The Structure of Hemicelluloses

The other principal hemicellulose in softwoods is arabino-4-O-methylglucurono-xylan (degree of polymerization of 70-180). The backbone is composed of about 150 P D 1,4 xylopyranose units which are partially substituted at the C2 position by 4-0-methyl-a-D-glucuronic acid groups (approximately one group for every 5-6 xylose units). Also an a-L-arabinofuranose unit is linked by a-1,3 bond on approximately every 6 to 10 xylose units. Arabinofuranose is so called because it is a furanoside having a five-membered ring. 

Xylans comprise 5-10% wt/wt of the cell wall mass in softwoods. 

In hardwoods xylose is by far the most important hemicellulose monomer followed by mannose, glucose and galactose, with smaller amounts of arabinose and rhamnose. The xylose occurs predominantly as O-acetyl-4-O-methylglucuronoxylan (degree of polymerization of 100-200). The basic skeleton of all xylans is a linear 

C) Partial chemical structure of O-acelyl-4-O-methylglucuronoxylan (a hardwood hemicellulose). 

Ac = acefyl, COCHj Me = methyl,-CHj U = uronic acid group,-COOH Ara/ - arabinofuranose 

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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. 

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 ... 

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... 

In the past, research activities in the field of hemicellulose were aimed mainly at utilizing plant biomass by conversion into sugars, chemicals, fuel and as sources of heat energy. However, hemicelluloses, due to their structural varieties and diversity are also attractive as biopolymers, which can be utilized in their native or modified forms in various areas, including food and non-food applications. 

The structural varieties of hemicelluloses offer a number of possibilities for specific chemical, physical, and enzymic modifications. Future advancements will be based on the synthesis of hemicellulose-based polymers with new functionalities and with a well-defined and preset primary structure both on the level of the repeating imit and the polymer chain. Hemicelluloses have also started to be attractive to synthetic polymer chemists as... 

Figure 3 shows the structure of the hemicellulosic polysaccharides obtained from the bran cell wall preparations. These structures were deduced from the results of the methylation analysis of purified fractions (13). In contrast to the endosperm cell wall, no / -l,3-,l,4-glucan was obtained from... 

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.

The current observations confirm previous studies on beechwood and sprucewood holocellulose (7,10,19). The attack of the hemicellulose proceeds from the primary wall/Si as well as from the tertiary wall into S2 the pit chambers constitute preferred paths of enzyme diffusion into the walls. Also, substances of the middle lamella, especially in the cell corners, are removed by the xylanase and the mannanase treatments. Parallel to the removal of hemicelluloses, the fibrillar structure of the cellulose and its lamellar arrangement in transections of cell walls became obvious. In samples treated with cellulases, the cellulose fibrils were often completely hydrolyzed in the Si layer, occasionally accompanied by complete dissolution of cell-wall portions. This is also in conformity with the previous conclusion that the cellulases hydrolyze highly ordered zones of cellulose and remove hemicelluloses by hydrolysis or by detachment. 

In contrast to cellulose, which is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base, but nature provides an arsenal of hemicellulase enzymes for its hydrolysis. Hemicellulases are commercially important because they open the structure of wood for easier bleaching and thus support the introduction of ECF or TCF methods. Many different pentoses are usually present in hemicellulose. Xylose, however, is always the predominating sugar. The pentoses are also present in rings (not shown) that can be five- or six-membered. 

Mannose, galactose, and xylose supplementation demonstrated significant inhibition of hydrolysis rate (Fig. 6). Hydrolysis rates decreased by 32, 38, and 45% after supplementation with galactose, xylose, and mannose, respectively, at 100 g/L. The hemicellulose-derived sugars seemed to have a direct inhibitory effect on the cellulase enzymes. The detailed inhibitory mechanisms of these sugars on the cellulase activities were not clear and are currently under investigation. However, it is likely that the mechanisms will depend on both the characteristics of the specific inhibitors and the structure of the different enzymes. 

Lignocellulose denotes the mixture of the carbohydrate biopolymers cellulose and hemicellulose with the aromatic polymer lignin that is found in plants. Wooden raw materials consist mainly of cellulose (30-50 wt%), hemicellulose (10-40 wt%), and lignin (15-30 wt%). As the structure of cellulose (C6 carbohydrates) and hemicellulose (C5 carbohydrates) is quite similar, they will be discussed together in Section 2.2.2.1.1, followed by lignin, which has a very different composition (Section 2.2.2.1.2). 

The amount of hemicelluloses of the dry weight of wood is usually between 20 and 30% (cf. Appendix). The composition and structure of the hemicelluloses in the softwoods differ in a characteristic way from those in the hardwoods. Considerable differences also exist in the hemicellulose content and composition between the stem, branches, roots, and bark. 

Lignin is a high-energy content biopolymer rich in phenolic components. It provides structural integrity to plants. The combination of hemicellulose and lignin provide a protective sheath around the cellulose and this sheath must be modified or removed before efficient hydrolysis of cellulose can occur. 

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