Uronic polysaccharide components

The characterization and quantitative determination of uronic acid components in polysaccharides faces the problem of complete release of the uronic acids without accompanying decomposition. This is a difficult task because of the acid resistance of the glycosiduronic bond. From a comparison of several methods, it was shown that methanolysis combined with trifluoroacetic acid hydrolysis is the best for the liberation of uronic acids.204 The identification can be performed by gas chromatography of the trimethylsilyl derivatives.205... 

Some polysaccharides carry negative charges, by virtue of their uronic acid components and/or modification of OH groups as sulfate esters or cyclic acetals of pyruvic acid (CH3COCOOH) these can often be precipitated by cationic surfactants such as cetyltrimethylammonium bromide. They can also be fractionated on weak anion exchangers. For neutral polysaccharides, elaborate precipitation protocols involving initial removal of proteins and nucleic acids have to be used - a useful precipitant of the polysaccharide is 50% ethanol. 

A number of zoopolysaccharides have been isolated in purified condition and the structures partially established. Many of these are bound with other tissue components in the natural condition. They are isolated from the macerated tissues or the secretions by extraction with alkalies or salts (especially calcium chloride). Mucins and many mucoids are precipitated by weak acids, and their polysaccharide components are subsequently recovered. If dissociable, proteins may be removed by precipitation with amyl alcohol - chloroform (Sevag) or formaldehyde (Masamime). Peptic or tryptic digestion may remove these and covalently bound proteins. Colorimetric, chromatographic, and isolation methods have been used for the identification of the component sugars (, 14, 15-16h), (See also imder Glucosamine, Chapter VIII, Uronic acids. Chapter VI and part I, Chapter XII.)... 

The condensation of hexuronic acid derivatives with hexosaminides to give amide-linked disaccharides is referred to in Chapter D-Galacturonic acid has been identified as a component of Pneumococcus type 25 capsular polysaccharide," and 2-O-methyl-D-glucuronic acid has been isolated for the first time in Nature from an extracellular polysaccharide of a species (Porphyridium cruentum) of red alga. The acidic sugar component in Klebsiella type 37 capsular polysaccharide has been identified as 4-0-[(5)-l-carboxyethyl]-D-glucuronic acid. Antibiotic substances and nucleosides that contain uronic acid components are referred to in Chapters 20 and 21, respectively. 

In tandem with his studies of polysaccharides, Stacey had a research group investigating the chemistry of the unusual sugars found as components of carbohydrate macromolecules. In this context, derivatives of simple sugars, di- and trisaccharides, uronic acids, deoxy sugars, amino sugars, sugar sulfates, and phosphates were studied. 

Rha, Ara and Gal are the neutral sugar components from all the fractions. Xyl is not present in Fla and is significantly present in the hemicellulose fractions, indicating that this monosaccharide is component of hemicellulosic polymers. Chemical composition of the water fractions were determined (Table V). High protein contents and the presence of O-acetyl-groups were observed in four aqueous fractions. Neutral sugar and uronic acid composition points to inclusion of these polymers in the class of pectic polysaccharides. 

The purification of a pneumococcal polysaccharide may be difficult, and the polysaccharide material is sometimes contaminated by a cell-wall component known as the C-substance. The methods for structural analysis of polysaccharides (which, like several of the pneumococcal polysaccharides, contain amino sugar and uronic acid... 

In this clas of materials is grouped the majority of the seed mucilages, the acidity of which is due to a uronic acid (usually D-galacturonic acid) or to a methyl ether derivative of a uronic acid. This presence of D-galacturonic acid as the acidic component of the polysaccharide differentiates the mucilages from the gums, the acidity of the majority of which is due to the presence of D-glucuronic acid, or to one of its methyl ether derivatives (see above). 

Uronic acid in the form of galacturonic acid is a major component of the pectic polysaccharide rhamnogalacturonan, which is present in large amounts in the cell walls of most fruits and vegetables. Small amounts of glucuronic acid and 4-O-methylglu-curonic acid have also been detected in cell walls. 

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