Polysaccharides xylan group

Many polysaccharides of the xylan group contain residues of D-glucuronic acid or 4-0-methyl-D-glucuronic acid. Since glycosiduronic acids are particularly resistant to acid hydrolysis, aldobiouronic acids may be isolated... 

D-xylopyranose units. This structural concept is substantiated by estimation of the formic acid obtained when the xylan is oxidized by periodate ions. On hydrolysis of the fully oxidized xylan there is obtained a small amount of D-xylose which presumably occupied the branch points in the polysaccharide and consequently was protected from periodate oxidation by possessing no adjacent free hydroxyl groups. 

In contrast to alkaline pulping, the acetyl groups of the hemicellu-loses are relatively stable at low pH, as are the glucuronic acid residues of the xylans. The main effect of high temperature and low pH is the hydrolysis of the glycosidic linkages in the polysaccharide... 

The hetero-l,4-linked xylans (or heteroxylans) constitute a well-characterized group of polysaccharides which form the major components of the hemicellulosic fractions of terrestrial plants (1-4). Softwoods are an exception, where the heteroxylans can be present as a minor component of the total hemicelluloses. They have been isolated from grasses, legumes, ferns, softwoods and hardwoods, and collectively may constitute up to 35% of the total dry weight of higher land plants (4). As such the heteroxylans rank second to cellulose in abundance as naturally occurring organic chemicals in the biosphere. The heteroxylans are closely associated with other... 

A xylan has been isolated from young internodes of the reed Arundo donax, a tissue in which most of the cells have primary walls.89 This polysaccharide possesses a backbone of / -(l— 4)-linked D-xylosyl residues with single 4-O-methyl-D-glucosyluronic and arabinofuranosyl groups separately attached to backbone xylose units they are bonded to 0-2 and 0-3, respectively, of separate xylosyl residues. A similar xylan was isolated from older tissues, containing secondary walls, of the same plant.89... 

---