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  • 1
    Electronic Resource
    Electronic Resource
    Combination of methods used in the structure solution of pyruvate:ferredoxin oxidoreductase from two crystal forms (1999)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 1546-1554 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of the homodimeric 267 kDa pyruvate:ferredoxin oxidoreductase (PFOR) of Desulfovibrio africanus was solved with data from two crystals forms, both containing two monomers per asymmetric unit. Phases were obtained from multiwavelength anomalous dispersion (MAD), solvent flattening (SF), molecular replacement (MR) using a 5 Å resolution electron-density search model, multiple isomorphous replacement (MIR) and, finally, electron-density averaging (DA) procedures. It is shown how the combination of all these techniques was used to overcome problems arising from incompleteness of MAD data and weak phasing power of MIR data. A real-space refinement (RSR) procedure is described to improve MR solutions and obtain very accurate protein envelopes and non-crystallographic symmetry (NCS) transformations from 5 Å resolution phase information. These were crucial for the phase extension to high resolution by DA methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444999008410
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  • 2
    Electronic Resource
    Electronic Resource
    Short-range order in Ni3Fe (1980)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 36 (1980), S. 1-7 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Short-range order parameters were obtained for a single crystal of 62Ni3Fe quenched from above Tc. These are the first absolute measurements of diffuse elastic neutron scattering complete enough to separate the effects of local order and atomic displacements (up to quadratic terms) and indicate that neutrons are ideally suited for such studies. The results are compared to those obtained with X-rays. The short-range order intensity of Ni3Fe is similar in shape to that for Cu3Au but in this alloy it is due to the plate-like nature of ordered domains, not the antiphase domain boundaries which are present in such regions in Cu3Au.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739480000010
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  • 3
    Electronic Resource
    Electronic Resource
    Low-resolution neutron diffractometry with a position-sensitive multidetector (1982)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 38 (1982), S. 670-679 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A data reduction system for neutron crystallography using a two-dimensional planar multidetector is described. The method is based on an a priori calculation of the intensity distribution for each reflection for any crystal orientation. The orientation matrix of the crystal has to be known. From the calculated intensity distribution a mask is derived for each reflection. The detection elements of the detector which are inside the mask are summed to determine the intensity of that reflection and the cells outside are used to determine the background. The size of the mask is adapted to the relative height of the measured intensity of the reflector compared to the background. The theoretical intensity distribution for each reflection is calculated in reciprocal space from the primary-beam divergence and wavelength spread and the crystal mosaic spread, on the basis of the kinematic theory of diffraction. The derivation of the necessary equations, i.e. the determination of the resolution function of the instrument, is presented. Some typical results of data collection are also presented. The advantages of this data collection method are the safe determination of weak reflections, the easy discrimination of reflections which nearly overlap, the determination of crystal mosaic spread, and the discrimination of inelastic scattering in the proximity of intense reflections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0567739482001363
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  • 4
    Electronic Resource
    Electronic Resource
    Best density maps in low-resolution crystallography with contrast variation (1987)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 43 (1987), S. 780-787 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method is presented of merging experimental diffraction data measured at different contrasts and structure factors calculated with models defined (and refined) at some of these contrasts (or at any other) in order to calculate an optimal density map at any contrast. It is based on a probabilistic approach which uses a joint probability distribution, for each [h, k, l], of the measured intensities and the calculated structure factors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767387098520
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  • 5
    Electronic Resource
    Electronic Resource
    On absolute scaling in protein crystallography using sums of low-resolution intensities and Wilson statistics at low resolution (1986)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 42 (1986), S. 230-240 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A method of absolute scaling of diffraction data is proposed, based on the calculation of the sum of the intensity diffracted at low resolution (Bragg d spacing 〉 15 Å). This sum is proportional to the mean-square deviation of the scattering-length density in the unit cell, and this property is used to determine the scale factor. The method is applied to the case of neutron diffraction using contrast variation experiments with biological molecules, and it is used to check the validity of some assumptions concerning the system under study, such as the global rate of H/D exchange or the uniformity of scattering-length density in the molecules. The use of this method requires an asymptotic correction of the sum of intensity. This correction is based on Porod's law, whose application to diffraction experiments is discussed, in particular for contrast variation experiments. An analysis of the spherical average of the diffracted intensity as a function of the scattering vector, compared to isotropic solution scattering, allows the conditions of applicability of Wilson statistics to be specified at low and medium resolution, i.e. the random statistical model underlying the Wilson statistics in this scattering range to be defined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767386099373
