Acetyl- 4-0-methylglucurono xylans

In an attempt to isolate a water-soluble 0-acetyl-(4-0-methylglucurono)-xylan from a birch chlorite holocellulose, Marchessault and coworkers kept the time of contact between the ammonia and the holocellulose at a minimum. Successive extractions of the pretreated holocellulose with water and methyl sulfoxide removed the xylan in yields of 7.1 and 14.6%, respectively, based on the weight of the original wood. The products still contained most of the iironic acid, but only one third of the 0-acetyl groups of the native xylan. 

In the ten years following this investigation, a dozen or more structural studies have been reported on similar xylans from various species of woody angiosperms of the temperate zones. It is now abundantly clear that the wood of these trees contains, as a preponderant hemicellulose, an 0-acetyl-(4-0-methylglucurono) xylan, the structural details and molecular properties of which vary only little, or not at all, from one species to another. Pertinent data at present available for these hemicelluloses are summarized in Table IV. It is readily seen that the general characteristics are the same... 

Specific rotation in aqueous alkali, usually sodium hydroxide. Number-average degree of polymerization (number of xylose residues in the xylan backbone). 0-Acetyl-(4-0-methylglucurono)xylan. In water. 

Few investigations have been carried out with native 0-acetyl-(4-0-methylglucurono)xylans. One such polysaccharide, from wood of white birch, has been reported to have a degree of polymerization of 180, as estimated from measurements of osmotic pressure on the fully acetylated polymer. A similar polysaccharide from wood of silver birch had a P value of 200 10, determined by osmotic-pressure measurements on an aqueous solution. 

Fig. 2.—Integral and Differential Distribution Curves of an 0-Acetylated (4-0-Methylglucurono)xylan. ...

The arabino-(4-0-methylglucurono)xylans present in the wood of gymnosperms received attention much later than the 0-acetyl-(4-0-methylglucurono)xylans in the wood of angiosperms. One reason for this was probably that, in softwoods, the xylan is not the preponderant hemi-cellulose (as it is in the hardwoods). However, a factor presumably more decisive was the fact that, of all polysaccharides of wood, the softwood xylans are the most difficult to isolate. Anderson and coworkers, in a pioneering study, failed to separate the D-xylose- and D-mannose-yielding portions of wood of white pine, but concluded that a mono-O-methyluronic... 

Acetyl signals at 2.2 ppm, as recently reported for 0-acetyl-(4-0-methylglucurono)xylan isolated from aspenwere not detected. Such O-acetyl groups are most probably removed under the alkaline conditions used here during the extraction of the xylan. 

For isolation of hardwood xylans by way of holocellulose, a choice has to be made between the different methods available, according to the objectives to be attained. If molecular properties are to be investigated, the modified chlorine-2-aminoethanol method and, possibly, also the method of Klauditz are suitable. For chemical studies, especially if the location of the 0-acetyl groups is to be established, only chlorine dioxide or chlorous acid can be used. The subsequent extraction of the holocellulose will also have to be adapted to the desired ends. For isolation of a pure (4-0-methylglucurono)xylan, aqueous potassium hydroxide is excellent, since it gives almost quantitative yields and seldom causes removal of the gluco-mannan. If necessary, further purification can be effected by precipitation with Fehling solution. - ... 

Kg. 24. Partial chemical structure of some hemicelluloses A — 0-acetyl-(4-0-methyl-glucurono)xylan from hardwood B = arabino-(4-0-methylglucurono)xylan from softwood C = 0-acetyl-galactoglucomannan D = arabinogalactan from larch wood (138). 

The glucuronoxylans are composed of linear chains of p-(l->4) xylopyranosyl residues branched with a-(l->2)-linked 4-0-methylglucuronopyranosyl residues (4-0-MeGlcA). The 4-0-MeGlcA-to-xylose ratio is -1 6 for softwood and -1 10 for hardwood xylans. (7-acetyl-(4-D-methylglucurono)-xylan is the major hemicellulose present in hardwood, whereas arabino-(4-6>-methylglucurono)-xylan is present in softwood. 

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