X-ray diffraction pictures

X-ray diffraction pictures taken with a flat-film camera show that crosslinked SE-BR samples crystallize on stretching. Sharp reflections are observed at an extension ratio of 4 1 (Figure 4). With samples having different degrees of stereoregularity the order for increasing strain-induced crystallization is the same as the order for the rate of low temperature crystallization. 

In X-ray diffraction pictures, atomic nuclei reflect X-rays onto photographic film, resulting in a pattern of dark-colored spots against a light-colored background. This value contrast in the picture allows the scientist to determine the position of atoms in a crystal or in a molecule. There are many uses for light-dark contrast. 

X-rays diffraction picture of washed sample 3 (Fig. 2c) contains intensive and wide asymmetrical peak corresponding to amorphous carbon and carbon nanofibres [6], set of Fe3C peaks and some weak peaks of y-Fe phase (Fig. 2c). 

The deformation of the lattice as a result of the mechanical working is seen from the broadening of the lines in the X-ray diffraction picture, which are narrow under normal circumstances (Debye-Scherrer or powder diagram). 

X-ray diffraction pictures made by Rosalind Franklin showed that the DNA structure was a helix (2 or more molecules spiraling around each other) the structure appeared to have a uniform thickness. 

The first X-ray diffraction pictures of a protein crystal were taken in 1934 by Bernal in Cambridge, but in those days the data quality was crude and the techniques for deriving a crystal structure of a macromolecule from the X-ray data were not sufficiently developed. The advent of the computer has been a critical development. 

Figure 19.29 X-ray diffraction picture of nevirapine (87) bound to HIV-1 reverse transcriptase (RT). The alpha-carbons of the key aspartic acid residues of the active site are represented by red spheres. The blue spheres represent sites of mutation that confer resistance to nevirapine and other NNRTi drugs (the larger blue spheres represent the positions of tyrosines 181 and 188). The orange spheres represent sites that confer resistance to nucleoside inhibitors. See text for additional discussion. (With permission from Proudfoot.)...

HtrII (1-159) under consideration consists of the C-terminal residue protruding from the cytoplasmic membrane surface (82-159) mainly as linker (Fig. 34) that may participate in the coiled-coil form responsible for phototaxis besides the two transmembrane a-helices. by analogy with the X-ray diffraction picture of four-helical-bundle structure of the cytoplasmic domain of a serine chemotaxis receptor. Indeed, each monomer is a 70 turn a-helix folded back on itself to form two long antiparallel coiled-coil structures. 

For historical reasons, such X-ray diffraction pictures are called fiber diagrams, although, of course, they are obtained also from drawn films. However, in contrast to crystal-rotation photographs, the fibers do not need to be rotated for fiber photographs, because many crystallites are already oriented. Reflections on the zero line are called equatorial, and correspond to crystal planes lying parallel to the molecular axis (draw direction). Crystal planes that lie vertical ( normal ) to the molecular axis produce what are called meridional reflections. Meridional reflections lie in a plane that bisects the equatorial line. When the crystallites are insufficiently oriented, the reflections (spots) degenerate into crescents (arcs) (see also Section 5.7). Thus, in shape, arcs lie between the spots of the fiber diagram with full crystallite orientation, and the circles produced by randomly oriented crystallites. 

The X-ray diffraction pictures on the aimealed samples do not show the rays typical of Sn02 (Figure 7.6). Nevertheless, the films deposited under a pressure close to 2 Pa, and therefore composed before firing of a mixture of metal and oxide, after firing contain different tin oxides, as well as a low quantity of metal. 

Drawn fibers and films show X-ray diffraction pictures which are similar to the Bragg crystal rotation photographs (Figure 5-5). The crystal rotation method was originally carried out in order to orient favorably as many crystal planes as possible with regard to the X-ray beam. In drawn fibers and films, the molecular axes he mainly in the direction of elongation (see Section 5.6). A ray that impinges normal to the draw direction will... 

Watson told the story of the discovery in his outrageously candid and captivating book. The Double Helix. With the self-confidence and ambition of his twenty-three years, he had formulated the modest plan to uncover the basis of heredity. He was in no doubt by then that DNA was indeed the genetic material, and at a meeting in Naples he had been transfixed by an X-ray diffraction picture of DNA fibres, which Maurice Wilkins had shown. It revealed the semi-crystalline nature of the DNA, and convinced him that the structure of DNA—of the gene, no less—could be cracked. Like many others of his generation, he... 

Figure 18. X-ray diffraction picture of Hycar at -100 C. Left is unstrained,right with 300% strain direction of strain parallel to long axis of page.

British molecular biologist. Maurice Wilkins was one of the key figures in the determination of the structure of DNA. He was originally a physicist but turned to biophysics after the end of World War II. He began to study DNA by x-ray diffraction. Some of the x-ray diffraction pictures produced by his colleague Rosalind FRANKLIN provided essential clues to Francis crick and James WATSON in their search for the structure of DNA. Wilkins shared the 1962 Nobel Prize for Medicine with Crick and Watson. Wilkins also determined the structure of ribonucleic acid (RNA) using x-ray diffraction. In 2003 Wilkins published his autobiography. 

---