X Ray Diffraction In Crystals Imperfect Crystals And Amorphous Bodies Pdf
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- Macromolecular diffractive imaging using imperfect crystals
- Correlated Motions from Crystallography Beyond Diffraction
- X-Ray Scattering and Diffraction
Macromolecular diffractive imaging using imperfect crystals
Over the past century, X-ray crystallography has been defined by a pursuit for perfection and high resolution. In next Holy Grail of crystallography is to embrace imperfection towards a dynamic picture of enzymes. Over its hundred-year history, X-ray crystallography has made enormous impacts on diverse fields by providing insight into the structures of countless molecules. Crystallographic structures represent some of the landmark achievements of biology in the past century. From the very first enzyme structure of lysozyme 1 in to the recent high-resolution structure of the oxygen-evolving complex of photosystem II, 2 crystallography has given detailed views into the intricate processes by which Nature catalyzes the reactions necessary for life. Indeed, one of the triumphs of crystallography in the last century was the rise of structural enzymology, in which changes in activity are correlated with changes in structure. Thus far, X-ray crystallography has embodied a quest for perfection.
The contribution of X-ray diffraction to conventional structural chemistry has been large. Much of our information about molecular geometry has come from this source, especially through its application to single crystals. As simple molecules become condensed into long chains, however, the resulting macromolecules rarely form good single crystals, and so the study of polymers by X rays involves mainly the study of fibres, or isotropic materials that are at best only partly crystalline, and are often totally amorphous. Less conventional mathematical formulae become involved in data analyses, and the information about structure becomes necessarily more limited and statistical. Despite this, however, the application of X-ray methods to polymeric materials is well established, 1—3 and is a sophisticated, and highly productive, activity. Unable to display preview.
Acta Cryst. A 70 , —] and examines the implications for the interpretation of experimental results and the estimation of structure factors. Further experimental evidence is included to justify the conclusions in the theory, showing that the residual intensity at twice the Bragg angle is a diffraction effect and not associated with the crystal shape. Because this new theory considers the intensity to be more distributed, it suggests that the entire structure factor can be difficult to capture by experiment. This article suggests some routes to achieve a good approximation of the structure factors for typical methods of data collection. Any measurement of intensity with background removal will exclude some of the distributed intensity, again leading to an underestimate of the structure factors, and therefore the missing intensity needs to be estimated. X-ray diffraction analysis has relied to an increasing extent on the accuracy of intensity measurements to reveal important structural information in complex molecules, e.
Correlated Motions from Crystallography Beyond Diffraction
The three-dimensional structures of macromolecules and their complexes are predominantly elucidated by X-ray protein crystallography. A major limitation is access to high-quality crystals, to ensure X-ray diffraction extends to sufficiently large scattering angles and hence yields sufficiently high-resolution information that the crystal structure can be solved. The observation that crystals with shrunken unit-cell volumes and tighter macromolecular packing often produce higher-resolution Bragg peaks 1 , 2 hints that crystallographic resolution for some macromolecules may be limited not by their heterogeneity but rather by a deviation of strict positional ordering of the crystalline lattice. Such displacements of molecules from the ideal lattice give rise to a continuous diffraction pattern, equal to the incoherent sum of diffraction from rigid single molecular complexes aligned along several discrete crystallographic orientations and hence with an increased information content 3. Although such continuous diffraction patterns have long been observed—and are of interest as a source of information about the dynamics of proteins 4 —they have not been used for structure determination. Here we show for crystals of the integral membrane protein complex photosystem II that lattice disorder increases the information content and the resolution of the diffraction pattern well beyond the 4. With the molecular envelope conventionally determined at 4.
X-Ray. Diffraction. In Crystals, Imperfect Crystals, and. Amorphous Bodies. A. GUINIER. UNIVERSITY OF PARIS. TRANSLATED BY Paul Lorrain. University of.
X-Ray Scattering and Diffraction
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Anomalous X-ray scattering AXRS or XRAS is a non-destructive determination technique within X-ray diffraction that makes use of the anomalous dispersion that occurs when a wavelength is selected that is in the vicinity of an absorption edge of one of the constituent elements of the sample. It is used in materials research to study nanometer sized differences in structure. In X-ray diffraction the scattering factor f for an atom is roughly proportional to the number of electrons that it possesses. However, for wavelengths that approximate those for which the atom strongly absorbs radiation the scattering factor undergoes a change due to anomalous dispersion. The dispersion not only affects the magnitude of the factor but also imparts a phase shift in the elastic collision of the photon.
Bragg diffraction; Bragg scattering; Wave interference from alattice; X-ray diffraction. Diffraction : The physical phenomenon of interference produced through the interaction of electromagnetic waves i. X-ray diffraction : The diffraction of X-rays by crystals, producing interference effects i.
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