Native cellulose structural complexity

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). 

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. 

The new information from spectroscopic studies sheds new light in two key areas. The first is related to the complexity of the structures of the native celluloses. The second is that of the relationship between the structures of celluloses I and II. 

It was observed In earlier studies of controlled alkall-mercerlzatlon of ramie cellulose that the crystal structure of native cellulose Is transformed to cellulose II through a series of crystalline alkali-cellulose complexes (1,2). The relationships between these "Na-celluloses" and their pathways of transformation are Illustrated In Fig. 1. It has further been observed that all of the transformations are crystal-to-crystal phase changes, not Involving Intermediate amorphous phases. All of the experimental evidence has suggested... 

At this point the major features of the structures of cellulose I and II can be considered to be solved. Nevertheless, questions remain, especially as regards the differences between the Infrared, Raman, and solid state NMR spectra obtained for different native celluloses, all of which appear to have the crystal structures shown in Figure 1. This has been a further motivation for our analyses of the structures of cellulose solvent complexes, which are described below. 

Native cellulose is a linear condensation homopolymer with a complex stmcmre. Three structural levels can be considered [4, 7] ... 

The native structure of lignocellulose, which is very complex, renders it resistant to enzymatic hydrolysis. The major component is cellulose, a pi -> 4 polymer of glucose (see Fig. 8.2 a), which accounts for 35-50% of the mass. Lignocellulose also contains 20-35% hemicellulose, a complex polymer of pentoses and hexoses and 10-25% lignin [10, 12]. Techniques to convert lignocellulose into a fermentable sugar mixture have been under development for over 20 years and are now entering use. 

Hemicelluloses are a structurally heterogenic group of polysaccharides, which vary in their monosaccharide composition, glycosidic linkage content, substitution pattern and degree of polymerisation (Table I) (i). The primary structure of hemicelluloses depends on the type of plant and may even vary between different parts of the same plant (J-5). The term hemicellulose itself is not very clear. It is rather loosely defined as plant cell wall polysaccharides which are closely associated with cellulose (6). Hemicelluloses are often water soluble in native form but extractable in larger amounts only with alkaline solutions due to the complex multilayer structure of the cell walls. 

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