Arabino- xylans softwoods

Arabino-xylan from wheat flour Arabino-glucuronoxylan from softwood Arabino- galactan Arabino- 13/6-- galactan Arabi- nan... 

The L-arabino-(4-0-methyl-D-glucurono)xylans are found in softwoods and annual plants. The L-arabinose is present primarily as a-L-arabinofuranosyl units, although P-L-arabinopyranosyl units may also be present. In either case, the arabinosyl units are often, but not always, present as single-unit side chains, as are the uronic acid units. 

The second important group of hemicelluloses found in softwoods are the arabino-(4-0-methylglucurono)xylans, which may make up from 5 to 10% of the dry weight of the wood. These consist of /3-1,4-linked D-xylopyranose units, partially substituted by L-arabino-furanose (at the 3 position) and by 4-O-methyl D-glucuronic acid (at the 2 position). The frequency of these substituent groups is around 1 to 2 residues per 10 xylose units. 

The other principal hemicellulose in softwoods is arabino-4-O-methylglucurono-xylan (degree of polymerization of 70-180). The backbone is composed of about 150 P D 1,4 xylopyranose units which are partially substituted at the C2 position by 4-0-methyl-a-D-glucuronic acid groups (approximately one group for every 5-6 xylose units). Also an a-L-arabinofuranose unit is linked by a-1,3 bond on approximately every 6 to 10 xylose units. Arabinofuranose is so called because it is a furanoside having a five-membered ring. 

Softwood xylans (arabino-4-O-methyl-glucuronoxylans) are composed of shorter chains with a degree of polymarization between 70 and 130 (Fig. 11.4-5). Unlike hardwood xylan, the softwood xylan has a higher content of4-0-methyl-glucuronic acid. The acetyl groups are replaced by a-L-arabinofuranose units, which are linked by a-l,3-glycosidic bonds to the C-3 position of xylose 143. ... 

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... 

As mentioned previously (see Part I, p. 255), only minor quantities of hemicelluloses can be removed from softwood by direct extraction with alkali. Very likely, the hemicelluloses are protected by the lignin, especially by that portion which surrounds the polysaccharides in the secondary wall. Isolation of softwood xylans by direct, alkaline extraction of tbe wood seems to have been used on only two occasions. Saarnio, using wood shavings of Norway spruce Picea abies), was able to obtain a relatively pure arabino-(4-0-methylglucurono)xylan in a yield of 3%, simply by extracting with 5% aqueous potassium hydroxide, a remarkable result. With wood of loblolly pine, Jones and Painter obtained a mixture of hemicelluloses, in a yield of about 1%, on direct extraction with 25% aqueous potassium hydroxide solution containing 4% of borate. 

A relationship has been developed for calculating the specific rotations of xylans containing both 4-0-methyl-a-D-glucuronic and L-arabinofuranose side-chains, such as those occurring in softwood xylans. In a comparison with arabino-(4-0-methylglucurono)xylans from six species, the calculated and found values agreed weU for three. In two of the other three xylans, the polysaccharides were of doubtful purity. 

All of the major polysaccharides in softwoods are now known these are cellulose, galactoglucomannans, arabino-(4-0-methylglucurono)xylan, ara-binogalactan, galactan, pectic material, and starch. It is clear, however, that other minor polysaccharide constituents, still unknown, must also be present. These polysaccharides have not yet been isolated, but their presence may be inferred from the formation of di- and tri-saccharides of unknown origin on partial hydrolysis of wood or crude hemicellulose preparations. 

Another major hemicellulose component in softwood is arabino-(4-(7-methylglucurono)-xyIan (Fig. 1), which represents about 7-15% of the wood [3]. Arabinoglucuronoxylans are composed of approximately 200 (1 4)-Iinked (1-D-xylopyranosyl units that are partially substimted by 4-0-methyl-a-D-glucuronic acid moieties at the C-2 position, with an average of (Mie glucuronic acid unit per 5-6 xylopyranosyl units [12]. In additimi, a-D-arabinofuianosyl units are bonded at the C-3 position of the main xylan chain, with an average of 1.3 arabinofuranosyl units per 10 xylopyranosyl units. 

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. 

---