Crystal-structure derivatives

Kojic acid crystallizes in the form of colorless, prismatic needles.3- 6 Data on its crystal structure, derived from x-ray investigations, were provided by Fox,111 and by McKinstry and coworkers.112... 

Structure file (in PDB format) containing the coordinates of the protein crystal structure derived from the gplip-HSDl cocrystal complex (Fig. 10.4a)... 

Computer Simulation of Pyramidalization. This hypothesis was tested by computer simulation of the pyramidalization at either Ca or Co of a single molecule surrounded by its stationary lattice neighbors. Using the X-ray crystal structure-derived coordinates of enones J arUl. as a starting point, the methyl groups attached to C and Co were rotated downwards by intervals of 11°, 22°, 33°,... 

Gervais, C., Coquerel, G., Simple model designed to generate new crystal structures derived from a mother phase, application to molecular compounds. Acta Crystallogr. Sect. B-Struct. Commun. 2002, 58, 662-672. 

SOURCE Modified from presentation of Marcetta Darensbourg, Texas A M University, based on crystal structures derived by (A) J.W. Peters and coworkers, 1998. X-ray crystal structure of the Fe-only hydrogenase from Clostridium pasteurianum to 1.8 Angstrom Resolution. Science 282(5395) 1853-1858 and (B) J.C. Fontecilla-Camps and coworkers, 1995. Crystal structure of the nickel-iron hydrogenase from Desulfovibrio gigas. Nature 373 580-587. 

Fig. 19 (a) Crystal structure of the HBS a-helix with electron density map superimposed onto the refined molecular model, (b) Putative i and i + 4 hydrogen bonds (magenta lines) in crystal structure-derived molecular model of HBS helix, (c) Overlay of crystal structure and a model of an idealized a-helix (Reprinted with permission from Patgiri et al. [71], Copyright (2008) American... 

In practice, one usually defines a training set of molecules and associated experimental properties and fits the relevant data with an assumed force field.The next step is to test the results on molecules and data outside the training set. Experimental data that depend on the energy surface and that may be used to determine the parameters of the intramolecular interactions consist mainly of gas-phase structural data derived from microwave spectra or electron diffraction patterns, crystal structures derived from X-ray and inelastic neutron scattering measurements, and vibrational frequencies obtained from infrared and Raman spectra. Torsional barriers are derived from NMR band shapes and relaxation times, whereas conformational energies are determined from spectroscopic and thermochemical data. Nonbonded parameters are determined mainly from... 

Yuen et al. determined atomic site charges on carbonate anion by fitting the crystal structures of calcium carbonate polymorphs calcite and aragonite. They obtained c = 0-95(26) and qo = -0.98(9). The 6-31G PD values are = 1-210(11) and qo = - 1.070(4), which are narrowly within the confidence limits of the crystal structure derived charges. Hirshfeld and Mirsky found qQ= - 0.410 e from the observed quadrupole moment of carbon dioxide, in good agreement with the PD value. 

III. Crystal Structures Derived From The Close-Packing of Spheres... 

Conceptually, crystals are infinite, three-dimensional, and periodic entities constructed from atoms, ions, and molecules. Periodicity implies that every object contained in its smallest repeating unit, the unit cell, is systematically and infinitely repeated in three dimensions. However, real crystals are prone to lattice and other defects and parts of molecules or even entire molecules often exist in multiple crystallographically independent (or energetically similar) orientations. Moreover, the crystal structure derived from the diffraction pattern is the spatial average over the whole crystal and any deviation from the ideal three-dimensional regularity of the crystal is presented as disorder. Generally, there are two categories of disorder substitutional and positional disorder. 

A series of superstructures Pr 4 (Fe4B4) have recently been observed by Braun et al. (1982) in the Pr-Fe-B system (table 2). Their crystal structures derive from the NdCo4B4-type of structure and are either incommensurate or have unusual large repeat units along the c-axis. Samples were prepared by arc melting Pr,, Fe4B4 alloys with subsequent heat treatment at 1000°C, 10 14 days. 

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