Ray and Electron Microscope Investigations

Further confirmation of the identity of bacterial and plant celluloses has been obtained by x-ray studies. From dried membranes prepared by the action of A. xylinum on sucrose, Eggert and Luft obtained x-ray diagrams similar to those of cotton cellulose. Hibbert and Barsha showed that a chloroform solution of the triacetate of cellulose 

Since bacterial cellulose from all suitable carbohydrate substrates is identical with natural cellulose, its industrial importance is obvious. Relatively large amounts of bacterial cellulose were produced in Germany during the first World War. More recently products similar to parchment, mercerized cotton, cellulose nitrate, acetate and viscose rayons have been produced from bacterial cellulose. 

From a theoretical standpoint, further study of the formation of bacterial cellulose might yield some information regarding the mechanism of plant synthesis. 

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We should also mention an early work by Slonimsky and Askadsky 74 who were apparently the first to observe structural changes taking place in extension under condition s of constant force. Three characteristic sections (see Fig. 20) were identified on the curves of strain versus tension time at F = const. These sections correspond to polymer flow in the amorphous state, the process of molecular ordering and crystallization, and, finally, to polymer flow in the crystalline state. The presence of crystalline formations on the latter section was detected with the help of X-ray-structural and electron-microscopic investigation of extended samples. As the tensile stress was lifted, the sample amorphised again and contracted. The occurrence of a drastic increase in strain on the second section was accounted for 74) by exhaustion of the longevity of supramolecular structures. 

Thus, the important question of the secondary structure of the transmembrane elements can only be addressed with models and by structural comparison with other transmembrane proteins for which the structure has been resolved. Detailed information on the structure of transmembrane elements is available for the photoreaction center of Rhodopseudomonas viridis (review Deisenhofer and Michel, 1989), cytochrome c oxidase (Iwata et al., 1995) and the OmpF porin of E. coli (Cowan et al., 1992 Fig. 5.3), amongst others. In addition, high resolution electron microscopic investigations and X-ray studies of bacteriorhodopsin, a light-driven ion pump with seven transmembrane elements, have yielded valuable information on the structure and configuration of membrane-spaiming elements (Henderson et al., 1990 Kimura et al., 1997 Pebay-Peyrula et al., 1997 Fig. 5.4). With the successful crystallization of the photoreaction center of Rhodopseudomonas viridis, a membrane protein was displayed at atomic resolution for the first time (Deisenhofer et al., 1985). The membrane-... 

The experiments consisted of three parts. In the first part, the characterization of minerals was explained by using x-ray diffractometer and electron microscope studies. Also, it was performed electrokinetic s studies of suspension. The effect of particle shape and size on the vacuum and pressure filtration of minerals has been investigated in the second part of the study. At the last part, the comparison of the particle shape and size effect on shear strength of the mineral filter cakes was performed. 

The dissociation and reconstitution of RubisCO subunits from prokar yotes(l,2) and eucaryotes(3) have been investigated recently. It has been demonstrated that the small subunit is a necessary component for RubisCO catalysis, and it is inferred that the role of the small subunit in maintenance of RubisCO activity is probably related to its spatial arrangment in the molecule. Up to now, insights into the quaternary structure of the LgSg RubisCO are almost solely based on X-ray crystallographic and electron microscopic studies(4,5). 

The structure and morphology of fibers and films can be described at various levels of structural dimensions, depending on the resolution of the diffraction equipment and of the microscopes used in the investigation. First of all the chain conformation in the solid state is discussed, then the packing modes of the chains in the crystallites determined by X-ray diffraction are reviewed, next the structural characteristics of the fibrils observed by X-ray and electron diffraction are examined, and finally the morphological features as seen by electron and optical microscopy are dealt with. After this survey of the structure and morphology, the formation of the fiber and film by coagulation from a lyotropic solution is discussed. 

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

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