Diffractometric studies

Gruszecki, W.I. and J. Sielewiesiuk. 1991. Galactolipid multibilayers modified with xanthophylls Orientational and diffractometric studies. Biochim. Biophys. Acta 1069 21-26. 

Hizukuri, S. (1961). X-ray diffractometric studies on starches. Agric. Biol. Chem. 25, 45 9. 

The primary sources of information concerning the molecular structure of cellulose have been x-ray and electron diffractometric studies, conformational analyses, and vibrational spectroscopy. The work up to 1971 was very ably reviewed by Jones (10), and by T0nnesen and Ellefsen (II, 12). They generally concluded that although much evidence can be interpreted in terms of cellulose chains possessing a two-fold axis of symmetry, in both Celluloses I and II, none of the structures proposed... 

The primary difficulty in diffractometric studies of polymeric materials in general is that the number of reflections recorded in a fiber diagram is usually less than 100 (15). This is in contrast with observation of 1000-2000 reflections in a typical study of the structure of a small... 

The considerations involving comparisons of the structures of cellobiose and / -methylcellobioside with the structures of mercerized and native cellulose, respectively, when taken together with the additional observation that the basic repeat unit derived from the diffractometric studies is 10.3 A rather than 5.15 A, require that data relating to the structure of cellulose be reexamined with the constraint that the anhydro-cellobiose unit, rather than the anhydroglucose unit, is the basic repeat unit. To the author s knowledge, no efforts have been made to interpret... 

Sankar, G. and Thomas, J. M. In situ combined X-ray absorption spectroscopic and X-ray diffractometric studies of solid catalysts, Top. Catal., 1999, 8, 1-21. 

In the high temperature x-ray diffractometric studies, Perri et al. (7J) indicate that WOg undergoes a phase transition from monoclinic to orthorhombic at approximately 593 K and from orthorhombic to tetragonal at 993 K. 

Saesmaa T, Makela T, Tannienen VP. Physical studies on the benzathine and emanate salts of some /3-lactam antibiotics. Part 1. X-ray powder diffractometric study. Acta Pharm Perm 1990 99 157-162. 

In summary then, the Raman and infrared spectral studies undertaken after the discovery of the composite nature of native celluloses point to the conclusion that the only difference between the two forms is in the pattern of hydrogen bonding between chains that possess identical conformations. Yet electron diffractometric studies have been interpreted to indicate that the two forms represent two crystalline phases with different crystal habits." More recently, diffractometric studies by Nishiyama have also been interpreted along... 

One of the discoveries growing out of the early diffractometric studies of cellulose was that it can occur in a number of allomorphic forms in the solid state, each producing distinctive X-ray diffractometric patterns. In addition to the cellulose II form, which has been discussed extensively, two other forms have been recognized these are cellulose III and cellulose IV. It is of interest to consider them briefly because they reflect the capacity of cellulose to aggregate in a wide variety of secondary and tertiary structures and because some of the higher plant celluloses produce diffraction patterns that are not unlike those of cellulose IV. Furthermore, they reflect the tendency for some of the celluloses to retain some memory of their earlier states of aggregation in a manner not yet understood. 

Quite early in the x-ray diffractometric studies of cellulose it was recognized that its crystallinity is polymorphic. It was established that native cellulose, on the one hand, and both regenrated and mercerized celluloses, on the other, represent two distinct crystallographic allomorphs (14). Little has transpired... 

Two classes of spectral studies have been applied for the first time during the past decade as the basis of structural studies of cellulose. These are Raman spectroscopy, and solid state NMR using the CP/MAS technique. Both have raised questions concerning the assumptions about symmetry incorporated in the diffractometric studies. And while they cannot provide direct information concerning the structures, they establish criteria that any structure must meet to be regarded as an adequate model. The information from spectroscopic studies represents one of the major portions of the phenomenology that any acceptable structural model must rationalize. 

In addition to their diffractometric studies reported in prior publications, they add in their contribution to the present symposium analyses of the infrared spectra as well as analyses of the CP-MAS 13c NMR spectra. Their thesis is not inconsistent with the proposals of Atalla and coworkers concerning differences between the conformations of celluloses I and II. However, Hayashi and coworkers go beyond this by proposing that the differences in conformation can be preserved in the course of heterogeneous derivati-zation reactions, and also in the process of generating the other allomorphs of cellulose, namely celluloses III and IV, from the two primary allomorphs 1 and II. 

It is clear that the new information developed from spectroscopic and multidisciplinary studies provides a basis for initiating diffractometric studies with a different set of constraints than those used in the past. The refinements are likely to be more complex, but the expectation is that the structures thus derived will more closely approximate the molecular structure of cellulose. Such models may then provide more comprehensive rationalizations of the phenomenology of cellulose. 

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