Xylan forms

D-xylose has a pyranose structure, while D-ribose has a furanose structure (Figure 3.7). The first is very widespread in wood, where it is associated with cellulose in a polysaccharide (xylane) form. It is also present in glycoside form. D-ribose is an essential component of nucleotides and nucleic acids. Arabinose is widespread in the plant kingdom, and the polysaccharide (gum)... 

Pentosans, of which the chief component is xylan, form some 16 to 20 % of the dry matter of grass and hay (Fraps, Hallsworth, and Ekelund ) and are digested to the extent of 50 % in the alimentary tract of the ruminant. 

DSC results suggest that native acetylated xylan and synthetically acetylated xylan form homogenous blends with commercially available cellulose acetate when cast from DMF solutions. 

Note. The name ending in -an refers to the unsubstituted polysaccharide. Thus xylan occurs in nature in unacetylated and partially acetylated forms. Xylan designates the unacetylated material, and xylan acetate an acetylated derivative. 

Most of the glucuronoxylans have single 4-0-methyl-o -D-glucopyranosyl uronic acid residues (MeGlcA) attached always at position 2 of the main chain Xylp units (Fig. 2). This structural type is usually named as 4-0-methyl-D-glucurono-D-xylan (MGX). However, the glucuronic acid side chain may be present in both the 4-0-methylated and non-methylated forms (GlcA). 

Several CHX samples were isolated from the leaves and barks of tropical dicots such as the Litsea species [3]. The mucilage-forming seeds of Plantago sp. contain very complex heteroxylans [53,54]. For the CHX from Plantago major seeds [53], a (1 3, 1 -> 4)-mixed-linkage xylan backbone has... 

The rheological behavior of xylans has rarely been investigated [4,114,115]. The water-insoluble hemicellulose from the viscose process (containing > 85% xylan) was reported to form thixotropic aqueous dispersions of high... 

Cross-linked xylan-based microparticles are produced by the emulsification of an alkaline solution of xylan with a lipophilic phase formed by a mixture of chloroform and cyclohexane by using 5% (w/v) sorbitan triesterate as the surfactant. Subsequently, the cross-linking reaction is carried out for 30 minutes with 5% (w/v) terephthaloyl chloride in order to yield a hard and rigid polymeric shell (Nagashima et al., 2008). 

Recently a quantitative method for the determination of carbonyl groups in oxidized xylan has been developed.106 The method involves the reaction of the carbonyl group with O-methylhydroxylamine hydrochloride to form the corresponding oxime with the liberation of one mole of titratable acid. 

The film-forming property of xylan derivatives also leads to the inference of an average D. P. greater than about 50. This deduction arises from the general observation that pliable polysaccharide acetate films are only formed from molecules with a D. P. of 50 or more. 

The end group which is produced contains a carboxylic acid functionality which has an influence on the anionicity of pulp fibres (Chapter 6) but, in this form, it is resistant to further alkaline degradation. The hemicelluloses are also able to undergo the same type of peeling reaction but at different rates from each other and from cellulose. The /3-1,4-xylans, for example, are more stable to alkaline degradation than the glucomannans. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

The particular wood species we chose for this study is aspen (Populus tremuloides), which is plentiful in Canada and in the northern U.S.A. The chemical composition we found to be glucan 53.4%, xylan 14.9%, total carbohydrate 79.0%, lignin 17.1% and extractives 3.8%. We would expect total fermentable sugars of about 56% in this sample of aspen in anhydro form (Timell has reported about 60% in another sample (15)) which upon hydrolysis would yield about 1,250 lb wood sugars per ton of wood (dry basis), from the stoichiometry. Theoretical conversion of this sugar to ethanol would yield 640 lb or 81.1 gallons of anhydrous... 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

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