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Thermal Displacement Parameters (thermal + displacement_parameter)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Following an isosymmetric phase transition by changes in bond lengths and anisotropic displacement parameters: the case of meta -carboxyphenylammonium phosphite

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009
El-Eulmi Bendeif
Crystal structure studies in the 100,345,K temperature range were performed to relate the molecular structure changes of meta -carboxyphenylammonium phosphite (m -CPAMP) to its first-order phase transition at Tc = 246,(2),K. Thermal displacement parameters and most bond distances show an abrupt jump at the transition. Such a structural change is related to collective effects leading to competition between intra- and intermolecular interactions. [source]

Ultrahigh-resolution crystallography and related electron density and electrostatic properties in proteins

JOURNAL OF SYNCHROTRON RADIATION, Issue 3 2008
Claude Lecomte
With an increasing number of biological macromolecular crystal structures measured at ultrahigh resolution (1,Å or better), it is necessary to extend to large systems the experimental valence electron density modelling that is applied to small molecules. A database of average multipole populations has been built, describing the electron density of chemical groups in all 20 amino acids found in proteins. It allows calculation of atomic aspherical scattering factors, which are the starting point for refinement of the protein electron density, using the MoPro software. It is shown that the use of non-spherical scattering factors has a major impact on crystallographic statistics and results in a more accurate crystal structure, notably in terms of thermal displacement parameters and bond distances involving H atoms. It is also possible to obtain a realistic valence electron density model, which is used in the calculation of the electrostatic potential and energetic properties of proteins. [source]

On the application of an experimental multipolar pseudo-atom library for accurate refinement of small-molecule and protein crystal structures

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2007
Bartosz Zarychta
With an increasing number of biomacromolecular crystal structures being measured to ultra-high resolution, it has become possible to extend to large systems experimental charge-density methods that are usually applied to small molecules. A library has been built of average multipole populations describing the electron density of chemical groups in all 20 amino acids found in proteins. The library uses the Hansen & Coppens multipolar pseudo-atom model to derive molecular electron density and electrostatic potential distributions. The library values are obtained from several small peptide or amino acid crystal structures refined against ultra-high-resolution X-ray diffraction data. The library transfer is applied automatically in the MoPro software suite to peptide and protein structures measured at atomic resolution. The transferred multipolar parameters are kept fixed while the positional and thermal parameters are refined. This enables a proper deconvolution of thermal motion and valence-electron-density redistributions, even when the diffraction data do not extend to subatomic resolution. The use of the experimental library multipolar atom model (ELMAM) also has a major impact on crystallographic structure modelling in the case of small-molecule crystals at atomic resolution. Compared to a spherical-atom model, the library transfer results in a more accurate crystal structure, notably in terms of thermal displacement parameters and bond distances involving H atoms. Upon transfer, crystallographic statistics of fit are improved, particularly free R factors, and residual electron-density maps are cleaner. [source]

The structure of orange HgI2.

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2002

The metastable orange crystals of HgI comprise three different crystal structures all of which are built from corner-linked HgI supertetrahedra. Two of the structures are end members with the maximum degree of order (MDO) of a polytypic layer structure. In this paper, the third structure (D) determined from X-ray diffraction, a crystal chemical discussion of the four known tetrahedral HgI structures, and a twinning model are presented. All the various diffraction results published during the past 70 years are now explained. The HgI supertetrahedra of the tetragonal structure D are corner-linked into two interpenetrating diamond-type networks. The stable red form and the three orange structures show the same cubic densest packing of I atoms and differ only in the distribution of Hg atoms in the tetrahedral voids. Transformations between the structures may involve only movements of Hg atoms, as implied by larger thermal displacement parameters of Hg than of I. A multiply twinned conglomerate of MDO1, MDO2 and D, each structure occurring in three orientations, results in metrically cubic crystals whose Bragg reflections are very close to reciprocal lattice points. [source]