Ray bundle

NaCl (d=0.56nm) or LiF (d=0.4nm) are frequently used as crystals. The X-rays bundles are collimated using systems of thin metal plates (thickness 50/zm) arranged parallel to each other at small separations (0.5mm). In this way the divergence can be limited to one degree or less. Alternatively, a curved crystal can be used this will cause radiation diverging from an entrance slit to be focused towards an exit slit. Curved crystals of alkali halides or mica can be used. 

FIGURE 7 Examples of image analysis methods, (a) Typical spot diagrams of well-corrected images in the focal plane of a 1-deg field. Each Image represents the ray bundle at that field location, (b) Computer-generated three-dimensional plot of intensity in a perfect image that has been diffracted by a circular aperture. 

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

Biindel, n. bundle, bunch parcel, packet (of rays) pencil, beam (of waves) packet, biindeln, v.t. focus, concentrate bundle, bunch. Bundes-rat, tn. federal council. -tag, tn. federal diet. 

Bilschel, tn. tuft, bunch, cluster (Elec.) brush bundle, pencil, beam (of rays), -ent-ladung, /. (Elec.) brush discharge, buschelfdrmig, a. tuft-like, tufted, clustered. Buschholz, n. undergrowth, underwood. 

X-rays being quanta, it follows that the effects used in x-ray detection are quantum effects. In most detectors important in analytical chemistry, these quantum effects eventually yield electrons. Under the simplest conditions, these electrons appear as separate, well-defined pulses (bundles), one pulse for each x-ray quantum. Under these conditions, the pulses can be counted as individuals, and each pulse (hence each x-ray quantum) will register as a unit. As the intensity increases,... 

Early biochemical studies supported the hypothesis that the HRl and the HR2 peptides would interact to form a helical structure (Chen et al. 1995 Lu et al. 1995). This hypothesis was strengthened when X-ray structures were resolved for co-crystals of HRl and HR2 peptides (Chan et al. 1997 Tan et al. 1997 Weissenhom et al. 1997). The results showed that in the six-helix bundle, three HRl domains were packed tightly together in the center of the bundle, with the HR2 domains bound in an antiparallel manner in grooves formed along the HRl core. 

In the x-ray structure of rhodopsin, an amphipathic helix runs parallel to the membrane from the intracellular end of TM-VII beneath the seven-helical bundle to the other side of TM-I and TM-II. At this point, one or more Cys residues are often found and are known to be subject to a dynamic posttranslational modification with palmitic acid residues. Like the phosphorylation event, the palmitoylation process appears to be dynamically regulated by receptor occupancy and is also involved in the desensitization phenomenon. The two posttranslational modifications can influence each other. For example, the conformational constraint induced by palmitoylation may alter the accessibility of certain phosphorylation sites. Like the phosphorylation process, the functional consequences of palmitoylation also appear to vary from receptor to receptor. 

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