Laue diffraction synchrotron

The five years since last considering specifically recent developments in X-ray and neutron diffraction methods for zeolites [1] have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal Laue diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. 

Now I return to X-ray diffraction to describe probably the oldest type of diffraction experiment, but one whose stock has soared with the advent of synchrotron radiation and powerful computer techniques for the analysis of complex diffraction data. The method, Laue diffraction, is already realizing its promise as a means to determine the structures of short-lived reaction intermediates. This method is sometimes called time-resolved crystallography, implying an attempt to take snapshots of a chemical reaction or physical change in progress. 

Fig. 41. The reaction of glycogen phosphorylase b studied by synchrotron Laue diffraction.

The original synchrotron Laue diffraction patterns from protein crystals recorded at Daresbury using a broad bandpass were conducted on this instrument with the monochromator removed (see Helliwell (1984)). Some preliminary multiwavelength experiments with a silicon double crystal monochromator (Si (111) triangle removed) were conducted. The growth of the Laue and MAD experiments has led to two further stations at Daresbury (SRS-3 and SRS-4). 

Figure 7.1 (a) Pea lectin synchrotron Laue diffraction pattern recorded on photographic film on station 9.6 of the SRS and then in (b) the corresponding predicted Laue pattern is shown followed by those parts of the pattern which are (c) energy overlap spots, (d) spatial overlap spots, (e) spots which are both energy and spatial overlaps and, finally, those that are (f) neither energy nor spatial overlaps. From Helliwell (1985) with the permission of Elsevier. See Table 7.1, note 5 for experimental conditions (crystal to film 95 mm)... 

It seems that a substantial part of the radiation damage in proteins and viruses (and some other crystals, too) is a function of time from the first exposure to the beam. The synchrotron Laue method allows complete data recording in a shorter time from first exposure than any other method. Of course, protein crystals differ greatly in their susceptibility to radiation damage. For example, some survive a single exposure while, at the other extreme, cases such as pea lectin can survive many exposures. Indeed, it now seems that many protein crystals (>50%) are able to yield high quality synchrotron Laue diffraction photographs. 

Bulk X-ray diffiraction, thus averaging the X-ray difiiraction signal over a large amount of sample ( mm ), ex situ or under operando conditions can be considered a standard technique using laboratory based or synchrotron based sources. Countless findings rely on the precise description of the atomic order obtained from Laue diffraction patterns to describe structure, phases, reaction pathways, strain, disorder etc. 

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. 

A Laue X-ray diffraction pattern from a protein crystal. A stationary crystal is irradiated with very intense white, multiwavelength X rays from a synchrotron source. The diffraction pattern is rich in information. A single 0.1 ms X-ray pulse may provide a pattern with enough information to determine a three-dimensional structure. 

While the methods for acquiring of X-ray diffraction data have not changed since the first diffraction studies by von Laue, improvements in their components have made them more efficient and user friendly, so that crystal structures of much larger and more complicated molecules can now be determined. For example, during the years since 1912, X-ray sources have evolved from sealed tubes that produce only a nominal flux of radiation to synchrotron sources that produce radiation that is several orders of magnitude more intense. The first structural studies were done on crystals of simple salts containing only a few atoms in each unit cell and therefore relatively small numbers of Bragg reflections were available... 

Enzyme-catalyzed reactions may be analyzed from structural studies of the active site of the enzyme and its perturbation and mode of binding of substrates and inhibitors. These analyses may also be aided by structure determinations of enzymes in which appropriate functional side chains have been mutated. Chemical reactions may be followed by observing changes in the diffraction pattern as a function of time. This may be seen in solids via powder diffraction, or in biological macromolecules by use of Laue photographs (in which large amounts of data are measured in a small amount of time). The availability of synchrotron radiation has greatly facilitated this. 

Specimens were extracted from the deformed samples at different radii from the cylinder axis, along the torsion axis direction. Some of the diffraction analyses were performed using a hard X-ray diffraction technique developped at Institut Laue Langevin, others were observed via synchrotron X-ray diffraction topography at ESRF (European Synchrotron Radiation Facilities) on the "ID19" beamline. 

Fleischmann and co-workers have carried out a number of in-situ x-ray diffraction studies at roughened electrodes.Their experiments are different from those previously discussed in that instead of a synchrotron source they employ a conventional x-ray tube ( 1.5 kW) with a position-sensitive proportional counter for the measurement of the diffracted intensity. They have carried out experiments in both transmission (Laue) and reflection (Bragg) modes. 

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