Mucilages uronic acid residues

Most of the known oligoglycuronic acids are aldobiouronic acids. Because of the stability of the glycosidic linkage of aldobiouronic acids toward acid hydrolysis, they are readily isolated after vigorous hydrolysis of polysaccharides that contain uronic acid residues. They have been obtained from wood hemicelluloses, plant mucilages, gums, bacterial and animal polysaccharides, and by synthesis. 

The occurrence in many polyglycosiduronic acids of relatively resistant linkages, usually those between uronic acid residues and adjacent residues, has resulted in the isolation of several aldobiouronic acids on graded hydrolysis of acidic polysaccharides. The isolation of such aldobiouronic acids, and subsequent conversion to their fully methylated derivatives, has become a standard procedure in structural studies on polyglycosiduronic acids, especially those of plant gums and mucilages.1 In other cases, partially methylated aldobiouronic acids have been isolated from the hydrolyzates of methylated polysaccharides. The sources and methods of isolation of the methyl ethers of aldobiouronic acids so far examined are given in Table I. Some properties of derivatives are recorded in Table Y. 

The closely related mucilages are widely distributed in intercellular material and in the cell contents, and they possess the same type of molecular structure as the gums. Mucilages may be either neutral or acidic (when uronic acid residues are present). Again, little is known of the finer details of molecular structure and it is often difficult to prove whether or not the material is homogeneous. 

Chem. Soc. 1953, 2332. Structure is a partially acetylated polysaccharide containing about 8% acetyl groups and about 37% uronic acid residues. Reviews F. Smith, R. Montgomery, The Chemistry of Plant Gums and Mucilages (Reinhold, New York, 1959)1 Goldstein, Alter, in Industrial Gums. R. L. Whistler, Ed. (Academic Press, New York, 2nd ed., 1973) pp 273-287. 

The dried exudate of the tree Sterculia urens Roxb., found in India, especially in the Gujerat region and in the central provinces. It is a partially acetylated polysaccharide that contains atout 8% acetyl groups and about 37% uronic acid residues. It is a finely ground white powder with a faint odor of acetic acid. It absorbs water rapidly to form viscous mucilages at low concentrations. Viscosity decreases on addition of acid or alkali. 

The heteroxylans (CHX) present in cereals, seeds, gum exudates, and mucilages are structurally more complex [6]. They have a (1 4)-)0-d-xylopyranose backbone decorated, except of the single uronic acid and arabinosyl residues with various mono- and oligoglycosyl side chains. Reinvestigations of CHX isolated from corn bran [52] have confirmed that the xylan backbone is heavily substituted (at both positions 2 and 3) with y0-D-Xylp, y0-L-Ara/, of-D-GlcpA residues and oligosaccharide side chains (1), (3) and (4). 

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