Xylan backbones

Fig. 1. A schematic representation of the xylan backbone of arborescent plants, where Xylp = xylopyranosyl unit ...

A similar molecular structure is also proposed82 for the gummy polysaccharide from corm sacs of Watsonia pyramidata in which the (1— 4)-xylan backbone is highly substituted with 2- as well as 3-linked L-arabinofuranosyl side... 

The heteroxylans (CHX) present in cereals, seeds, gum exudates, and mucilages are structurally more complex [6]. They have a (1 4)-)0-d-xylopyranose backbone decorated, except of the single uronic acid and arabinosyl residues with various mono- and oligoglycosyl side chains. Reinvestigations of CHX isolated from corn bran [52] have confirmed that the xylan backbone is heavily substituted (at both positions 2 and 3) with y0-D-Xylp, y0-L-Ara/, of-D-GlcpA residues and oligosaccharide side chains (1), (3) and (4). 

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... 

Figure 3.8(a) Possible ester linkage between an a-OH of lignin and a carboxyl group of a 4-0-methyl-/3-d-glucuronic acid residue on the xylan backbone. 

Among the plant polysaccharides there may be mentioned the hemi-celluloses the most common of these contain 4-O-methyl-D-glucuronic acid as branch units linked to a /3-D-xylan backbone.110 The commercially important gum arabic, a soluble polysaccharide produced by Acacia trees and widely used in foods and pharmaceuticals, also contains glucuronic units.111 D-Glucuronic acid has been found in sulfated complex polysaccharides from brown algae.112... 

XII or XIII, it is reasonable to assume that the xylan backbone is unbranched. In the case, however, of xylans containing L-arabinofuranose residues, it is much more difficult to obtain direct evidence for the order of linkage of sugar residues. It is doubtful if it would be possible by present methods to detect a small proportion of L-arabinofuranose residues terminating main xylan chains (as in XIV) if these residues were also attached as side-chains (I), as commonly found. 

Kong, F., Engler, C. R., and Soltes, E. J., Effects of cell-wall acetate, xylan backbone, and lignin on enzymatic hydrolysis of aspen wood. Appl Biochem Biotechnol 1993, 34/35, 23-35. 

The D-xylanases of rumen microbial origin are all of the endo type, and degrade D-xylan, arabinoxylan, hemicellulose B, and xylo-oligosaccharides of d.p. 2=3, mainly to X2 and D-xylose X2 remains unattacked. The D-xylanases were found38,42 to have a higher affinity for linear D-xylans than for the branched hemicelluloses, namely, arabinoxylan and a branched B fraction from Trifolium pratense (red clover). The presence of L-arabinose, D-galactose, and uronic acid substituents on the D-xylan backbone thus appears to inhibit the action of these D-xylanases. The D-xylan was not effectively hydrolyzed until the side-group substituents had mostly been removed, for example, by arabinofuranosidase action on arabinoxylan.44... 

Fig. 2 Hemicellulose (xylan backbone with arabinose, glucuronic acid and feruloyl groups) [32]... 

Specific rotation in aqueous alkali, usually sodium hydroxide. Number-average degree of polymerization (number of xylose residues in the xylan backbone). 0-Acetyl-(4-0-methylglucurono)xylan. In water. 

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