Xylan, repeat unit

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

Figure 1. Possible repeat unit structures for the condensation of xylan with triphenylantimony dichloride.

Second, it is hoped that the reaction with xylans may give products that show some solubility since they have only two hydroxyls per repeat unit. Further, the two hydroxyls are adjacent which may encourage cyclization with the dihaloantimony rather than crosslinking. Third, information gained from such studies should allow a better understanding of the reaction dependencies involved in the use of natural polyhydroxylic materials as feedstocks in the production of useful materials. 

Fig. 6.15. Tentative molecular conformation and repeat unit of the xylan molecule in the hydrate unit cell. (Marchessault and Liang, 1962.)...

Fujita M., Shoda S., Kobayashi S., Xylanase-catalyzed synthesis of a novel polysaccharide having a glucose-xylose repeating unit, a cellulose-xylan hybrid polymer, J. Am. Chem. Soc., 120, 1998,6411-6412. 

Kobyashi S., Makino A., Tachibana N., Ohmae M., Chitinase-catalyzed synthesis of a chitin-xylan hybrid polymer A novel water-soluble P(1 <— 4) polysaccharide having an JV-acetylglucosamine-xylose repeating unit, Macromol. Rapid Comm., 27,2006, 781-786. 

Hemicelluloses are noncellulose materials produced by plants and found in straw, wood, and other fibrous tissues. Xylans are the most abundant hemicelluloses. They consist of l,4-)3-linked o-xylopyranoses. Draw the structure for the repeating unit in xylans. 

Moreover, 2,3-dialdehyde xylan may be obtained in the well-known glycol cleavage oxidation of vicinal diol units with sodium periodate, which can be further oxidized to give 2,3-dicarboxyl xylan. Figure 3 summarizes the structures of the possible oxidized repeating units. 

The former four are called homopolysaccharides since the fimdamen-tal sugar repeating unit is of the same structure o-glucose is linked via a j6(l 4) glycosidic bond in cellulose and a(l 4) in amylose, xylose and W-acetyl-D-glucosamine being via 4) in xylan and chitin, respectively. 

Hemicellulose HemiceUulose is predominantly found in the primary and second cell walls in wood. Compared to cellulose, hemicellulose usually exhibits lower molecular weight and lower degree of polymerization, which may be tens of hundreds of repeating units. Hemicellulose comprises of polysaccharides, e.g., arabino-xylans, gluco-mannans and galactans, of which xylans are predominant [21]. Unlike cellulose, the composition of hemicellulose varies between trees and species. Besides, hemicelluloses are more hydrophilic than cellulose and are extremely susceptible to alkali and acid hydrolysis because of their branched and amorphous nature [22]. The combination of cellulose and hemicellulose is known as holocellulose. 

Xylans are a group of polymers based on a structure analogous to that of cellulose wherein xylose is the repeating unit. The simplest representative contains only D-xylose with /3-1-4 linkages and is a common component of plant walls. Several heteropolysaccharides utilize the xylan backbone and have various other saccharides as branches. Xylans are often associated with cellulose in plant cell walls. 

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

Chitinase from Bacillus sp. was added to a solution of Xyl (l— 4)GlcNAc oxazoline 13 dissolved into a phosphate buffer. The consumption rate of 13 was accelerated by the enzyme addition and chitin-xylan hybrid polysaccharide 14 was produced. During the polymerization, the reaction proceeded homogenously throughout the reaction. The yield was 76% and its average molecular weight value of 14 was M = 1500 with MJMa = 1.76, determined by GPC. Furthermore, MALDI-TOF mass spectrum of 14 indicated the peaks at every m/z 335, which corresponds to the molecular mass of the repeating disaccharide unit. 

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