Structural models, cellulose

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

FIGURE 20-32 A plausible model for the structure of cellulose synthase. The enzyme complex includes a catalytic subunit with eight transmembrane segments and several other subunits that are presumed to act in threading cellulose chains through the catalytic site and out of the cell, and in the crystallization of 36 cellulose strands into the paracrystalline microfibrils shown in Figure 20-29. 

The constitutional formula and molecular weight of cellulose determined on the basis of chemical and physico-chemical experiments has been confirmed by X-ray analysis, which has also led to the discovery of the microcrystalline structure of cellulose. Today the structural model proposed by Meyer and Mark [21] and Mark and Misch [22] based on the X-ray measurements of Polanyi [23] and Sponsler and Dore [24] and taking into consideration Haworth s conclusions about the existence... 

This paper is a review of x-ray diffraction work in the authors laboratory to refine the structures of cellulose I and II, and a- and B-chitin, concentrating on the methods used to select between alternate models. Cellulose I is shown to consist of an array of parallel chains, and this conclusion is supported by a separate refinement based on electron diffraction data. In the case of cellulose II, both parallel and antiparallel chain... 

Figure l.(Top) Structure of cellulose molecule, (bottom) model of structure... 

Figure 30. Possible models for the structure of cellulose and nitrocellulose (after Ref. 18J...

Figure 7.43. SIMS analysis of lignin, the second most abundant biopolymer in nature, following cellulose. Shown is the phenylpropane subunits and a structural model of softwood Ugnin (top). The secondary-ion mass spectra of pine (softwood) and beech (hardwood) milled wood Ugnin (MWL, a solvent-extracted form of Ugnin from beech wood) are also shown (bottom). Reproduced with permission from Saito, K. Kato, T. Tsuji, Y. Fukushima, TL. Biomacromolecules 2005, 6,678. Copyright 2005 American Chemical Society.

Ball-and-stick models showing the three-dimensional structures of cellulose and starch were given in Figure 5.2. 

A start has been made toward solving the structural problems of crystalline cellulose by the same method as that used for proteins. Jones (1046a) gave a set of criteria quite similar to that in Table 10-IV namely, (a) the sugar residues have standard bond lengths and angles, (b) all the O—H groups are H bonded with C—O 0 between 100° and 135°, and, (c) the residues are screw related. This paper also contains a helpful review of previous work on model cellulose structures. 

Vogt, U. Zugenmaier, P. Structural models for some liquid crystalline cellulose derivatives. Berichte der Bunsen-Gesellschaft 1985, 89 (11), 1217-1224. 

In addition to the disallowed reflections in the electron diffraction patterns that placed the crystallographic models in question, new spectral evidence was developed pointing to the need for further refinement of structural models, particularly for native celluloses. The models derived from the crystallographic studies could not rationalize many features of the spectral data known to be quite sensitive to structural variations. 

With the recognition that all plant celluloses are composites of the and I,g forms it was possible to rationalize many of the earlier difficulties in developing suitable structural models. It became clear that the efforts to reconcile the diffraction patterns in terms of a unique unit cell for native celluloses were frustrated by the reality that the celluloses were composites of two crystalline forms that were blended in different proportions in celluloses produced by different organisms. 

T,he chemical structure of cellulose chains was established by Haworth and Hibbert more than 40 years ago. Native cellulose occurs in solid state and in partly crystalline form. A schematic model for the native cellulose lattice was worked out about 1930 by Meyer, Mark, and Misch. The basic morphology of native cellulose could not be resolved, however, before electron microscopy with high resolution was developed and applied. During the last two decades, it has been amply shown and is now generally accepted that native cellulose basically is composed of microfibrils of a width 100 A. or less as was reviewed in 1956 (12). 

Figure 5. Structural model of a cellulose microfibril containing folded cellulose chains (9)...

Figure 9 Schematic structure models of native cellulose and cupra rayon fibers-...

Thus, they provide information complementary to the diffractometric data in that it serves to constrain the acceptable structural models to a smaller subset than that otherwise admissible on the basis of diffractometric observations alone. In this respect, the spectroscopic information complements the diffractometric data in the same way as the assumptions concerning the symmetry of the unit cell. Furthermore, it appears that the structures suggested by the spectroscopic studies represent relatively small although significant departures from those derived on the basis of diffractometry alone. In anticipation of future directions in studies of celluloses, it is noted that multidisciplinary approaches, similar to some described in later chapters, hold great promise for future progress in understanding the structural diversity that is characteristic of cellulose. 

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. 

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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