Home About us Contact
|
Home About us Contact | ||
|
|
||
Systematic Absences (systematic + absence)
Selected AbstractsX-Cell: a novel indexing algorithm for routine tasks and difficult casesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2003Marcus A. Neumann X-Cell is a novel indexing algorithm that makes explicit use of systematic absences to search for possible indexing solutions from cells with low numbers of calculated reflections to cells with high numbers of reflections. Space groups with the same pattern of systematic absences are grouped together in powder extinction classes, and for a given peak number range an independent search is carried out in each powder extinction class. The method has the advantage that the correct cell is likely to be found before the rapid increase of possible solutions slows down the search significantly. A successive dichotomy approach is used to establish a complete list of all possible indexing solutions. The dichotomy procedure is combined with a search for the zero-point shift of the diffraction pattern, and impurity peaks can be dealt with by allowing for a user-defined portion of unindexed reflections. To rank indexing solutions with varying numbers of unindexed reflections, a new figure of merit is introduced that takes into account the highest level of agreement typically obtained for completely incorrect unit cells. The indexing of long and flat unit cells is facilitated by the possibility to search for rows or zones in reciprocal space first and then to use the lattice parameters of the dominant row or zone in the unit-cell search. The main advantages of X-Cell are robustness and completeness, as demonstrated by a validation study on a variety of compounds. The dominant phase of phase mixtures can be indexed in the presence of up to 50% of impurity peaks if high-quality synchrotron data are available. [source] The commensurate composite ,-structure of ,-tantalumACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2003Alla Arakcheeva The single-crystal investigation of the self-hosting ,-structure of ,-tantalum (,-Ta) at 120,K (low-temperature, LT, structure) and at 293,K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same (or an integer multiple) lattice constants but different space groups. The space groups of both host (H) and guest (G) components cause systematic absences, which result from their intersection. The highest symmetry of a ,-structure can be described as [H: P42/mnm; G: P4/mbm (cG = 0.5cH); composite: P42/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In ,-Ta, two components modify their symmetry during the thermal process 293,K (RT-I) , 120,K (LT) , 293,K (RT-II): [H: P21m; G: P21m; composite: P21m] , [H: P, G: P4/mbm (cG = 0.5cH), composite: P] , [H: P21m, G: P4/mbm (cG = 0.5cH), composite: P21m]. Thus, the phase transition is reversible with respect to H and irreversible with respect to G. [source] The correct space group of NaPF6·H2OACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2003Ilia A. Guzei The structure of sodium hexafluorophosphate monohydrate, NaPF6·H2O, has been inadvertently redetermined, revealing that the previously reported space group, Imma, was assigned incorrectly, with the a and b axes interchanged. The correct space group is Pnna. The program PLATON [Spek (2003). J. Appl. Cryst.36, 7,13] suggested both Imma and Pmma as possible space groups, but only Pnna is consistent with the systematic absences. The inter-ionic and hydrogen-bonding interactions in the lattice form a three-dimensional network. [source] Crystallization and preliminary X-ray data investigation of the bacterial enterocin A immunity protein at 1.65,Å resolutionACTA CRYSTALLOGRAPHICA SECTION D, Issue 7 2003Bjørn Dalhus Crystals of the bacterial enterocin A immunity protein have been prepared by the hanging-drop vapour-diffusion technique at 293,K. The crystals diffract to better than 1.7,Å resolution and X-ray diffraction data to 1.65,Å have been collected at 110,K using synchrotron radiation. The enterocin A immunity protein crystals belong to the monoclinic crystal system, with unit-cell parameters a = 116.32, b = 42.35, c = 66.17,Å, , = 111.3°. The symmetry and systematic absences in the diffraction pattern are consistent with space group C2. The presence of two molecules in the asymmetric unit with a molecular weight of ,12.2,kDa gives a crystal volume per protein mass (VM) of ,3.1,Å3,Da,1 and a solvent content of ,60% by volume. [source] Expression, purification, crystallization, data collection and preliminary biochemical characterization of methicillin-resistant Staphylococcus aureus Sar2028, an aspartate/tyrosine/phenylalanine pyridoxal-5,-phosphate-dependent aminotransferaseACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 5 2007Jaldappagari Seetharamappa Sar2028, an aspartate/tyrosine/phenylalanine pyridoxal-5,-phosphate-dependent aminotransferase with a molecular weight of 48,168,Da, was overexpressed in methicillin-resistant Staphylococcus aureus compared with a methicillin-sensitive strain. The protein was expressed in Escherichia coli, purified and crystallized. The protein crystallized in a primitive orthorhombic Laue group with unit-cell parameters a = 83.6, b = 91.3, c = 106.0,Å, , = , = , = 90°. Analysis of the systematic absences along the three principal axes indicated the space group to be P212121. A complete data set was collected to 2.5,Å resolution. [source] | ||||||||||