Sheet crystallization

Zaleski-Ejgierd, P., Hakala, M. and Pyykkd, P. (2007) Comparison of chain versus sheet crystal structures for the... 

To a stirred solution of NH4C1 (1.25 g) in H20 (40 mL) was added fluoronitrobenzene (2.85 g, 23 mmol). Zn powder (2.95 g, 45 mmol) was added portionwise over 15 min and the mixture was heated to reflux for some time. ZnO was filtered off from the warm mixture and the filtrate was mixed with a solution of FeCl3 (5.5 g) in ice water (135 mL). The precipitate was filtered off, dried between filter paper sheets, crystallized (EtOH), and steam-distilled to give pure 8 yields l-fluoro-2-nitrosobenzene, 49% mp 19 C l-fluoro-3-nitrosobenzene, 87% mp 51 C l-fluoro-4-nitrosobenzene, 89% mp 39 C. 

Model systems that simulate the photographic emulsion have been used extensively as simpler systems for mechanistic studies. These include macroscopic sheet crystals grown from the melt (55-58), "evaporated layers" formed by evaporating silver... 

Sensitization of Model Systems. Silver bromide sheet crystals can be sulfur-sensitized easily, and have been studied extensively (56-58). Evaporated silver halide layers and sprayed emulsions likewise can be sulfur-sensitized. 

Zhelev and Astrov (125) studied the effects of S-sensitiza1-tion on the stability of silver deposited on evaporated films of silver bromide. The bulk distribution of development centers formed on the grain surface by deposition of silver atoms from vacuum was identical with that formed by exposure to high-irrad-iance light flashes or by the action of hydrogen atoms (126) on the silver bromide. As in experiments with unsensitized sheet crystals (127), surface developability disappeared upon storage... 

Unlike rhodium, iridium can be an effective sensitizer for latent image formation. Mitchell and associates (187), and West and Saunders (188) used chloroiridite to sensitize sheet crystals of silver bromde. Silver chloroiridite, formed by adsorption of chloroiridite ions by the crystal surface, provides an effective hole trap. However, gelatin interferes with its use in this way in photographic emulsions. 

Mitchell s views on the mechanism of latent image formation are based on theoretical calculations and extensive experimental work on silver bromide sheet crystals (55,56). He accepts the participation of separate electronic and ionic processes in the formation of latent image centers, which he termed the Gurney-Mott principle, but changes the order of the two processes. 

The specific value of the crossover depends on both the emulsion used and the conditions of exposure. For exposures made in air, the crossover occurs at a reduction potential that is less negative the lower the pAg.of the emulsion (248). Honda and associates (264) attribute this dependence to "anodic and cathodic shifts of the band edge of the silver halide, respectively." They support this explanation by experiments on the effects of silver ions and halide ions on the sensitized photocurrent in an electrochemical cell having an AgBr or AgCl sheet crystal window. 

Figure 14.22 plots the oxygen permeability of PET nonoriented sheets and biaxially oriented bottles as a function of the degree of crystallinity. Indeed, the effect of crystallinity is larger than that of biaxial orientation. But in practical terms, nonoriented sheets crystallize much more slowly than the biaxially oriented bottle walls, because deformation-induced orientation proceeds at higher rates at any temperature between Tg and Tm. That is, the schematically represented crystallization rate curve in Fig. 14.21 extends vertically upwards. 

Figure 2 Schematic representation of the repetitive structure of (a) fibroin (Sehnal and Zurovec, 2004), (b) the crystalline and amorphous regions in silk, and (c) the structure of the parallel P-sheet crystals (Zhou et al., 2001). The bullnose symbols in (a) depict the nonrepetitive termini. Each pair of rectangle and triangle represents a second-order repeat unit. Each line on the symbols is the unit of third-order repeat. Crystalline regions are shown in black (Gosline et al., 1986 Heslot, 1998).

It is interesting that the mode of cleavage of a silicate may reflect its structure. The asbestos minerals, which may be broken down into fibres, are related to the chainlike amphiboles, whereas the micas (composed of layers) may be easily cleaved to thin sheets. Crystals of the framework silicates are much more difficult to break, for they have chemical bonds extending over three dimensions. 

The serpentine group of minerals, which include chrysotile asbestos, are almost identical in composition. The chemical composition of unit cell is Mg6(OH)8Si40io. Chrysotiles have layered or sheeted crystal structure containing a silica sheet of (Si20s) in which silica tetrahedra point one way (Streib 1978). A layer of brucite, Mg(OH)2, joins the silica tetrahedra on one side of the sheet structure. Two out of every three —OH are replaced by oxygen atoms. X-ray and electron microscope studies indicate... 

Keten S, Xu Z, Ihle B, Buehler MJ (2010) Nanoctnrfinement controls stiffiiess, strength and mechanical toughness of p-sheet crystals in silk. Nat Mater 9(4) 359-367... 

Keten, S., Xu, Z., Ihle, B., and Buehler, M.J. (2010) Nanoconfinement controls stiffness, strength and mechanical toughness of beta-sheet crystals in silk. Nature Materials, 9, 359-367. 

Crystalene, PE sheet, Crystal-X Plastics Cryslalfll, Ultrafine powdered glass. Potters Industries Inc. 

Numata, K., Cebe, P., Kaplan, D.L., 2010. Mechanism of enzymatic degradation of beta-sheet crystals. Biomaterials 31 (10), 2926—2933. 

The intended role of the glycylalanine repeats in this design is the formation of p-sheet crystalline stems,7.8 while residues X and Y are chosen to form reverse tums.9,10 In each case, the selection of local sequence elements was based on information about the conformational properties of known polypeptides poly(glycylalanine) has been reported to form p-sheet crystals, , and Sibanda and Thornton have identified five X,Y pairs that appear to favor p-hairpin formation in globular proteins.9,10 As of this writing, we have expressed six different sequence variants of 1, and several chain-length variants as well. Detailed structural analysis by x-ray scattering and spectroscopic methods is underway and will be repotted elsewhere. 

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