Variants, crystal

Natural cellulose is now referred to as cellulose I. Cellulose 11 is obtained from cellulose I by the mercerization process, i.e., soaking and wash out with aqueous NaOH. The crystal structure of cellulose is shown in Fig. 1.20. Later researches have proposed variant crystal structures of cellulose (cellulose I [118], cellulose II [119, 120], and indeed a cellulose III [121], and cellulose IV [122]) different from the Meyer-Misch model. 

All static studies at pressures beyond 25 GPa are done with diamond-anvil cells conceived independently by Jamieson [32] and by Weir etal [33]. In these variants of Bridgman s design, the anvils are single-crystal gem-quality diamonds, the hardest known material, truncated with small flat faces (culets) usually less than 0.5 nun in diameter. Diamond anvils with 50 pm diameter or smaller culets can generate pressures to about 500 GPa, the highest static laboratory pressures equivalent to the pressure at the centre of the Earth. 

We consider first the Maier-Saupe tlieory and its variants. In its original foniiulation, tills tlieory assumed tliat orientational order in nematic liquid crystals arises from long-range dispersion forces which are weakly anisotropic [60, 61 and 62]. However, it has been pointed out [63] tliat tlie fonii of tlie Maier-Saupe potential is equivalent to one in... 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

In another process variant, only 88% of the nitrobenzene is reduced, and the reaction mixture then consists of two phases the precious metal catalyst (palladium on activated carbon) remains in the unreacted nitrobenzene phase. Therefore, phase separation is sufficient as work-up, and the nitrobenzene phase can be recycled direcdy to the next batch. The aqueous sulfuric acid phase contains 4-aminophenol and by-product aniline. After neutralization, the aniline is stripped, and the aminophenol is obtained by crystallization after the aqueous phase is purified with activated carbon (53). 

Detailed, critical surveys of the variants and complexities of crystal growth from the melt were published for low-melting metals by Goss (1963) and for high-melting metals (which present much greater difficulties) by Schadler (1963). 

Transmission electron microscopes (TEM) with their variants (scanning transmission microscopes, analytical microscopes, high-resolution microscopes, high-voltage microscopes) are now crucial tools in the study of materials crystal defects of all kinds, radiation damage, ofif-stoichiometric compounds, features of atomic order, polyphase microstructures, stages in phase transformations, orientation relationships between phases, recrystallisation, local textures, compositions of phases... there is no end to the features that are today studied by TEM. Newbury and Williams (2000) have surveyed the place of the electron microscope as the materials characterisation tool of the millennium . 

IBM in California by Eigler and Schweizer (1990), who manipulated individual xenon atoms aeross a niekel (110) crystal surfaee. In the immediate aftermath of this achievement, many other variants of atom manipulation by STM have been published, and DiNardo surveys these. 

Rosenow, M. A., Huffman, H. A., Phail, M. E., and Wachter, R. M. (2004). The crystal structure of the Y66L variant of green fluorescent protein supports a cyclization-oxidation-dehydration mechanism for chromophore maturation. Biochemistry 43 4464 1472. 

To a certain extent, the formation of the tetrabasic variant is desired, because 4PbO PbS04 forms fairly large crystals when transformed into lead dioxide (PbO,). This results in a mechanically stable active material, but there are disadvantages, because it is more difficult to transform this material into lead dioxide, i.e., the formation process (see below) is more expensive (and takes longer) and the initial capacity is slightly reduced (cf., e.g., Ref. [19]. For "long-life batteries" (Bell systems cell), a special process has been developed to produce pure tetrabasic material [20]. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

Courtney M., Jallat S., Tessier L-H., Benavente A., Crystal R.G. Lecocq J-P. (1985) Synthesis in E. coli of alpha,-antitrypsin variants of therapeutic potential for emphysema and thrombosis. Nature, 313, 149-151. 

While wild-type PAMO was unable to convert 2-phenylcyclohexanone efficiently, all deletion mutants readily accepted this ketone as substrate. All mutants also displayed a similar thermostability when compared with the parent enzyme. The most active mutant (deletion of S441 and A442) was used for examining its enantioselective properties. It was found that the mutant preferably formed the (/ )-enantiomer of the corresponding lactone E = 100). While CHMO also shows a similar enantioselective behavior, this PAMO deletion mutant is a better candidate for future applications due to its superior stability. This clearly demonstrates that PAMO can be used as parent enzyme to design thermostable BVMO variants. It also illustrates that the available crystal structure of PAMO will be of great help for BVMO redesign efforts. ... 

There are many variants of this system which can be envisaged as means by which the current possibilities for automation in data collection can be applied for specific purposes. There are considerable dangers in this approach in that it may be all too easy to build in restrictions which predetermine the results. These dangers, however, are not likely to be worse than those normally encountered in electron microscopy or single crystal diffraction where the one particularly "good-looking picture is taken as being "typical" of a sample. 

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