Methylglucuronic acid residues

The distribution of the acid side-chains in softwood xylans is not known. No oligosaccharides containing more than one 4-0-methylglucuronic acid residue were liberated on partial hydrolysis of a hemlock xylan, suggesting that few of the acid groups occur on contiguous D-xylose residues. 

Figure 4. MALDI-MS spectrum (negative-ion mode) of a 4-0-methyl-glucuronoxylan from a sulfite pulp (spruce). The mass peaks correspond to xylooligosaccharides of increasing lengths containing an odd (o) or even (x) number of4-0-methylglucuronic acid residues. Reproduced from Reference 3, Copyright 2001 American Chemical Society,...

The xylan of hardwoods (O-acetyl-4-O-methylglucuronoxylan) consists of at least 70 p-xylopyranose residues (average degree of polymerization between 150 and 200) linked by P-l,4-glycosidic bonds (Fig. 11.4-4) 139. Every tenth xylose residue carries a 4-O-methylglucuronic acid attached to the C-2 of xylose 131. In addition, hardwood xylans are highly acetylated e. g. birchwood xylan contains more than 1 mol of acetic acid per 2 mols of xylose 140. Acetylation occurs usually at the C-3 rather than the C-2 position of xylose. Acetylation at both positions has also been reported 141, 142. The presence of these acetyl groups is responsible for the partial solubility of xylan in water 133. The alkali extraction of xylan leads to the deacetylation of this substrate 140. ... 

Carbohydrate analysis using enzymatic hydrolysis and subsequent capillary zone electrophoresis (ENZ/CZE) revealed the number of 4-0-methylglucuronic acid units to be approximately one unit per nineD-xylose residues (Table 4). No glucuronic acid units were detected. The carbohydrate composition of the polysaccharide isolated here from aspen wood is quite similar to that previously reported for typical hardwood xylans . SEC analysis indicated the molecular weight (Mw) to be 15000 g/mol. 

The xylan obtained was studied by NMR spectroscopy. One major and two minor signals were observed in the anomeric proton region between 4.4 and 5.5 ppm in the H NMR spectrum (Fig. 3). The chemical shifts and three-bond coupling constants in the H NMR spectrum indicate that the major structural element in the xylan was a P-(l->4)-D-xylopyranosyl residue and that a 4-0-methylglucuronic acid side-group and a disubstituted xylopyranosyl residue were minor structural elements. The spectrum resembles the spectrum for alkali-extracted 4-0-methylglucuronoxylan from aspen described earlier. It can thus be concluded that the isolated... 

Figure 8, MALDI mass spectrum (negative-ion mode) of the acidic xylooligosaccharides obtained by endoxylanase treatment of a hexenuronoxylan isolated from alkaline peroxide bleached hardwood kraft pulp. The spectrum consists of two series of peaks assigned to the molecular ions [M-Hf of xylooligosaccharides with varying chain lengths and containing one hexenuronic acid (HexA) residue or one 4-O-methylglucuronic acid ( 4-0-...

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