Native cellulose crystalline structures

D. L. A. VanderHart andR. H. Atalla, Further C NMR evidence for the co-existence of two crystallines forms in native celluloses, The Structures of Cellulose, Characterization of the Solid States, ACS Symposium Ser., 1987, pp. 88-118. 

In native cellulose, the structure develops under conditions of thermodynamic equilibrium and occurs very slowly. For regenerated cellulose, however, not only must the structure be formed rapidly, but also the organization of the macromolecules by crystallization is constrained by the extent of tangling present in the solution. It was suggested by Baker [261] that the structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than by definite polymorphic forms of cellulose. This view is supported by the observation that the x-ray diffraction pattern of rayon often reveals both cellulose II and IV components to an extent, depending on the conditions used to make the fiber. Hindeleh and Johnson [262] have described an x-ray diffraction procedure to measure crystallinity and crystallite size in cellulose fibers by which the relative proportions of cellulose II and IV in rayon can be determined. 

Polymers with rigid, cyclic structures in the polymer chain, as in cellulose and poly(ethy-leneterephthalate), are difficult to crystallize. Moderate crystallization does occur in these cases, as a result of the polar polymer chains. Additional crystallization can be induced by mechanical stretching. Cellulose is interesting in that native cellulose in the form of cotton is much more crystalline than cellulose that is obtained by precipitation of cellulose from... 

Fungal cellulase enzyme systems capable of efficiently catalyzing the hydrolytic degradation of crystalline cellulose are typically composed of endo-acting cellulases (EGs), exo-acting cellulases (CBHs), and at least one cellobiase (1-6). The CBHs are typically the predominant enzymes, on a mole fraction basis, in such systems (7). Consequently, the CBHs have been the focus of many studies (8). The three-dimensional structure of prototypical CBHs is known (9-12) and their specificities are, in general, well characterized (13,14). However, mechanism-based kinetic analyses of CBH-catalyzed cellulose saccharification are rather limited (15,16). Studies of this latter type are particularly difficult owing to the inherent complexity of native cellulose substrates. 

Corn stover, like lignocellulosic materials in general, is resistant to enzymatic hydrolysis, because of both the tight network in the lignocellulose complex and the crystalline structure of the native cellulose. These difficulties can be overcome by employing a suitable pretreatment (7). 

No critical discussion of biosynthesis can be attempted without consideration of the information available on the structure of native cellulose. As for studies on biosynthesis, much controversy has existed in the field of structural analyses of cellulose. There seem to be only two points on which all workers are in agreement, namely, that (a) native cellulose is a composite of linearly extended chains of (l->4)-j6-D-glu-can (1), and (b) strong intrachain and interchain hydrogen-bonding between D-glucosyl residues occurs in such a way as to create a highly insoluble, and partially crystalline, fibrillar structure. 

Although the chemical structure of cellulose is understood in detail, its supermolecular state, including its crystalline and fibrillar structure is still open to debate. Examples of incompletely solved problem areas are the exact molecular weight and polydispersity of native cellulose and the dimensions of the microfibrils. 

The crystalline structure of cellulose has been characterized by X-ray diffraction analysis and by methods based on the absorption of polarized infrared radiation. The unit cell of native cellulose (cellulose I) consists of four glucose residues (Figs. 3-6 and 3-7). In the chain direction (c), the repeating unit is a cellobiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a... 

Crystallinity long has been recognized as one of the characteristics of native cellulosic fibers. Indeed, native cellulose was observed to diffract x-rays, in the manner characteristic of three-dimensionally ordered molecular systems, before the hypothesis of polymeric structure had been proposed by Staudinger (1,2). 

Cellulosic materials usually form crystal structures in part, and water cannot penetrate the inside of crystalline domains at room temperature. Native celluloses form crystalline microfibrils or bundles of cellulose chains 2-5 nm in width for higher plant celluloses and 15-30 nm for algal celluloses, which are observable by electron microscope. Almost all native celluloses have X-ray diffraction patterns of cellulose I with crystallinity indexes (Cl) 13] of about 40-95 %. 

In the crystalline part, the cellobiose units are closely packed to form Cellulose I in native cellulose fibres and Cellulose II in regenerated cellulose fibres. In Cellulose I the chain molecules are parallel to one another [16]. The folded chain occurs at Cellulose II, in the crystalline regions the chain molecules are antiparallel. Thus, the basis for helical structure for Cellulose I is preferably extended to the structure of Cellulose II [17]. 

The first attempt to rationalize the spectra was in terms of information that they might provide concerning a more complex structure for the unit cell of cellulose I, perhaps along the lines suggested by Honjo and Watanabe in the 1950s. However, it soon became obvious that such a rationalization was not possible because the relative intensities within the multiplets were neither constant nor were they in ratios of small whole numbers as would be the case if the same unit cell prevailed throughout the crystalline domains. The conclusion was that the multiplicities were evidence of site heterogeniety within the crystalline domains, and that therefore native celluloses must be composites of more than one crystalline form. 

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