Arabino- 4-O-methylglucurono xylans

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. 

As the chlorine-2-aminoethanol method offers no obvious advantages in the case of softwoods, most investigators have used chlorite holocelluloses for isolating the hemicelluloses. The most common procedure has been to subject the holocellulose to a preliminary extraction with aqueous potassium hydroxide or sodium hydroxide solution. Low concentrations of alkali favor removal of the acidic xylan, as does the use of potassium 

Immergut and R8.nby have resolved a mixture of a xylan and a gluco-mannan from a spruce sulfite pulp by a series of alkaline extractions and 

In several cases, separation of acidic xylans from neutral glucomannans has been achieved by way of their acetates, by a method originally suggested by Perlin. As mentioned earlier (see Part I, p. 295), the acetates of acidic xylans are seldom readily soluble in chloroform. On the other hand, acetylated hexosans dissolve readily. When a chloroform solution of a glucuronoxylan acetate and a glucomannan acetate is shaken with water, an emulsion is formed which contains the pentosan acetate while the hexosan acetate remains in the chloroform. This method has been successfully applied by other workers, but it is somewhat time-consuming. 

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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. 

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