Diffraction methods data collection

Radoslovich tried to answer these problems using a crystal structure refinement technique involving X-ray diffraction intensity data collected by the multiple-film Weissenberg method. His refinement involved a modest use of an electronic computer with two-dimensional Fourier syntheses using intensity data transferred to perforated tape... 

Lane diffraction, stroboscopic data collection and femtosecond flash photolysis. NucL Instrum. Methods Phys. Res. A 398, 69 (1997)... 

M. Wulff, F. Schotte, G. Naylor, D. Bourgeois, K. Moffat, G. Mourou Time-resolved structures of macromolecules at the ESRF Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis. Nucl. Instrum. Methods Phys. Res. A 398, 69 (1997)... 

Quantitative Phase Analysis. Once the identity of the components in a sample are known, it is possible to determine the quantitative composition of the sample. There are several different methods for doing a quantitative analysis, but the most rehable method is to use mixtures of known composition as standards. The computer can determine quantitatively the relative amounts of each component in the unknown sample. For accurate calculations of relative amounts in the unknown sample, it is necessary that the sample and standards have uniform distributions of crystaUites. Often the sample and standards are rotated during data collection to provide a more even distribution of crystaUites which diffract. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

When a diffracted X-ray beam hits a data collection device, only the intensity of the reflection is recorded. The other vital piece of information is the phase of the reflected X-ray beam. It is the combination of the intensity and the phase of the reflections that is needed to unravel the contributions made to the diffraction by the electrons in different parts of the molecule in the crystal. This so-called phase problem has been a challenge for theoretical crystallographers for many decades. For practical crystallography, there are four main methods for phasing the data generated from a particular crystal. 

Otwinowski Z, Minor W. Processing of X-ray diffraction data collected in oscillation mode. Methods Enzymol 1997 276 307-26. 

The molecular structure of 1,2,9,10-tetrastanna[2.2]paracyclophane 22 was determined by the X-ray diffraction method. The crystal belongs to the space group P2,/a, and the data collection was carried out at 13°C. The ORTEP drawing of 22 is shown in Fig. 8. 

There are three methods of collecting high-resolution X-ray diffraction data diffractometry, photographically, and by electronic area detector. Each method has advantages and disadvantages for a particular crystalline protein, but for very accurate data acquisition beyond 2 A... 

The multiple-wavelength anomalous diffraction method (MAD) relies upon sufficiently strong anomalously scattering atoms in the protein structure itself In this method, diffraction data must be collected at a number of different wavelengths, usually requiring data collection with... 

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. 

Garman, E. F (1991). Modern methods for rapid X-ray diffraction data collection from crystals of macromolecules. In Methods in Molecular Biology, vol. 56, Crystallographic Methods and Protocols, Jones, C., MuUoy B. and Sanderson, M. R., eds. Humana Press. 

Otwinowski, Z. and Minor, W. (1997). Processing of X-ray Diffraction Data Collected in the Oscillation Mode. Method Enzymol. 276A, 307-326. 

For high-throughput data collection, sample centring via loop detection is currently the method of choice for placing crystals in the X-ray beam. Alternative methods rely on the direct detection of the crystal itself by monitoring the intensity of either X-ray diffraction from the crystal or X-ray fluorescence from an element in the crystal that is not present in the cryoprotectant (Pohl et al., 2004). Both of these approaches have drawbacks. First, some of the limited X-ray lifetime of the crystal (Section 12.6) must be committed to detection instead of data collection. Second, the loop itself has to be placed close to the X-ray beam prior to crystal detection, followed by step-wise translation of the crystal through the beam, thereby increasing the time required to centre the sample. 

The SR spectnun of an intense energy continuum of radiation makes possible rapid data collection in the sub-second regime by the Laue diffraction method. This rate of data collection is sufficient to investigate some of the crystal kinetics and catalytic intermediate reaction states that can be produced. 

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