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  • 1
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Crystals have been obtained of succinylated concanavalin A complexed to a novel bidentate synthetic ligand. The crystals are the first example of a lectin with a synthetic multivalent ligand and the first report of crystallization of succinylated concanavalin A. The crystals were obtained by sitting-drop vapour diffusion equilibrating with a solution of 20% polyethylene glycol, pH 5, 293.5 K. Crystals are orthorhombic, belonging to space group C2221 with unit-cell dimensions of a = 99.1, b = 127.4, c = 118.9 Å. The asymmetric unit contains a dimer, with over 65% of the volume occupied by water. The ligand cross links concanavalin A monomers. Succinylated concanavalin A is known to be a dimer in solution, yet it is found as the typical concanavalin A tetramer in the crystal. The contacts holding together the tetramer appear extensive and suggest that a fine balance between dimer and tetramers exists. Data to 2.65 Å have been collected and the structure determined by the molecular replacement method.
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  • 2
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In the paper by McMahon, Leonard, Buchanan, Giraud & Naismith [Acta Cryst. (1999). D55, 399–402] an author's error has resulted in the fifth sentence of the Abstract being incorrect. The sentence should read `They are monoclinic, space group P21, with unit-cell dimensions a = 71.12, b = 58.42, c = 96.38 Å, β = 96.38°. 92% (native) and 94% (selenomethionine) complete data sets have been recorded to 2.9 Å (Rmerge = 5.0%) and 3.0 Å (Rmerge = 6.9%), respectively.'
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 49 (1993), S. 418-423 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 1 (1994), S. 69-73 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The sample volume in diamond-anvil pressure cells suitable for X-ray powder diffraction studies is very small (≤ 100 μm across). The resulting low signal-to-noise ratio has made it very difficult to obtain useful results with monochromatic angle-dispersive techniques, and the alternative white- beam energy-dispersive techniques have limited resolution and generally give unreliable peak intensities. The situation has been transformed recently by the introduction of the image-plate two-dimensional detector, which allows angle-dispersive methods to be used with a greatly increased signal and improved powder averaging. A short review is given of this development, the experimental techniques, and the principal advantages, particularly as found in results obtained at SRS Daresbury over the past two or three years.
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  • 5
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 3 (1996), S. 112-119 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A novel approach to the detection and modelling of preferred orientation is presented, based on the interpretation of two-dimensional powder patterns. A simple graphical construction is introduced to aid interpretation, and the application of this construction to some standard diffraction geometries is discussed. It is also shown in outline how a standard preferred-orientation model can be adapted to describe two-dimensional data.
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  • 6
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: This paper describes a new technique for indexing powder diffraction data. The lattice parameters (unit-cell dimensions) {a,b,c,α,β,γ} define the parameter space of the problem, and the aim is to find the optimal lattice parameters for a given experimental powder diffraction pattern. Conventional methods for indexing consider the measured positions of a limited number of peak maxima, whereas this new approach considers the whole powder diffraction profile. This new strategy offers several advantages, which are discussed fully. In this approach, the quality of a given set of lattice parameters is determined from the profile R-factor, Rwp, obtained following a Le Bail-type fit of the intensity profile. To find the correct lattice parameters (i.e. the global minimum in Rwp), a genetic algorithm has been used to explore the Rwp(a,b,c,α,β,γ) hypersurface. (Other methods for global minimization, such as Monte Carlo and simulated annealing, may also be effective.) Initially, a number of trial sets of lattice parameters are generated at random, and this `population' is then allowed to evolve subject to well defined evolutionary procedures for mating, mutation and natural selection (the fitness of each member of the population is determined from its value of Rwp). Examples are presented to demonstrate the success and underline the potential of this new approach for indexing powder diffraction data.
