Diffraction patterns helical

Figure 18.14 The diffraction pattern of helices in fiber crystallites can be simulated by the diffraction pattern of a single slit with the shape of a sine curve (representing the projection of a helix). Two such simulations are given in (a) and (b), with the helix shown to the left of its diffraction pattern. The spacing between the layer lines is inversely related to the helix pitch, P and the angle of the cross arms in the diffraction pattern is related to the angle of climb of the helix, 6. The helix in (b) has a smaller pitch and angle of climb than the helix in (a). (Courtesy of W. Fuller.)...

Clathrate structures have been recently obtained also for s-PS [8] and s-PPMS [10]. In particular for s-PS, the treatment of amorphous samples, as well as of crystalline samples in the a or in the y form, produces clathrate structures including helices having s (2/1)2 symmetries, which present similar diffraction patterns, independently of the considered solvent. The treatment of samples of s-PPMS with suitable solvents also produces clathrate structures including s(2/l)2 helices however, the large differences in the X-ray diffraction patterns suggest different modes of packing, depending on the included solvent. 

Crystalline samples of syndiotactic poly(methyl methacrylate) (st-PMMA) may be obtained from chloroacetone 178). This guest could be completely replaced by a variety of other guest molecules such as acetone, 1,3-dichloroacetone, bromoacetone, pinacolone, cyclohexanone, acetophenone and benzene. The X-ray diffraction patterns for these inclusion compounds were similar. These data indicate that the st-PMMA chains adopt a helical conformation of radius about 8 A and pitch 8.85 A. The guest molecules are located both inside the helical canals and in interhelix interstitial sites. 

Another example of symmetry breaking is provided by the crystal structure of form III of iP4MP. Electron diffraction patterns of the square single crystals of form III have indicated that chains in fourfold helical conformation... 

As already shown in Figure 2.29a, in the ideal limit-ordered model of form I of sPP, right- and left-handed twofold helical chains alternate along the a and b axes of the orthorhombic unit cell.59 In the electron diffraction patterns of single crystals grown at low temperatures, reported by Lovinger et al.,145 the... 

CPMAS spectrum typical of the regular twofold helical conformation (Figure 2.45b).147 191 Samples of sPP which present X-ray powder diffraction patterns typical of the isochiral form II (Figure 2.45c) show a different solid-state 13C NMR spectrum with additional signals in the region of the methylene and methyl carbon resonances (Figure 2A5d). These additional resonances have... 

While a polar-zipper model was initially proposed for Asp2Glni3Lys2 (Perutz et ah, 1994), more recently a water-filled nanotube was proposed in which the homopolymer in the / -conformation forms a helical array having 20 residues per turn (Perutz et ah, 2002a,b). (In the earlier work, the diffraction pattern had been interpreted as a fiber pattern—that is, with the 4.8-A reflection in the fibril direction.) Rather than being attributed to intersheet stacking, the 8.4-A reflection was not accounted for. Further,... 

A prime example of a Refolding model is that of the insulin protofilament (Jimenez et al., 2002). Insulin is a polypeptide hormone composed of two peptide chains of mainly o -helical secondary structure (Fig. 3A Adams et al., 1969). Its chains (21- and 30-amino acids long) are held together by 3 disulfide bonds, 2 interchain and 1 intrachain (Sanger, 1959). These bonds remain intact in the insulin amyloid fibrils of patients with injection amyloidosis (Dische et al., 1988). Fourier transform infrared (FTIR) and circular dichroic (CD) spectroscopy indicate that a conversion to jS-structure accompanies insulin fibril formation (Bouchard et al., 2000). The fibrils also give a cross-jS diffraction pattern (Burke and Rougvie, 1972). 

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