4- 0-Methyl-D-glucuronic acid

Fractionation and Chemistry of Citrus Pectic Polysaccharides. Pectic polysaccharides, commonly known as pectin, appear early in plant cell-wall formation. A series of complex biochemical steps results in the formation of cell plates followed first by its growth in area (primary cell wall) then in thickness (secondary cell wa.ll). Exclusive of randomly oriented cellulose fibrils, primary cell wall is composed mainly of pectic polysaccharides (34). These pectic polysaccharides are rich in D-galacturonic acid, D-galactose and L-arabinose residues. With growth in thickness of cell wall (secondary cell wall),there appears to be a replacement of pectic polysaccharide deposition with polysaccharides rich in D-glucuronic acid or 4-0-methyl-D-glucuronic acid,... 

Classic methods in use to fractionate plant cell wall polysaccharides are mainly based upon the differential solubilities of various cell wall polysaccharide constituents (32). To solubilize pectic polysaccharides, the cell wall material is extracted with water, ammonium oxalate or disodium EDTA. The solubilization of pectic polysaccharides could then be followed by solubilization of hemicelluloses, i.e., polysaccharides rich in D-glucuronic acid 4-0-methyl-D-glucuronic acid, D-xylose and D-glucose residues. The solubilization of hemicellulosic polysaccharides is accomplished by treatment of water, ammonium oxalate or EDTA insoluble material with a strong alkali. 

Methylated uronic acids are usually separated from methylated aldoses by virtue of the insolubility of their barium salts in chloroform. Recently, 4-0-methyl-D-glucuronic acid has been separated from a mixture of non-acidic reducing sugars by absorption on a weakly basic... 

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... 

Xylans from cereals and grasses are generally characterized by the presence of L-arabinofuranose residues linked to the backbone as single-unit side-chains, usually to position 3 of D-xylose, but, in many cases, D-glu-curonic acid or 4-0-methyl-D-glucuronic acid residues, or both, are also present in a smaller proportion. 

Hamilton and N. S. Thompson, by fractionation of a partial hydroly-zate of a xylan on filter paper, isolated 4-0-methyl-D-glucuronic acid, the aldobiouronic acid mentioned above, and a crystalline aldotriouronic acid identified as 0-(4-0-methyl-a-D-glucopyranosyluronic acid)-(l —> 2)-0-/3-d-xylopyranosyl-(l 4)-D-xylopyranose. As the isomeric aldotriouronic acid was virtually absent, the authors suggested that, in the xylan polyuronide... 

This explanation also applies to the hydrolysis of oligosaccharides and polysaccharides (see Section V p. 85). A case in point is the tetrasac-charide (16). Bond A is hydrolyzed preferentially over bond B, even when the 4-0-methyl-D-glucuronic acid residue is reduced to 4-0-methyl-D-... 

Table II lists derivatives of D-glucuronic acid which have been obtained by catalytic oxidation. A ready preparation of 3-0-methyl-D-glucuronic acid is by the oxidation of l,2-0-isopropylidene-3-0-methyl-D-gluco-furanose and subsequent hydrolysis of the product." 4-0-Methyl-D-glucuronic acid, a compound of considerable interest because of its occurrence as a fundamental unit in hemicelluloses" and polysaccharides," cannot be obtained by the simple catalytic oxidation of methyl 4-0-methyl-D-glucopyranoside. f...

For the preparation of 4-0-methyl-D-glucuronic acid, Wacek and co-workers have worked out several synthetic methods which involve the... 

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. 

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