Structure of hemicelluloses

While the hemicelluloses obtained from the germ, aleuron, and caryopsis coat cell walls all showed a similar monosaccharide composition, this was not the case for the endosperm tissue. Thus, a major difference in the structure of hemicellulosic polysaccharides exists between the preparations obtained from the endosperm cell walls and those from the cell walls of the other parts of the grain, i.e., rice bran. (Rice bran consists of the caryopsis coat, aleuron layer and germ.) Comparison of the detailed structural features of the hemicellulosic polysaccharides of endosperm and bran cell walls will be discussed in the following sections. 

Figure 2. Structure of hemicellulosic polysaccharides obtained from the rice endosperm cell wall, (a) From ref. 12 (b) and (c), deduced from the results described in ref. 11.

Little has been done in studying the size and structure of hemicellulose molecules. Some that have been isolated from hardwoods and separated from lignin and pectic materials consist of a monomethyluronic acid combined with a series of molecules of n-xylose. The difficulty with... 

Figure 4.2.24 Typical structures of hemicelluloses or polyoses hardwood D-xylan and glucomannan and softwood galactan.

The chemical structure of hemicelluloses is the most important factor that influences their properties [1,4,7]. 

Figure 7.2 Typical chemical structure of hemicelluloses. The structure of hemicel-lulose is typically composed of five different sugar monomers D-xylose, L-arabinose, D-galactose, D-glucose, and D-mannose.

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. 

Due to the complex structures of hemicelluloses, several different enzymes are involved in their enzymatic degradation and modification (Table II) (7-70). Enzymes are nature s own catalysts, which act in mild conditions. Consequently, the use of enzymes offers an excellent alternative to engineer the properties of hemicelluloses in a controlled way. This paper discusses properties of hemicellulases and the possibilities for enzyme-aided modifications of hemicelluloses. 

What are the differences between the structures of hemicellulose and cellulose, and how do these differences contribute to the role of hemicellulose in the cell wall ... 

Figure 1.3b Structures of hemicelluloses. Reprinted with permission from Thakur and Thakur [2014]. Copyright 2014 Eisevier.

Raw knowledge about the main chemical elements in the composition of wood is insufficient to model gasification processes. The elements in question are mainly components of long chain hydrocarbons, like fignin, cellulose, and hemicelluloses (see Table 5.15 and Fig. 5.101). The structure of hemicelluloses is very similar to... 

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