X-ray fiber

The F-actin helix has 13 molecules of G-actin in six turns of the helix, repeating every 360 A. Oriented gels of actin fibers yield x-ray fiber diffraction patterns to about 6 A resolution. Knowing the atomic structure of G-actin it was possible for the group of Ken Holmes to determine its orientation in the F-actin fiber, and thus arrive at an atomic model of the actin filament that best accounted for the fiber diffraction pattern. 

The value of the angle of tilting of the texture can be determined from the analysis of x-ray fiber diagrams. As Urbanczyk noted [18], the position of layer reflexes oil and ill or the position of equatorial reflexes 010 and 100 can be analyzed. In the first case, the tilting angle of the texture (v ) can be determined from the equation ... 

Five articles on polysaccharide helices solved prior to 1979 have appeared in the volumes published between 1967 and 1982.2-6 The first was a review on X-ray fiber diffraction and its application to cellulose, chitin, amylose, and related structures, and the rest were bibliographic accounts. Since then, X-ray structures of several new polysaccharides composed of simple to complex repeating units have been successfully determined, thanks to technological advances in fiber-diffraction techniques, the availability of fast and powerful computers, and the development of sophisticated software. Also, some old models have been either re-... 

The Pn conformation of poly-L-proline (PP) or collagen in the solid state could be identified from X-ray fiber diffraction results (Cowan and McGavin, 1955). Persistence of this basic structure in solution was inferred from the resemblance between the CD spectra of solutions and films of the polypeptide. The CD spectra of the charged forms of PGA and PL closely resemble that of Pn (compare Fig. IB, 1C, and ID) however, these spectra differ significantly from those of PP peptides at high temperature or in the presence of high concentration of salts... 

Figure 6. X-ray fiber diagram of poly(5-methyl-1,4-hexadiene) prepared with a Etj,AlCl/S-TiCl, catalyst at 0°C in pentane solvent. Compression molded sample cold drawn to four times its original length.

Different kinds of disorder may affect differently the X-ray diffraction pattern of the crystals. Depending on the features present in the X-ray fiber diffraction pattern, it is useful to distinguish three main classes of disordered structures170 ... 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

Amyloid fibrils form from a variety of native proteins with diverse sequences and folds. The classic method for the structural analysis of amyloid has been X-ray fiber diffraction amyloid fibrils exhibit a characteristic diffraction signature, called the cross-/) pattern. This cross-/ pattern suggested a repeating structure in which /1-sheets run parallel to the fiber axis with their constituent /1-strands perpendicular to that direction (Sunde and Blake, 1997). This diffraction signature pointed to an underlying common core molecular structure for the amyloid fibril that could accommodate diverse sequences and folds. A number of groups have proposed amyloid folds that are consistent with the experimental data and these can be linked to repeating /1-structured units. 

Cross-/] structure has been demonstrated for Sup35pNM filaments. Serio et al. (2000) observed a 0.47-nm reflection by X-ray diffraction, and subsequently this reflection was shown to be meridional both by X-ray fiber diffraction (Kishimoto et al., 2004) and electron diffraction (King and Diaz-Avalos, 2004). In the Ure2p system, cross-/ structure has been established by electron diffraction from prion domain filaments preserved in vitreous ice (Fig. 7 Baxa et al, 2005). In addition, a 0.47-nm reflection was detected by both X-ray diffraction and electron diffraction from filament preparations of full-length Ure2p and the Ure2p1 65-GFP fusion, indicating that they contain the same structure (Fig. 7 Baxa et al, 2005). 

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