Heteropolysaccharides hemicellulose fractions

Occurrence. D-Xylose (10) is present in widely abundant polysaccharides of plant tissues. Xylan is the main carbohydrate found in the hemicellulosic fraction, and accounts for one third of all renewable organic carbon available on earth. The structure and composition of xylans are variable, from linear (3-(l->4)-linked xylose chains to highly branched heteropolysaccharides. The branches may involve short oligosaccharides, usually of L-arabinofuranosyl units. Xyloglucans are also important hemicellulose polysaccharides consisting of a backbone of ( I — 4)-(i-D-glucopyranosyl residues heavily substituted with a-D-Xylp. Other monosaccharides may also be present.44... 

Hemicellulose refers to a chemically ill-defined group of heteropolysaccharides of plant cell-walls that are closely associated with cellulose. They are composed of different hexoses and pentoses, and uronic acids. The hemicelluloses may be separated into two fractions, A and B however, there is no clear demarcation of structure between the two fractions. Hemicellulose B usually contains a higher proportion of uronic acid, chiefly 4-0-methyl-D-glucuronic acid, than the A fraction. The methyl ethers of D-aldobiouronic acids containing D-glucuronic acid, D-galacturonic acid, or D-mannuronic acid are isolated chiefly as the 4-O-methylaldobiouronic acids, and are hydrolyzed with difficulty by acid. 

Hemicelluloses are non-cellulosic and short-branched chain heteropolysaccharides, which consist of various different sugar units. They can be arranged in different proportions and with different substituents. Large amounts of hemicelluloses with a wide variation in content and chemical structure are found in plant cell walls. Hemicelluloses generally consist of several populations of polysaccharide molecules, varying in structural characteristics. Several fractionation techniques have been employed in order to obtain more homogeneous fractions as well as exploring structure-property relationships for the hemicellulosic polymers. ... 

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