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.

The syntheses, spectroscopic characterisation and in one case (X = Br) the single-crystal structure of the novel PdI-Pd' dimers [Pd2(p-X)2(PB~'3)2] (X = Br or I) have been determined; preliminary results on their reactions with CO, H,, CNC6H3Me2 and C2H2 have also been obtained.Zero oxidation state palladium compounds are efficient catalysts in several organic syntheses ' and an important step in the catalytic cycle is the oxidative addition of organic halides to the metal centre.2 Recently considerable interest has been shown in palladium(1) dimers such as I and I1 (see Scheme 1) as possible intermediates in these organic transformation^.^'^ Interestingly the corresponding simple halogen bridged dimers I11 have not previously been reported. We have recently established ' 1 6

To understand the molecular origins of diseases caused by ultraviolet and visible light, and also to develop photodynamic therapy, it is important to resolve the mechanism of photoinduced DNA damage. Damage to DNA bound to a photosensitizer molecule frequently proceeds by one-electron photo-oxidation of guanine, but the precise dynamics of this process are sensitive to the location and the orientation of the photosensitizer, which are very difficult to define in solution. To overcome this, ultrafast time-resolved infrared (TRIR) spectroscopy was performed on photoexcited ruthenium polypyridyl-DNA crystals, the atomic structure of which was determined by X-ray crystallography. By combining the X-ray and TRIR data we are able to define both the geometry of the reaction site and the rates of individual steps in a reversible photoinduced electron-transfer process. This allows us to propose an individual guanine as the reaction site and, intriguingly, reveals that the dynamics in the crystal state are quite similar to those observed in the solvent medium.

Strikingly different TRIR spectra are recorded for the complex in D2O or CD3CN or when DNA-bound.