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  • 6
    Electronic Resource
    Electronic Resource
    Data Compression for Diffraction Patterns (1998)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 54 (1998), S. 184-199 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Efficient coding (lossless) and compression (lossy) of diffraction patterns is important in protein crystallography experiments because of storage and transmission limitations. The goal is to reduce the bit-rate significantly while keeping diffraction peak intensity distortion at an acceptable level. This paper presents an overview of coding and compression techniques more or less adapted to such problems. A large part of this study is dedicated to time-frequency-transform based compression algorithms and some of their extensions. Wavelet based software has been developed and tested. Results are compared with the discrete cosine transform (DCT) and other classical algorithms. These tools seem attractive and very promising for analyzing and compressing signals with singularities and transient phenomena such as diffraction peaks. Tests were performed on a standard protein crystallography data set coming from the CCD detector of D2AM beamline at the European Synchrotron Radiation Facility at Grenoble. These were compressed with DCT and wavelet-based algorithms. It appears that alterations of the result of the processing of restored images remain very weak for compression rates up to 10. These preliminary results indicate that the proposed wavelet method is a good standard technique for efficient compression of diffraction patterns.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444997007257
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  • 7
    Electronic Resource
    Electronic Resource
    The small-angle scattering of neutrons by surface imperfections (1977)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 10 (1977), S. 172-176 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: It is shown that the neutron small-angle scattering of pure polycrystalline metals is strongly connected with surface imperfections in the samples (surface irregularities and oxide layers) rather then with the grain boundaries. It is also shown that the asymptotic form of the scattering cross section of the surface imperfections is q−4 like the Porod law.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889877013181
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  • 8
    Electronic Resource
    Electronic Resource
    Neutron small-angle scattering in aged Al–Mg (1975)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 8 (1975), S. 535-537 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Neutron small-angle scattering in polycrystalline Al–Mg alloys aged nine months at 0°C has revealed the existence of Guinier–Preston zones in an Al–11.5% Mg alloy, but no G. P. zones in an Al–7% Mg alloy. The evaluation of the data for the 11.5% Mg alloy gives the following parametric values: mean radius of the zones, 19 Å; mean distance between them, 56 Å; Mg concentration in the zones, about 19% in the matrix, about 8%. A qualitative model is proposed for the process of dissolution of the zones by annealing at 100°C.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S002188987501120X
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  • 9
    Electronic Resource
    Electronic Resource
    An Algorithm for Automatic Mosaic Spread Analysis of a Protein Crystal (1998)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 31 (1998), S. 523-532 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The present paper describes the program MOSAICITY which assembles tools developed for the mosaic spread analysis of protein crystals. This analysis, performed in an automatic and nonparametric way, combines wavelet filters and morphological treatment in order to localize reflections in a diffraction pattern. The reflection intensities, rescaled according to incidence-beam intensity and corrected for the background, are measured using a fine rotation angle slicing. A Gaussian profile fitting is performed. The angular width can be corrected for the Lorentz factor, beam divergence and energy resolution in order to obtain an estimate of the mosaic spread of the crystal. For this last stage, a new analytical expression for the φ extent of the reflection range has been derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889897015045
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  • 10
    Electronic Resource
    Electronic Resource
    Small-angle neutron scattering of single crystals of aged aluminium–13 at.% magnesium alloy (1979)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 12 (1979), S. 429-431 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: X-ray, neutron and electron diffraction experiments have revealed the existence of internally ordered Guinier–Preston zones in Al-(8 to 15 at.%)Mg alloys; diffuse satellites oriented along (100) directions are observed near Bragg peaks and give rise to modulated contrasts by TEM. A small-angle neutron scattering experiment has been carried out on single crystals and a model is proposed for the interpretation of the observed pattern. The existence of concentration modulations is related to alignments of spherical GP zones along (100) directions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889879012966
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