4-O-Methyl-D-glucuronic acid residues

Mesquite gum, the exudate of Prosopis piliflora D.C. is of little commercial importance, but structural studies have been made which suggest that it has a (i -> 3)-linked main chain of D-galactop5n anose residues, with 6-linked side chains. These may be short (i - 3)-linked chains of L-arabinofuranose residues, or similar chains attached through D-galactose or 4-O-methyl-D-glucuronic acid residues. 

The second important group of hemicelluloses found in softwoods are the arabino-(4-0-methylglucurono)xylans, which may make up from 5 to 10% of the dry weight of the wood. These consist of /3-1,4-linked D-xylopyranose units, partially substituted by L-arabino-furanose (at the 3 position) and by 4-O-methyl D-glucuronic acid (at the 2 position). The frequency of these substituent groups is around 1 to 2 residues per 10 xylose units. 

Conversion of the reducing end-unit in 2-0-(4-0-methyl-D-glucosyl-uronic acid)-D-xylose to a tartronic semialdehyde residue (as in LXXVII) facilitated the ready isolation of 4-O-methyl-D-glucuronic acid by acid hy-... 

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... 

In hardwood, the xylan, or glucuronoxylan, consists mainly of D-xylose residues, most of which are acetylated, and 4-0-methyl-D-glucuronic acid residues (Fig. 2). Minor amounts of other constituents, such as L-rhamnose and D-galacturonic acid, can also be found. The molar ratio of 4-0-methyl-a-D-glu-curonic acid, acetyl groups, and xylose has been estimated to be 1 7 10 [59]. Hardwood also contains small amounts (2-5% dry weight (DW)) of gluco-mannan, composed of D-glucose and o-mannose. 

O. 3 0.6 0.5, respectively, as well as 2,4-di-O-methyl-D-galactose. Each polysaccharide contained, in addition, 10% half-ester sulphate and ca. 0.5% protein. Structural studies showed, inter alia, that the D-glucuronic acid residues were linked to 0-3 of the D-galactose residues and that 2-0-methyl-D-glucuronic acid residues were linked to 0-4 of the L-galactose residues. The polysaccharide from... 

D-Xylp = D-Xylopyranose, L-Ara/ = L-arabinofuranose, o-Gp = D-glucopyranose, and Galp = galactopyranose residues. 6 (Me) indicates that D-glucuronic acid residues are present, wholly or in part, as the 4-methyl ether. 

The extracellular polysaccharide of Porphyridium cruentum contained D-xylose, D-glucose, D- and L-galactose, 3-O-methyl-D-xylose, 3 (or 4)-0-methyl-D-galactose, D-glucuronic acid, 2-0-methyl-D-glucuronic acid, and a 2-O-methyl hexose in the molar ratio 3 1 2.5 0.13 0.13 0.8 0.2 0.13, whereas that from P. aerugineum contained the first six sugar residues in the molar ratio 1.7 1.0 1.1 ... 

---