The Structure of Cellulose

Study of the structure of cellulose (Figure 22.2) leads one to expect that the molecules would be essentially extended and linear and capable of existing in the crystalline state. This is confirmed by X-ray data which indicate that the cell repeating unit (10.25 A) corresponds to the cellobiose repeating unit of the molecule. 

FIGURE 7.27 The structure of cellulose, showing the hydrogen bonds (blue) between the sheets, which strengthen the structure. Intrachain hydrogen bonds are in red and interchain hydrogen bonds are in green. 

The CP/MAS NMR spectra are an important source of information regarding the structure of cellulose and its polymorphos. A number of groups have investigated these spectra 11 15) and also reviews on the subject have been published 16 17>. For an orientation in the field Table 1 shows the most important features of the solid-state NMR spectra of cellulose I, II and IV and in Fig. 3 the numeration of the carbon atoms of the cellulose basic unit is given. It is evident that the polymorphs... 

The incorporation of water in the structure of cellulose influences. Upon the hydrogen bond structure of the macromolecule. A great deal of work has been done in this area. Calorimetric methods have been invaluable in helping to solve the problem 23 It is, however evident that solid-state NMR spectroscopy may also give valuable information. 

The structure of cellulose/solvent system complexes has been described by several schemes, differing essentially in the role played by the Ii+ and Cl ... 

The length-to diameter ratio strengthens the assumption that pectic acid is a fairly rigid, rod-hke molecule and comparable to the structure of cellulose... 

The structure of cellulose reveals the two key components for hydrogen bonding OH functional groups (the X-H unit) and additional oxygen atoms (the Y atom). Hence, water molecules may form intermolecular hydrogen bonds with cellulose in one of two ways ... 

In Geneva, he resumed with new energy his studies of macromolecules. He was able to obtain the cooperation of A. J. H. van der Wijk, who was one of his most devoted coworkers the latter s realistic criticisms were a valuable balance to Meyer s great enthusiasm. Studies on the thermodynamics of large molecules in solution, and on the structure of cellulose and chitin, were pursued with C. Boissonnas, W. Lothmar, and L. Misch. A theory of the elasticity of rubber evolved from his work with C. Ferri and his previous observations with Susich and Valk6. 

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. 

Xanthan. Xanthan, known commercially as xanthan gum [11138-66-2], has a main chain of (1 — 4)-linked (3-D-glucopyranosyl units therefore, the chemical structure of the main chain is identical to the structure of cellulose [9004-34-6]. However, in xanthan, every other (3-D-glucopyranosyl unit in the main chain is substituted on 0-3 with a trisaccharide unit. The trisaccharide side chain consists of (reading from the terminal, nonreducing end in towards the main chain) a P-D-mannopyranosyl unit linked (1 — 4) to a P-D-glucopyranosyluronic acid unit linked (1 — 2) to a... 

At the present moment it is difficult to decide which of the two hypotheses concerning the structure of cellulose is correct the idea of an amorpho-crystalline structure, or that postulating solely an amorphous texture. Nikitin assumes that the first hypothesis is the more probable, more especially as it is well in line with the most recent work of Zaydes and Sinitskaya [45] who conclude on the basis of electron diffraction investigations that in the natural cellulose fibre of Chinese nettle, there exist phases having a distinct microcrystalline structure. This suggests that structures shown in Figs. 78, 79 and 80 are the most probable. 

Changes in the conformation of the different forms of cellulose have been investigated157,158,195,196 by use of Raman spectroscopy. Celluloses I and II were found157 to have different, and distinct, molecular-chain conformations. No assignments of the frequencies were proposed, but the correlation between the spectra and the structure of celluloses was discussed. The major differences in the Raman spectra were observed below 800 cm-1, in the... 

In order to place the discussion in perspective, the next section will be devoted to a brief review of current ideas concerning the structure of cellulose, as well as to consideration of the implications of two crystallographic studies of cellobiose and 0-methylcellobioside. The response of different celluloses to acid and enzymatic hydrolysis will then be examined in light of the structural considerations. 

The findings summarized above, together with the information assembled by Rees and Skerrett on conformational analyses, led to a reassessment of the available information concerning the conformation of the glycosidic linkage, its variability, and the assumptions about it implicit in past investigations of the structure of cellulose. 

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... 

FIGURE 25 The structure of cellulose tris(phenylcarbamate). (From Ref. 1.)... 

Dyeing on a cellulose layer is totally different to dyeing directly on the textile, because the structure of the textile is different from the structure of cellulose. Depending on the type of dye used, there is either a chemical, an electrochemical18, an ionic18, an electrostatic19 or an adsorptive20 interaction with the textile structure. It is clear that this interaction would be totally different if the dye were to be used directly on the cellulose layer. The interaction could be weaker, or even zero, or could be based on a different mechanism. 

This symposium presented an unusual opportunity in that we discussed the methods used to determine polymer structures from fiber diffraction data, rather than concentrating on the actual structures derived and their possible implications. At Case Western Reserve University we have been involved in determination of the structures of cellulose and chitin. This paper describes our analyses (1-6) of the structures of cellulose I and II and a- and 6-chitin, emphasizing the manner in which structural decisions were taken in each case. Efforts to determine these structures have a history of over 60 years, and it has only been with the advent of least squares techniques for the refinement of polymer structures (7) [notably the LALS method (8)], and the development of our present knowledge of polysaccharide stereochemistry, that solutions have become possible. In what follows we will look first at our methods for measuring intensities and thereafter will review the work on each of the four structures. 

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