Aldobiouronic acids 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. 

Like the parent compounds, the methyl ethers of aldobiouronic acids are resistant to acid hydrolysis, and it is difficult to carry out hydrolysis without some decomposition of the product. This difficulty has recently been overcome by reduction of the uronic acid residue with lithium aluminum hydride66-67 the resulting disaccharide then undergoes hydrolysis without difficulty. The first reduction of the uronic acid residue of a methylated aldobiouronic acid methyl ester was accomplished by Levene, Meyer and Kuna,69 who reduced the methylated aldobiouronic acid from gum arabic with hydrogen in the presence of copper chromite catalyst under the conditions previously used701 for reducing the acety-... 

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... 

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

Wood xylans are characterized by the presence of residues of 4-O-methyl-D-glucuronic acid linked to position 2 of D-xylose in the main xylan chains (XXVI), and, accordingly, the aldobiouronic acid, 0-(4-0-methyl-a-D-glu-cosyluronic acid)-(l —> 2)-D-xylose (IX), is commonly isolated after the graded hydrolysis of such xylans. Partial acid hydrolysis of Monterey pine wood69 furnishes this compound together with the isomeric aldobiouronic acid, 0-(4-0-methyl-a-n-glucosyluronic acid)-(l—>3)-D-xylose, as a minor product, but the role of the (1 — 3)-linked aldobiouronic acid unit in the... 

The method of hydrolysis should, of course, be efficient enough to de-polymerize the polysaccharide to the monomeric state, but care should be taken to choose conditions that will give the least possible further degradation of the resultant monomers. Uronic acid residues appear in the hy-drolyzate mainly as methylated aldobiouronic acids, because of the great resistance of glycosiduronic linkages to hydrolysis. The hydrolysis of methylated polysaccharides is, under normal conditions, always accompanied by slight demethylation and, in some cases, this may make the results difficult to interpret. 

Fig. 5.—Sugars in Hydrolyzates of Total Hemicellulose from Oat Plants of Different Ages, Excluding Glucose but Including Uronic Acids Separately Determined.167 (It is assumed that, after hydrolysis, all uronic acids were present in aldobiouronic acids containing a xylose residue the proportions of xylose have been altered to include xylosyl residues equivalent to the uronic acid determined. Reproduced, by permission, from Phytochemistry.)...

The hemicelluloses of wood contain uronic acid residues which frequently bear methyl ether groups. Hydrolysis of the hemicelluloses yields aldobioLironic and aldotriouronic acids. In researches referred to previously (page 300), O Dwyer " investigated the hemicelluloses from oakwood. She isolated an aldobiouronic acid as a barium salt after combined enzymic and acidic hydrolysis. Analyses of this compound were in agreement with the empirical formula for a barium salt of an 0-methyl-xylo-aldobiouronic acid. This compound was isolated from a polysaccharide material which contained xylose (6 parts), uronic acid (1 part), and methoxyl (1 part). Studies by Anderson on the hemicelluloses of a number of hardwoods also showed that 0-methylhexuronic acid residues are present in glycosidic union with xylose, in ratios extending from 8 to 19 xylose units per uronic acid unit. 

There is some evidence that a small, perhaps variable, fraction of the acid side-chains in softwood xylans lacks the 4-0-methyl group. Roudier, in his extensive investigations on the uronic acids present in maritime pine, " isolated and identified 2-0(-a-D-glucopyranosyluronic acid)-D-xylose after partial hydrolysis of the wood. Hydrolysis of the xylan gave a small proportion of the same aldobiouronic acid. As previously mentioned (see Part I, p. 273), D-glucurono-6,3-lactone has been isolated from the wood of black spruce. " It has become evident that, despite the mild conditions often used, demethylation of the 4-0-methyl-D-glu-curonic acid residues may occur during hydrolysis of the above xylans. The presence or absence of D-glucuronic acid side-chains in hardwood or softwood xylans has, therefore, yet to be ascertained. 

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