Arabino-4-O-methylglucuronoxylan

Fig. 9. Partial structural formulas and shorthand notations for principal hemiceUuloses found in wood, where the sugar units ate noted as P-D-xylopyranose (Xylp), 4-Omethyl-a-D-glucopyranosyluronic acid (GlupU), a-L-arabinofuranose (Araf), P-D-glucopyranose (Glup), P-D-mannopyranose (Manp), and P-D-galactopyranose (Galp) for (a) arabino-4-O-methylglucuronoxylan from softwood, (b) 0-acetyl-galactoglucomannan from softwood, and...

Figure 3. Distribution of polysaccharides through the cell wall. C, cellulose M, galacto-glucomannan X, arabino-4-O-methylglucuronoxylan.

The main polyoses of softwoods are galactoglucomannan, with an average ratio of glucose to mannose of 1 3 and an average acetyl content of 1.3% (based on wood), and arabino-4-O-methylglucuronoxylan. 

Softwoods galactoglucomannan with 1.3 (%) acetyl hardwoods glucomannan. 6Softwood arabino-4-O-methylglucuronoxylan hardwoods 4-O-methylglucuro-noxylan with 4 (%) acetyl. 

Mannobiose was predominant throughout the enzymatic reaction its amount relative to mannose decreased, however, with incubation time (Table II). Besides mannobiose and mannose, glucose was present in the reaction solutions (Figure 2). The amount of glucose relative to the mannosaccharides increased with time it was about 1 8 after 5 hr of incubation and about 1 4-5 after 23 to 80 hr. No cellobiose and no degradation products from the arabino-4-O-methylglucuronoxylan were found (cf. Figure 2 and Table I, Columns 3 and 4). 

Several xylo-oligosaccharides of d.p. 2-7 have been isolated from enzymic hydrolyzates of terrestrial-plant D-xylans, arabinoxylans, arabino-4-O-methylglucuronoxylans, and 4-O-methylglucuronoxylans by the action of endo-xylanases,5,6,38-40,42-44,47,181,215-217,222,228 236,247-250, 254,256,258-261 an(j have been shown to constitute a series of /3-D-(l — 4)-linked D-xylo-oligosaccharides. 

Figure IL The SEC molar mass distribution curves for arabino-4-O-methyl-glucuronoxylan (from spruce holocellulose), arabinohexenuronoxylan (kraft pulp) and two different 4 0-methylglucuronoxylans (sulfite and dissolving pulp) obtained by the SEC/MALDI-MS procedure. Each xylan was analyzed separately employing the alkaline (pH 13) buffer SEC system and subsequently graphed on the same MALDI-MS calibrated absolute molar mass scale.

---