge of how the binding of derivatives compares to the parent compound is highly desirable, this information can be difficult to obtain. Here we report the synthesis of three new methylated complexes, [Ru(TAP) 2 (dppz-10-Me).2Cl, [Ru(TAP) 2 (dppz-10,12-Me 2)].2Cl and [Ru(TAP) 2 (dppz-11-Me)].2Cl, and examine the consequences for DNA binding through atomic resolution X-ray crystallography. We find that the methyl groups are located in discrete positions with a complete directional preference which may help to explain the quenching behavior which is found in solution.

The crystal structure of the ruthenium DNA ‘light-switch’ complex Λ -[Ru(TAP) 2 (11-Cl-dppz)] 2+ (TAP=tetraazaphenanthrene, dppz=dipyrido[3,2- a ′:2′,3′- c ]phenazine) bound to the oligonucleotide duplex d(TCGGCGCCGA) 2 is reported. The synthesis of the racemic ruthenium complex is described for the first time, and the racemate was used in this study. The crystal structure, at atomic resolution (1.0 Å), shows one ligand as a wedge in the minor groove, resulting in the 51 ° kinking of the double helix, as with the parent Λ -[Ru(TAP) 2 (dppz)] 2+ . Each complex binds to one duplex by intercalation of the dppz ligand and also by semi-intercalation of one of the orthogonal TAP ligands into a second symmetrically equivalent duplex. The 11-chloro substituent binds with the major component (66%) oriented with the 11-chloro substituent on the purine side of the terminal step of the duplex.

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