Amorphous xylan

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. 

With the exception of cellulose and chitin, plant polysaccharides are usually hydrated. Hydration often occurs in the crystalline regions as well as in the amorphous areas. When water of hydration is found in the crystallites, it may or may not affect the conformation of the polysaccharide backbone and in most cases, it affects the unit-cell dimensions, while in a few cases, the water appears to have no effect on unit-cell dimensions. The structures of six hydrated neutral polysaccharides will be examined with regards to the state of water of hydration in the structure. It wi 11 be seen that water may occur as columns or as sheets in these structures. The structures that will be discussed are (1 4)-3-p-xylan, nigeran, amylose, galactomannan, (1 3)-3-p-gTucan and (1 3)-s-P-xy1 an. The chemical structures of these polysaccharides are shown in Figure 1. 

The actual state of the xylans and glucomannans in the living tree is still largely unknown. Since they do not crystallize readily without some chemical modification, such as partial depolymerization or removal of side chains, or both, it would seem likely that they occur in the amorphous state in the wood. The possibility that they could form microfibrils, as does mannan B in vegetable ivory, cannot be entirely excluded.More probably, however, they occur as a powder between and around the cellulose microfibrils. Using a polarized infrared technique, Marchessault and coworkers have obtained indications that, not only the cellulose, but also the xylans and glucomannans, may be oriented in the direction of the fiber axis in wood. 

Chemically, about 35-55% of the dry material is the glucose polymer cellulose, much of which is in a crystalline structure while another 25—35% is hemicellulose, an amorphous polymer. The remainder is mostly lignin plus less amounts of minerals, waxes, and other compounds [3]. Cellulose is formed by beta-[l, 4] glucosyl linkages in a linear backbone, whereas hemicelluloses are branched polymers composed of several monosaccharides [5]. Fig. 1 shows the schematic illustration of the cellulose chain, while Fig. 2 shows the schematic illustration of xylans from Gramineae [5, 6]. 

In ARP pretreatment of hybrid poplar, deUgnification is increased with the treatment severity, but xylan removal occurs only to a certain extent. Increase of temperature from 175 to 195 °C in ARP does not affect xylan/lignin removal in hybrid poplar, yet the glucan digestibility of treated solid is increased. This is caused by an increase of cellulase accessibility to cellulose created by breakage of certain bonds and reconfiguration of the components in hemicellulose-lignin matrix. Crystallinity index of hybrid poplar increases after the ARP treatment. It is primarily caused by the removal of amorphous components,... 

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. 

Morphology. The cell wall of algae consists of chitin, lipids, polysaccharides and proteins. It is usually made of a multi-layered (at least two layers) microfibrillar framework generally consisting of cellulose and interspersed with amorphous material. This additional material is composed predominantly of other polysaccharides such as pectin, xylans, mannans, alginic acids or fucinic acid. ... 

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