Hemicelluloses arabinogalactans

Figure 8.2 Acid-catalyzed hydrolysis of a hemicellulose (arabinogalactane).

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

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. 

Figure 1. Schematic representation of pectin structure indicating stabilization of catenated polygalacturonic acid chains through Ca + (O) cross-bridging. Non-bridging sequences of a(l-4) linked /S-galacturonic acid methylester extend from L-rhamnose via (1-4) linkage to another rhamnose via a(l-2) linkages. Arabinogalactan side chains are linked to rhamnose residues and couple the RG structure to hemicellulose.

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

The noncellulose (1-n-glucans in hemicellulose are generally 1,3- and 1,4-bonded arabinans, xylans, arabinoxylans, arabinogalactans, etc. Their aqueous dispersions are quite viscous to the detriment of filters used to clarify cereal-based fermented beverages. Oat (3-glucans are Theologically... 

Mesophyll, endosperm, and, possibly, primary, or other thin, cell walls may all have similar hemicelluloses in any one species and variety of grass. If this is so, it is of interest to consider whether thin-walled, non-endospermic cells similar to those of the endosperm possess water-soluble arabinogalactans and arabinose-rich xylans similar to those present in, or associated with, the endosperm (see Section IV). 

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. 

Hemicellulose Samples. Two water-soluble hemicellulosic samples were used (1) acetyl-4-O-methylglucuronoxylan (molecular weight = 30,000) from white birchwood, isolated by the extraction of chlorine holocellulose with dimethyl sulfoxide and (2) arabinogalactan, Stractan from St. Regis Paper Company (arabinose galactose = 1 6 molecular weight = 60,000), prepared by the water extraction of heartwood of western larch. Both samples were kindly provided by T. E. Timell (SUNY). 

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]. 

Hemicelluloses have short side-chains, apart from arabinogalactan which is heavily branched. In contrast, cellulose is a long unbranched polymer. 

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. " ... 

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