4-O-Methylglucuronic acid

The ratios of arabinose xylose and 4-O-methylglucuronic acid xylose in the reaction solution after acid hydrolysis were about the same as those of holocellulose before xylanase treatment (cf. Experimental). Oligomer or polymer mannan fragments were present in the reaction solution. They were detected after additional acid hydrolysis, and they represented about 12% of the mannan present in the starting material (Table I, Columns 2 and 6). 

The 4-O-methylglucuronic acid groups prevent the peeling of xylan chains at lower temperatures (< 100°C) but they offer only a partial protection at higher temperatures. Since the 4-O-methylglucuronic acid groups are bound to the C-2 position in the xylose units, no conversion of this carbon atom to a carbonyl group can take place. Instead, HO-3 is eliminated directly (/8-hydroxy elimination). 

Fig. 7-34. Loss of 4-O-methylglucuronic acid groups (Johansson and Samuelson, 1977). P denotes fragmentation products formed.

Tables XIII and XIV present more detailed analyses of woods Table XIII contains data on 30 North American species, and Table XIV contains data on 32 species from the southeastern United States. The lignin values in Table XIV are the sum of Klason and acid-soluble lignins. Pectin (Table XIV) is mainly galacturonic acid. It is the measured total uronic acid value minus the estimated glucuronic acid value. Glucuronic acid content can be estimated from the xylan content by assuming a ratio of xylose to 4-O-methylglucuronic acid of 10 1 see Table I and Figure 5). The reported values of the carbo-...

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. ... 

The final major hemicellulose is an arabinoglucuronoxylan, which accounts for 5-10% of softwood (4), This polysaccharide has a linear xylose chain onto which arabinose and 4-O-methylglucuronic acid units are individually attached. For every 10 xylose units, the molecule contains approximately one arabinose and two 4-O-methylglucuronic acid substitutions. The side-chain substituents make this hemicellulose susceptible to acid hydrolysis, but resistant to alkali-catalyzed degradation (2). 

The linear backbone of xylan consists of xylopyranose units linked by P-1,4-glycosidic bonds. Hardwood xylans are partially acetylated and branched with randomly distributed 4-O-methylglucuronic acid (MeGlcA) groups. ... 

Aldopentose Deoxyaldohexose Aldohexose Ketohexose Hexuronic adds 4-O-Methyl-hexuronic adds 173 187 203 203 217, 239 231, 253 132 66 146 73 162 81 162 81 176 88 190 95 Ribose, xylose, arabinose Rhamnose, fucose Glucose, mannose, galactose Fructose Glucuronic add, galacturonic add 4-O-Methylglucuronic acid ... 

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-...

Figure 1 HPAEC (pH 5) elution pattern of an Eucalyptus wood hydrolysate a gradient was used of 0-17 mM of sodiumacetate buffer (pH 5) during 5 min followed by 17-30 mM of buffer during 35 min at 0.5 ml/min (X = xylose GlcAfne - 4-O-methylglucuronic acid Ac = O-acetyl).

From this research it was concluded that by performing hydrothermal treatments of the xylan-rich by-products a variety of differently substituted XOS (DP 2-50) and xylan-fragments (DP >50) can be obtained. These XOS included linear XOS, xylan-fragments substituted with arabinoses, O-acetylated XOS and (O-acetylated) XOS substituted with one or two 4-O-methylglucuronic acid(s). 

Figure 1. Enzymatic hydrolyses of different xylans (-X-X-) and mannans (-M-M-) by T. reesei enzymes. The purified oligosaccharides are shown in the boxes. Hydrolysis points of enzymes are marked end0 l,4-fi-xylanase II, A /J-xylosidase, H endO I,4-fi-xylanase I, A endO I,4-fi-mannanase, X fi-D-xylopyranose unit, M fi-D-mannopyranose unit, A a-L-arabinofuranoside unit, G fi-D-galactopyranose unit, MeGlcA a-D-4-O- methylglucuronic acid, HexA...

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