Glucuronoxylan structure

The possibility of an interaction between glucuronoxylan and lignin could give rise to aggregation behaviour in solution. However, one could not rule out the contribution of H-bonding between polysaccharide chains. We therefore perform a fractionation of the heterogeneous glucuronoxylan structure. 

Most of the glucuronoxylans have single 4-0-methyl-o -D-glucopyranosyl uronic acid residues (MeGlcA) attached always at position 2 of the main chain Xylp units (Fig. 2). This structural type is usually named as 4-0-methyl-D-glucurono-D-xylan (MGX). However, the glucuronic acid side chain may be present in both the 4-0-methylated and non-methylated forms (GlcA). 

The specificity of a xylanase preparation may be determined from the isolation and characterization of oligosaccharides formed from xylan hydrolysis. While the oligosaccharides released were mainly dependent on the specificity of the xylanase, the complex structure of the heteroxylan also determines the extent of hydrolysis and the site of cleavage. Two types of xylooligosaccharides are usually produced acidic (from glucuronoxylans) and neutral (from arabino- and arabinoglucurono-xylans). The specificity of the action of several purified xylanases on various heteroxylans is shown below ... 

Figure 18 O-Acetyl-4-O-methyl-D-glucuronoxylan, a type of hemicellulose, structure from angiosperms (Dekker, 1985) (reproduced by permission of Academic Press from Biosynthesis and Biodegradation of...

In the past SO years wood chemists have learned much about the composition and physical properties of hardwood xylans. Partially acetylated glucuronoxylans are model native hardwood xylans. They exhibit thermoplasticity, film forming properties, crystallization potential and are oriented in the secondary cell wall. Their crystal structure has been determined and the hydration of this crystalline polysaccharide has been defined. The structural regularity of these abundant polysaccharides can be interpreted using the principle of optical superposition. 

Since the days of Pasteur and Freudenberg, the value of optical rotation [a]o, as a means of characterizing polysaccharides has been appreciated. Timell isolated and purified glucuronoxylans and arabinoxylans to provide a series of samples whose optical rotation could be recorded in 10% aqueous sodium hydroxide. Figure 1 is a schematic of the molecular structure of the 4-0-methyl-glucuronoxylans which he isolated with a variable content of uronic acid. Figure 1 also shows the Equation which we used to account for the observed molar rotation of xylans (3). 

Figure 3.8 Principal structure of the main softwood hemicellulose type, galactoglucomannan, and of the main hardwood hemicellulose type, 4-Omethyl glucuronoxylan.

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