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  • 7
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: 3-Methylaspartase (E.C. 4.3.1.2) catalyses the reversible anti elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mesaconic acid as well as a slower syn elimination from the (2S,3R)-epimer, L-erythro-3-methylaspartic acid. The anti-elimination reaction occurs in the second step of the catabolic pathway for glutamic acid in Clostridium tetanomorphum. The reverse reaction is of particular interest because the addition of ammonia to substituted fumaric acids is highly stereoselective and gives highly functionalized amino acids. The mechanism of the transformation is unusual and of considerable interest. 3-Methylaspartase from C. tetanomorphum has been overexpressed and purified from Escherichia coli. Crystals of the enzyme have been obtained by sitting-drop vapour diffusion. Two native data sets have been collected, one in-house on a rotating-anode generator to 3.2 Å and one at the European Synchrotron Radiation Facility to 2.0 Å. A 2.1 Å data set has been collected on a crystal of selenomethionine protein. Combining the data sets identify the space group as P21212, with unit-cell parameters a = 110.3, b = 109.9, c = 67.2 Å, α = β = γ = 90°. The asymmetric unit contains two monomers with 42% solvent. A self-rotation function indicates the presence of a twofold axis, consistent with a biological dimer.
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  • 8
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 1415-1420 
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of UDP-galactopyranose mutase, the enzyme responsible for the conversion of UDP-galactopyranose to UDP-galactofuranose, has been solved. The structure solution required the use of two crystal forms and a selenomethionine variant. Crystal form P21 was used to collect a complete MAD data set, a native data set and a single-wavelength non-isomorphous selenomethionine data set. A starting set of MAD phases was then improved by non-crystallographic averaging and cross-crystal averaging of all P21 data. The initial maps were of such low quality that transformation matrices between cells could not be determined. It was therefore assumed that although there were large changes in unit-cell parameters, the molecule occupied the same position in each cell. This starting assumption was allowed to refine during the averaging procedure and did so satisfactorily. Despite a visible increase in the quality of the map allowing some secondary-structural elements to be located, the overall structure could not be traced and refined. The rediscovery of the second crystal form, P212121, allowed the collection of a native data set to 2.4 Å. Molecular placement of electron density was used to determine the relationship between the two unit cells. In this study, only the already averaged P21 experimental density could be placed in the P212121 map. Less extensively density-modified maps did not give a clear solution. The study suggests even poor non-isomorphous data can be used to significantly improve map quality. The relationship between P21 and P212121 could then be used in a final round of cross-crystal averaging to generate phases. The resulting map was easily traced and the structure has been refined. The structure sheds important light on a novel mechanism and is also a therapeutic target in the treatment of tuberculosis.
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  • 9
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: L-Rhamnose is an essential component of the cell wall of many pathogenic bacteria. Its precusor, dTDP-L-rhamnose, is synthesized from α-D-glucose-1-phosphate and dTTP via a pathway requiring four distinct enzymes: RmlA, RmlB, RmlC and RmlD. RmlC was overexpressed in Escherichia coli. The recombinant protein was purified by a two-step protocol involving anion-exchange and hydrophobic chromatography. Dynamic light-scattering experiments indicated that the recombinant protein is monodisperse. Crystals were obtained using the sitting-drop vapour-diffusion method with ammonium sulfate as precipitant. Diffraction data were collected on a frozen crystal to a resolution of 2.17 Å. The crystal belongs to either space group P3121 or P3221, with unit-cell parameters a = b = 71.56, c = 183.53 Å and α = β = 90, γ = 120°.
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  • 10
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 23 (1990), S. 392-396 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The integrated intensities measured in X-ray single-crystal high-pressure structural studies using a diamond-anvil cell are shown to be reduced substantially when the diamonds diffract at the same setting as the sample – by as much as 50% in some cases. The pressure and wavelength dependence of this process have been studied and also the effect of changing the beam divergence by the use of a synchrotron beam. The consequences for the accuracy of structural information derived from data sets collected at high pressure are considered and a data-collection strategy for detecting and avoiding the effects of diamond diffraction is proposed.
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