Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Chemical Basis of Life
Sugars and Polysaccharides
Cellulose

Cellulose is the main structural element of all plant Cells, from Algae to trees, and the most abundant organic compound on Earth. Typical Examples of Materials consisting mainly of cellulose are cotton and wood. It is estimated that 1011 tons of cellulose are produced annually in the biosphere. Each cellulose molecule is a long, unbranched chain built from d-glucose residues, with a molecular weight ranging from 50,000 to 1 million or more.

Although the glucose residues in cellulose are linked by 1,4-glycosidic bonds, these bonds differ from those involved in The Structure of amylose (compare the structural formula below with that of amylose). This seemingly minor difference has major consequences. The α-1,4-glycosidic bonds characteristic of starch and Glycogen are easily cleaved (hydrolyzed) by Enzymes of many microorganisms, plants, and animals; in contrast, only very few Living organisms are capable of hydrolyzing the β-1,4-bonds of cellulose. One of the common products of Enzymatic Hydrolysis of cellulose is the disaccharide cellobiose, whose molecule is composed of two glucose residues linked by a β-1,4-glycosidic bond.

The resistance of cellulose to degradation in both natural and laboratory environments is due not so much to the Specific features of the β-1,4-glycosidic bond, but rather to the crystalline structure of cellulose and the unique packaging of its molecules in biological structures. As shown in Fig. 2.4, cellulose is characterized by intrachain Hydrogen Bonds between the hydroxyl group at C-3 and the oxygen atom of the pyranose ring, and occasionally also by interchain hydrogen bonds. These bonds lead to The formation of large supramolecular structures from individual cellulose chains, called crystallites, which are easily visible in electron micrographs.

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Several different models of the crystalline structure of cellulose have been proposed. For our purposes, it is sufficient to outline the Main Features of these models, schematically shown in Fig. 2.5. Most of the cellulose is concentrated in highly ordered crystalline regions, where the cellulose chains or fibrils are packed so tightly that even Water molecules penetrate them with great difficulty. For this reason, cellulose cannot dissolve in water. Less ordered regions, called amorphous, typically make up about 15% of the cellulose microstructure. Amorphous regions are relatively easily hydrolyzed, for example, by acids; crystalline regions are much more resistant to degradation.

FIG. 2.4. Schematic representation of the hydrogen bonding system between glucose residues in cellulose. The symbol R denotes the site of possible chemical modification of cellulose. Thus, in methylcellulose, acetylcellulose, and carboxymethylcellulose, R = CH₃, COCH₃, and CH₂COONa, respectively.

The chains of cellulose molecules are grouped into microfibrils, which are often shown in cross-section as depicted in Fig. 2.5, a. Here, the solid lines represent the planes of the monomeric glucose units, while the dashed lines indicate the orientation of another important polysaccharide called hemicellulose. In wood and other cellulosic materials, hemicellulose molecules surround clusters of microfibrils; these complex structural elements are, in turn, enclosed in a Lignin sheath reinforced by numerous cross-links.

FIG. 2.5. Structure of cellulose microfibrils. [Reproduced from: Mülethaner K., Ann. Rev. Plant Physiol., 18, 1 (1967).]

Materials of this structure are often referred to as lignocellulose. The data presented in Table 2.3 show that plant biomass, pulp and paper products, and their waste contain significant amounts of lignocellulose, in which the relative content of cellulose, hemicellulose, and lignin can vary widely. These data also indicate that plant biomass is a self-renewable source of not only cellulose-based materials but also of vast quantities of other potentially valuable raw materials. In this regard, we will examine The properties of hemicellulose and lignin below.

Table 2.3. Relative content (wt. %) of cellulose, hemicellulose, and lignin in plant biomass, pulp and paper products, and their waste

The composition of hemicelluloses varies widely depending on the plant source. Generally, hemicelluloses are short, branched polymers composed of pentose residues (xylose and arabinose) and certain hexose residues (glucose, galactose, and mannose). These monomeric units, which typically contain A large number of acetyl groups, are linked by 1,3-, 1,6-, and 1,4-glycosidic bonds. The main Features of the Chemical Structure of hardwood hemicellulose are shown in the following diagram:

In this diagram, the dashes represent β-1,4-glycosidic bonds, and the numbers indicate the positions of the carbon atoms involved in the Formation of the various bonds.

We have not previously mentioned glucuronic acid. It is a typical representative of the uronic acid class, which are formed by the selective oxidation of one of the primary hydroxyl groups of a monosaccharide to a carboxyl group; specifically, glucuronic acid is a glucose derivative with a carboxyl group at position 6.

Hemicellulose is relatively easy to hydrolyze into soluble compounds, for example, by Treatment with dilute (0.05–3%) sulfuric acid or even hot water. In contrast, the lignin sheath surrounding the polysaccharide components of biomass is much more resistant to hydrolysis.

Lignin is a polyphenol of variable composition. An idea of the complexity and heterogeneity of this substance is provided by the structural model of spruce lignin shown in Fig. 2.6. Such a disordered combination of various structural units is highly resistant to chemical and enzymatic agents. Some of the currently used Methods for Processing lignin will be briefly discussed in Chapter 4. In concluding this Brief Overview of biomass chemistry and structure, it should be noted that the products of partial degradation of lignin are themselves potentially valuable chemicals and feedstocks for the chemical industry.

FIG. 2.6. Schematic two-dimensional representation of the chemical structure of spruce lignin. [Reproduced with permission from: Freudenberg K., Lignin: Its Constitution and Formation from p-Hydroxycinnamyl Alcohols; Science, 148, 595 (1965). © 1965 by the American Association for the Advancement of Science.]