Heteropolysaccharides, methylation

Xylans are the major hemicelluloses of many plant materials, where they often contribute to the rigidity of plant cell walls. Most xylans are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked j8-D-xylo-pyranose units. The degree and t pe of substitution of the backbone is dependent on the plant origin of a xylan. In addition to xylose, xylans may contain L-arabinose, D-glucuronic acid or its 4-O-methyl ether, and acetic, p-coumaric, and ferulic acids. 

The observation that the methyl ether or ester groups of plant heteropolysaccharides are introduced at the macromolecular level is similar to the finding that C- and N-methyl groups of RNA and DNA are also introduced into preformed macromolecules. 

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. 

The methylated galactoses obtained from tamarack galactan were analyzed as the per (trimethylsilyl) derivatives by Jiang and Timell, and the same derivatives were used by Larsson and Samuelson in a study of a galacturonic acid anhydride. 3-O-Methyl-n-galactose has been estimated in the hydrolyzate of a fungal heteropolysaccharide by using trimethylsilyl derivatives. ... 

The location of inter-residue cyclic acetals has been determined following the oxidation by periodate ion of heteropolysaccharides. These acetals protected hydroxy-groups during methylation analysis. Helix-helix aggregations and intermolecular associations through double helices have been demonstrated by vacuum-u.v.-circular dichroism spectra of agarose films. 

Potentiometric titration and other physical data indicated that the native polysaccharide possesses an ordered secondary structure that is stabilized by nonionic interactions, which outweigh the repulsion between adjacent carboxy-groups. An acidic heteropolysaccharide from Xanthomonas SI 9 is composed of residues of D-glucuronic acid, D-glucose, o-galactose, D-mannose, and acetate. Although methylation analysis showed that this polysaccharide possesses many structural features similar to those of other Xanthomonas polysaccharides, it is unusual in that D-mannosyl and D-galactosyl residues occur in the structure. 

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