Fibers diffraction pattern

Polymer or Fiber Diffraction. Polymers and fibers are often ordered ia one or two dimensions but not ordered ia the second or third dimension. The resulting diffraction patterns have broad diffuse diffraction maxima. The abiHty to coUect two dimensional images makes it possible to coUect and analyze polymer and fiber diffraction patterns. 

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 occurrence of the mesophase in the fiber is confirmed by x-ray diffraction examination. The occurrence of three equatorial reflections 010, 110, and 100, the absence of layer and meridional reflections, and the manifestation of the intensity maximum of diffusively scattered radiation at 20 = 19 in the fiber diffraction pattern are the criterion for the presence of the mesophase. The... 

A fiber-diffraction pattern is recorded on a flat-film camera in which the fiber-to-photographic film distance is typically in the range of 3 to 4 cm. During exposure to X-rays, the specimen chamber is continuously flushed with a slow and steady stream of helium gas that has been bubbled through a saturated salt solution so that (a) the fiber is maintained at a constant desired r.h. and (b) fogging of the photographic film from air scattering is reduced. 

In contrast to single-crystal work, a fiber-diffraction pattern contains much fewer reflections going up to about 3 A resolution. This is a major drawback and it arises either as a result of accidental overlap of reflections that have the same / value and the same Bragg angle 0, or because of systematic superposition of hkl and its counterparts (-h-kl, h-kl, and -hkl, as in an orthorhombic system, for example). Sometimes, two or more adjacent reflections might be too close to separate analytically. Under such circumstances, these reflections have to be considered individually in structure-factor calculation and compounded properly for comparison with the observed composite reflection. Unobserved reflections that are too weak to see are assigned threshold values, based on the lowest measured intensities. Nevertheless, the number of available X-ray data is far fewer than the number of atomic coordinates in a repeat of the helix. Thus, X-ray data alone is inadequate to solve a fiber structure. 

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. 

Fig. 2. The differences that might be observed in the fiber diffraction patterns from oriented samples of antiparallel /1-structures depending on whether the chains are aligned along (A, B) or perpendicular to (C, D) the fiber axis. Color coding on (A, B) as in Fig. 1.

The notion of a common core structure has been further supported by synchrotron X-ray fiber diffraction patterns of several amyloid fibrils the patterns show common reflections in addition to those at 4.7 and 10 A (Sunde et al., 1997). Although these data give some insight into the arrangement of the amyloid fibril core, the exact molecular structure and organization of the proteins making up this common core have yet to be uniquely defined. The inherently noncrystalline, insoluble nature of the fibrils makes their structures difficult to study via traditional techniques of X-ray crystallography and solution NMR. An impressive breadth of biochemical and biophysical techniques has therefore been employed to illuminate additional features of amyloid fibril structure. 

In a later publication, Kishimoto et al. (2004) proposed the water-filled nanotube as a model for the fibrillar N-terminal domain of the yeast prion Sup35p. The authors find that hydrated Sup35p fibrils show no 10-A equatorial reflection in the fiber diffraction pattern, but that dried fibrils... 

A fiber diffraction pattern of the potassium salt is shown in Figure 2 ( 8). Sharp Bragg reflections extend to approximately 3.0 A resolution with meridional intensities on the 6th and 9th layer lines. The diffraction pattern can be indexed on the basis of a... 

Figure 2. X-ray fiber diffraction pattern from K " chondroitin 4-sulfate. (Reproduced with permission from ref. 18. Copyright 1983 Academic Press Inc.)...

Figure 9.4 Fiber diffraction patterns from A-DNA (left half of figure) and B-DNA (right). A-DNA was microcrystalline and thus gave discrete, but overlapping, Bragg reflections. B-DNA was noncrystalline and thus gave continous variation in intensity along each layer line. Image kindly provided by Professor Kenneth Holmes.

---