Diffraction structure determination

Water on Vermiculite. For low water contents (that is, one or two water layers), the evidence for highly structured water in the interlayer spaces of smectites and vermiculites is most easily seen in X-ray diffraction structure determinations of ordered hydrate structures such as the two-water layer hydrate of Ca-vermiculite (14. 15) and Na-vermiculite (15., 16). 

Key words electron diffraction, structure determination, low-dose CBED, protein strueture... 

The theoretical density of a crystal can be obtained from the volume of the unit cell and the mass of the unit cell contents. The results of an X-ray diffraction structure determination gives both of these data, as the unit cell dimensions are accurately measured and the type and number of formula units in the unit cell are also determined. An example of this type of calculation for FeO follows ... 

Jdnsson PG, Kvick A (1972) Precision neutron diffraction structure determination of protein and nucleic acid components. III. Crystal and molecular structure of the amino acid a-glycine. Acta Cryst B 28 1827 -1833... 

Ellison RD, Levy HA (1965) A centered hydrogen bond in potassium hydrogen chloro-maleate A neutron diffraction structure determination. Acta Cryst 19 260-268... 

Frey MN, Koetzle TF, Lehmann MS, Hamilton WC (1973) Predsion neutron diffraction structure determination of protein and nucleic acid components. XII. A study of hydrogen bonding in the purine-pyrimidine base pair 9-methyladenine and 1-methylthymine. J Chem Phys 59 915-924... 

Similarly, the absolute stereochemistry of the conversion of fu-marate to malate by the enzyme fumarase was confirmed by a neutron-diffraction structure determination of the structure of the (- -)-i -a-methylbenzylammonium salt of malate (see Figure 14.6). The malate was prepared enzymatically with the use of heavy water (D2O) so that... 

The residual difference after a successful DDM refinement or/and decomposition can be considered as a scattering component of the powder pattern free of Bragg diffraction. The separation of this component would facilitate the analysis of the amorphous fraction of the sample, the radial distribution function of the non-crystalline scatterers, the thermal diffuse scattering properties and other non-Bragg features of powder patterns. The background-independent profile treatment can be especially desirable in quantitative phase analysis when amorphous admixtures must be accounted for. Further extensions of DDM may involve Bayesian probability theory, which has been utilized efficiently in background estimation procedures and Rietveld refinement in the presence of impurities.DDM will also be useful at the initial steps of powder diffraction structure determination when the structure model is absent and the background line cannot be determined correctly. The direct space search methods of structure solution, in particular, may efficiently utilize DDM. 

An electron-diffraction structure determination is nowadays based exclusively on comparing experimental, sME(s), and theoretical, sMT(s), molecular intensity curves. The structure parameters are adjusted until the... 

ADP, 7mM MgCla, 6.5% polyethylene glycol 6000, 1.0% ethylene glycol, 1 week, cryoprotection with 25% glycerol. X-ray diffraction structure determination [32]) [32]... 

A co-crystal of celecoxib with nicotinamide (Vitamin B3) was identified [49]. The co-crystal has a 1 1 stoichiometry and is comprised of hydrogen bonded nicotinamide dimers that sit within channels formed by celecoxib molecules, as elucidated by powder diffraction structure determination (Figure 3.5). The form rapidly dissociates when dispersed in aqueous solutions and the parent, celecoxib, precipitates in one of several forms depending on the excipients present. Although complex, the dissolution behaviour of the co-crystal has been evaluated and through the use of a spring and parachute approach, the in vitro performance of celecoxib nicotinamide was shown to be superior to that of the parent compound. 

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