Wood, arabinogalactans hemicelluloses

Smaller amounts of other hemicellulose polysaccharides are also found in softwoods. In particular, larch contains an unusually large amount of arabinogalactan, which is usually only a minor components of other wood species. 

Shimizu, K., Teratini, F., Hashi, M. and Miyazaki, K. (1972). Effect of the thermal treatment on wood hemicelluloses. VI. Studies on the thermal analysis of arabinogalactan, and O-acetyl and deacetylated-galactoglucomannans. Mokuzai Gakkaishi, 18(2), 79-84. 

Hemicelluloses in reaction woods are quite different from those in the normal woods, namely, galactan and P-(l-3)-gIucan in compression wood and galac-tan in tension wood. It is also well known that a remarkable amount of a water-soluble polysaccharide, arabinogalactan, is contained in the heartwood of larch. Since this polysaccharide occurs mainly in the lumen of tracheids and is not a cell wall component, it may not be included in hemicelluloses. Although structures and distributions of hemicelluloses have been comprehensively studied in the last 20 years, their physiologic meanings in a cell wall are not known yet. This must be the most important point for the future study of hemicelluloses. 

Wood Solubility. The solubility of wood in various solvents is a measure of the extraneous components content. No single solvent is able to remove all of the extraneous materials. Ether is relatively nonpolar and extracts fats, resins, oils, sterols, and terpenes. Ethanol/ benzene is more polar and extracts most of the ether-solubles plus most of the organic materials insoluble in water. Hot water extracts some inorganic salts and low molecular weight polysaccharides including gums and starches. Water also removes certain hemicelluloses such as the arabinogalactan gum present in larch wood see Table I). 

Some hemicelluloses have practical applications such as larch arabinogalactan, which is a water-soluble gum found in the heartwood of trees from Larix genus, and which is used in processed food as an emulsifier, or in pudding mixes, etc. A Py-GC/MS study of arabinogalactan from larch wood indicated the presence of the galactose unit and of arabinose unit in the ratio 6 1, as it is known in this material. The arabinose units were identified mainly by the formation of 1,4-anhydro-L-arabinopyranose, which elutes faster than the corresponding hexoses. The structure shown below was therefore confirmed by Py-GC/MS [64]. 

The exo-anomeric effect enforces a value of cp around 90° for anomeric axial linkages. This ensures a proclivity towards helical structures (as distinct from the ribbons of 1,4-diequatorially linked polysaccharides), as in starch and glycogen. p-(1 4)-Linked galactans, 1,4 axial-equatorial in the other sense, are known as hemicellulose components of compression and tension wood, the storage polysaccharides of lupins and as arabinogalactans attached to the rhamnogalacturonan I component of pectin, but have not been subject to conformational studies they appear to be biosynthesised from UDP-Gal. " ... 

Other hemicelluloses such as arabinans, galactans, arabinogalactans, rhamnogalactans, etc are regarded as pec-tic substances of plant cell walls. They are a complex mixture of colloidal polysaccharides, more abundant in the soft tissues of some fruit, as well as in sugar beet pulp, but less frequent in wood tissues. 

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). 

Table 7.1 compares the chemical composition of extractive-free hardwoods and softwoods. As cellulose and the hemicelluloses are both polysaccharides, it is obvious that the polysaccharide of wood is by far the dominant component, making up approximately 70 percent of both hardwoods and softwoods. Additional polysaccharides may occur as extraneous components of wood, which are not part of the cell wall for example, the heartwood of species of larch can contain up to 25 percent (dry weight) of arabinogalactan, a water-soluble polysaccharide that occurs only in trace quantities in other wood species. ... 

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