The crystal structure of a brominated oligonucleotide d(CGCG(Br)CG), chemical formula C(114)N(48)O(68)P(10)Br(2), has been analysed by multiwavelength anomalous dispersion (MAD) methods. The oligonucleotide crystallizes in space group P2(1)2(1)2(1) with a = 17.97, b = 30.98, c = 44.85 A, alpha = beta = gamma 90 degrees . Data to a resolution of 1.65 A were collected at four wavelengths about the K-absorption edge of the bromine atom (lambda(1) = 0.9323 A, a reference wavelength at the long-wavelength side of the edge; lambda(2) = 0.9192 A, at the absorption-edge inflection point; lambda(3) = 0.9185 A, at the ;white line' absorption maximum; lambda(4) = 0.8983 A, a reference wavelength at the short-wavelength side) using synchrotron radiation at Station PX9.5, SRS, Daresbury. Multiwavelength data could be collected on a single-crystal as the sample was radiation stable. Anomalous and dispersive Patterson maps were readily interpretable to give the bromine anomalous scatterer positions. Phase calculations to 1.65 A, resolution, using all four wavelengths, gave a figure of merit of 0.825 for 2454 reflections. The electron-density map was readily interpretable showing excellent connectivity for the sugar/phosphate backbone and each base was easily characterized. The two nucleotide strands paired up as expected in an antiparallel Watson-Crick-type manner. The structure was refined to 1.65 A using all the data (R-factor = 17.0% based on 3151 reflections, with a data-to-parameter ratio of 2.6). In addition to the four-wavelength analysis, a variety of other phasing strategies, and the associated quality of the resulting electron-density maps, were compared. These included use of either of the reference wavelength data sets in the two possible three-wavelength phasing combinations to assess their relative effectiveness. Moreover, the time dependence upon measuring the Bijvoet differences and its effect upon phasing was also investigated. Finally, the use of only two wavelengths, including Friedel pairs, is demonstrated (the theoretical minimum case); this is of particular interest when considering overall beam time needs and is clearly a feasible experimental strategy, as shown here.

Large area flat crystal x-ray spectrometer with high integrated intensity for an electron beam ion trap Rev. Sci. Instrum. 71, 4065 (2000)

OppA is a 58.8 kDa bacterial transport protein involved in the transport of peptides across the cytoplasmic membrane of Gram-negative bacteria. It binds peptides from two to five residues in length but with little sequence specificity. OppA from Salmonella typhimurium has been cloned and expressed in E. coli and the protein cocrystallized with uranyl acetate, producing two distinct crystal forms with different uranium sites. Multiple-wavelength data collected about the uranium L(III) edge have been collected at the Daresbury Synchrotron Radiation Source (SRS) to a nominal resolution limit of 2.3 A. Maximum-likelihood phasing methods have been used in phase determination from the multiple-wavelength data giving a readily interpretable electron-density map, without any density modification. The electron-density map, calculated at 2.3 A resolution shows OppA to be a bilobal, principally beta-stranded, three-domain protein. The tri-lysine ligand molecule can be clearly seen in the peptide-binding site between the two lobes.

The small-angle scattering facility at Daresbury has been constructed for diffraction studies of a wide range of naturally occurring and synthetic materials. The high brightness of the SRS is combined with focusing optics, resulting in exposure times that can be two or three orders of magnitude less than those required on a conventional source. Spacings of 2000 A in the vertical direction and 300 A in the horizontal direction can be observed, while the resolution between diffraction orders is 5000 and 600/~. In addition, preliminary results have been obtained on a double-crystal diffractometer that has a resolution, in one dimension, of better than 26000A. For highangle fibre diffraction studies, a camera with pinhole collimation has been constructed. Examples from solution scattering and fibre diffraction are used to illustrate the performance of these facilities.

X-ray data have been collected from a monoclinic hen egg-white lysozyme (HEWL) crystal, rapidly frozen at 120 K on the end of a glass fiber, using high-intensity synchrotron X-radiation. A comparison of oscillation photographs taken at the beginning and end of data collection shows no evidence of radiation damage even though the crystal was exposed in the synchrotron beam for -8 h. Not only are the low-resolution data unaffected by radiation damage but the high-resolution data remain unaffected as well. There is also a marked improvement in the resolution of the diffraction pattern of the rapidly frozen crystal when compared to a similarly sized crystal irradiated in a capillary tube at room temperature.