Diffraction pattern data collection

F. X-ray Powder Difi action Search System. Compounds that fail to crystallize may still be examined by X-ray diffraction, because non-crystalline materials, as powders, give characteristic diffraction patterns. A collection of powder diffraction patterns proves to be a very effective means by which to identify materials and indeed, one of the very earliest search systems in chemical analysis was based upon such data by Hanawalt (21) over forty years ago. The importance of these data in TSCA can be seen by examining the TSCA Inventory regulations for treatment of confidential chemicals (22). Section 710.7 of these regulations indicates that EPA intends to rely on powder diffraction data to assure the validity and seriousness of a manufacturers request for treating information on a chemical as confidential. 

Princeton Instruments (now Roper Scientific), and was used to store and display the x-ray diffraction data in real time. By incrementally jogging the weld to new locations in 200 pm intervals, a series of space-resolved x-ray diffraction patterns was collected along a lineal scan direction perpendicular to and away from the centerline of the weld. A schematic of the SRXRD setup is shown in Figure 2. Details of the apparatus has been described in detail elsewhere. ... 

Multiple isomorphous replacement allows the ab initio determination of the phases for a new protein structure. Diffraction data are collected for crystals soaked with different heavy atoms. The scattering from these atoms dominates the diffraction pattern, and a direct calculation of the relative position of the heavy atoms is possible by a direct method known as the Patterson synthesis. If a number of heavy atom derivatives are available, and... 

In addition to the dynamic disorder caused by temperature-dependent vibration of atoms, protein crystals have static disorder due to the fact that molecules, or parts of molecules, do not occupy exactly the same position or do not have exactly the same orientation in the crystal unit cell. However, unless data are collected at different temperatures, one cannot distinguish between dynamic and static disorder. Because of protein crystal disorder, the diffraction pattern fades away at some diffraction angle 0max. The corresponding lattice distance <7mm is determined by Bragg s law as shown in equation 3.7 ... 

When an equimolar solution of the two salts is used, the reaction is shown to be extremely rapid - the host peak had decayed completely before data collection commenced. Bragg reflections with d-spacings of 14.8 and 13.4 A appear in the diffraction pattern, corresponding to the 1,2-BDA and 1,4-BDA intercalates respectively (Fig. 20). 

Rietveld refinement [25, 26] is a method of whole pattern refinement, where a calculated diffraction pattern for a structure model is a least-squares fit to an observed diffraction pattern. Originally, it was used as a means of verifying proposed structure models. For zeolites, Rietveld refinement is still used for the same purpose and provides details of the structure including atomic positions of framework atoms and cation sittings. Data with accurate intensities and well-resolved peaks are needed for the most accurate work, and so often a synchrotron source is used for data collection since it can provide higher intensity and peak resolution than an in-house diffractometer. However, modern in-house diffractometers often provide good enough data for some refinements. 

Otherwise commercially interesting proteins, which are not readily crystallizable. The high intensity and collimation of the SR is currently allowing data to be collected and several structures to be studied where samples as small as 100 x 100 x 15 pm and 200 x 50 x 50 pm (Ealick, Helliwell and Cook, unpublished) with unit cell volumes in the range of 10 A or so.. Sample volumes as small as (20 pm) have yielded very strong diffraction patterns with the white SR beam, (Fig. 6) (Hedman et al. 1985). In this application area efforts to reduce the X-ray background at the detector include replacement of traditional glass capillaries in the sample with thin film capillaries. 

Fig. 7. The instrumental set up at XI1 /DORIS for time resolved data collection with a linear detector for CO myoglobin following laser photolysis of the ligand. A section of the diffraction pattern with stationary crystal, stationary detector is recorded with a linear detector and (b) shows the time course of three reflections before and after the laser flash (from Bartunik et al. 1982)...

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