Hemicelluloses

Polyoses are the linking material between cellulose and lignin. Unlike cellulose, hemicelluloses consist of different monosaccharide units. In addition, the polymer chains of hemicelluloses have short branches and are amorphous. Because of the amorphous morphology, hemicelluloses are partially soluble or swell in water. The backbone of the chains of hemicelluloses can be a homopolymer (generally con­sisting of single sugar repeat unit) or a heteropolymer (mixture of different sugars). Formulas of the sugar component of hemicelluloses are listed in Fig. 1.5. Among the most important sugars of hemicelluloses component is xylose. In hardwood xylan, the backbone chain consists of xylose units which are linked by b-(1, 4)- glycosidic bonds and branched by b-(1, 2)-glycosidic bonds with 4-O-methyl glucuronic acid groups. In addition, O-acetyl groups sometimes replace the OH groups in positions C2 and C3 (Fig. 1.4). This leads to the lack of crystalline structure (Kirk-Othmer 2001). For softwood xylan, the acetyl groups are fewer in the backbone chain. However, softwood xylan has additional branches consisting of arabinofuranose units linked by b-(1, 3)-glycosidic bonds to the backbone (Fig. 1.4).

1.2.1.1 Lignin

By contrast, Lignin is a three-dimensional polyphenolic network built of di — methoxylated (syringyl), monomethoxylated (guaiacyl), and nonmethoxylated (p-hydroxyphenil) phenylpropanoid units, derived from the corresponding p-hydroxycinnamyl alcohols, which give rise to a variety of subunits including different ether and C-C bonds. Lignin is hydrophobic and highly resistant toward chemical and biological degradation. It is located in the middle lamella, acting as cement between the plant cells, and in the layers of the cell wall, forming, together with hemicellulose, an amorphous matrix in which the

ACIDS

COOH

Подпись: 3-D-XyIose

image6
Подпись: PENTOSES

3-D-Glucose p-D-Glucuronic acid a-L-Rhamnose

COOH

a-L-Aiabmopyranose p-D-Mannose a-D-4-0-Methylgmcuroiuc a-L-Fucase

acid

CH. OH

HO№C a-L-Arabmofuranose a-D-Galactose a-D-Galacniromc acid

Fig. 1.5 Formulas of the sugar component of hemicelluloses (Mustafa et al. 2009)

image7cellulose fibrils are embedded and protected against biodegradation. Lignin content and composition vary among different plant groups. Moreover, the lignin composition varies between the different wood tissues and cell wall layers. The monomeric building units of lignin are shown in Fig. 1.6. The guaiacyl unit is dominant in the softwoods. In contrast, syringyl units are dominant in hardwoods (Kirk-Othmer 2001) (Fig. 1.7)

image011

Fig. 1.6

 

image9

Fig. 1.7 Structure of lignin (Glazer et al. 1995)

 

image8

Table 1.2 Overview of linkages between the monomer units and polymers to form lignocellu — lose (Harmsen et al. 2010)

Bonds within different components (intrapolymer linkages)

Подпись:Ether bond

Carbon to carbon

Hydrogen bond

Ester bond

Bonds connecting different components (interpolymer linkages)

Подпись:Cellulose-lignin

Hemicellulose lignin

Hemicellulose-lignin

Cellulose-hemicellulose

Hemicellulose-lignin

Cellulose-lignin

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