Hemicelluloses

Hemicelluloses (cellulosans) is the family name of polysaccharides that includes pentosans (C5Hg04) , made up of the pentose units and hexosans (CeHmOg), made up of hexose units. The pentosans include such substances as xylan and araban which are hydrolysed to xylose and arabinose respectively. On dehydration, furfuraldehyde is formed  

The hemicelluloses also include the polyuronides, or polyuronic acids, for instance a polymer of a hexuronic acid such as galacturonic acid. There is a possible generic link between the polyuronides and the pentosans since the latter might be produced as the result of the decarboxylation of hexuronic acids. The possibility of transforming hexuronic acids into pentosans by the removal of carbon dioxide 

All hemicelluloses dififer from cellulose in that they are soluble in dilute alkalis it is self-evident that hexuronic acids dissolve particularly readily. 

Cellulose and lignin are the structural components of most plant cell walls. 

The constitution of lignin and its biosynthesis have been extensively studied but these problems are not yet solved. 

The term hemicelluloses refers to substances which occupy the spaces between the cellulose fibrils within the cell walls of plants. Various studies, e. g., on apples, potatoes, and beans, show that xyloglucans dominate in the class Dicotyledoneae. A section of the structure of a xyloglucan from runner beans is presented in Formula 4.161. 

In the class Monocotyledoneae, the composition of the hemicelluloses in the endosperm tissue varies greatly, e. g., wheat and rye contain mainly arabinoxylans (pentosans, cf. 15.2.4.2.1), while P-glucans (cf. 15.2.4.2.2) predominate in barley and oats. 

Hydrolyses of hemicelluloses yield mixtures of glucose, glucuronic acid, xylose, arabinose, galactose, galacturonic acid, mannose, and rhamnose. Some common polymers of pentoses, also known as pentosans, are xylan, galactan, araban, and others. Pentosans are found in large amounts (20-40%) in cereal straws and in brans. Large-scale industrial preparations of furfural, for instance, are based on these materials. 

one of the better known hemicelluloses, is a component of plant cell membranes. This pentosan occurs in association with cellulose. The structure of xylan was shown to be 1,4-polyxylose  

Another hemicellulose, Galactan, is a minor component of some coniferous and deciduous woods. Larch wood was shown to contain about 8% of this polymer.  

Plant gums and mucilages are high molecular weight polysaccharides composed of hexoses and pentoses. They also contain some uronic acid units. Among the gums are gum arabic, gum tragacanth, and many others. 

Principles of Polymer Chemistry, DOI 10.1007/978-l-4614-2212-9 8, 1st and 2nd editions (C) Kluwer Academic/Plenum Publishers 1995, 2000, 

In addition to cellulose certain related substances, namely hemicelluloses and lignin, occur widely in plants. 

There are several polysaccharides that have structures similar to cellulose, with P-1 4 linked monosaccharide units. They are the hemicelluloses, chitin, 

The hemicelluloses are a heterogeneous group of polysaccharides that vary from plant to plant and from one plant part to another. There are four basic types of hemicellulose polysaccharides D-xyloglucans, composed of D-xylopyranose attached to a cellulose chain D-xylans, composed of D-xylose D-mannans, composed of D-mannose and D-galactans, composed of D-galactose. These polysaccharides are similar to cellulose in having their main chains linked p-1 - 4. Most of the hemicelluloses are, however, heteropolysaccharides with one to three monosaccharides units linked to the main monosaccharide chains. 

D-Xyloglucan is an extensively studied hemicellulose [57,58]. The structure consists of P-1 4-linked D-glucopyranose residues with single D-xylopyranose residues linked a-1 6 to the glucan chain [59,60] (see Fig. 6.9A). There are some xyloglucans from rape, nasturtium, and Tamarindus indica seeds that also have D-galactopyranosyl units linked P-1 2 to the D-xylopyranosyl residues 

D-Xylan is composed of D-xylopyranose linked P-1 4 with various kinds of 

The main chain of D-xylan is very much like that of cellulose except that each sugar residue lacks a primary alcohol group. The absence of the primary alcohol group reduces the formation of intermolecular hydrogen bonds and the formation of microfibrils. Further, the substitution of uronic acids onto the xylan chain makes them acidic polysaccharides and much more water soluble than cellulose chains. 

Hemicellulose is a collective name for the non-cellulosic polysaccharides, which are a large constituent in the structure of the wooden cell wall. Hemicelluloses are generally considered to cover the elementary cellulose fibrils and function as an amorphous glue binding the cellulose fibrils together. Hemicelluloses comprise typically 25-35% of the wood, and are thus the second most abundant component group in wood. Hemicelluloses have only a very weak surface activity but are, nevertheless, of scientific and technical interest because of their ability to function as emulsion stabilizers through a steric stabilization mechanism. 

The predominant hemicellulose type in softwoods is galactoglucomannan (GGM), having a linear backbone of mannose and glucose units with single galactose units as side groups 

In the manufacture of mechanical pulps, either by grinding of logs or by refining of chips, part of the hemicelluloses will be dissolved in the process waters. Spruce mechanical 

GGMs can be concentrated and recovered by ultraflltration of the process waters [44-46]. GGMs can also be extracted directly from softwoods using plain water at temperatures of 160-170 °C [47], at conditions where the acetyl groups are preserved as much as possible. However, hydrolytic chain cleavage cannot be avoided and the dissolved GGMs exist partly as monomers and oligomers. 

Xylans can be extracted from hardwoods, also using plain water [48]. Moreover, xylans can be extracted from hardwood black liquors, especially from liquor taken out in the early stage of the cook [49]. 

Some wood polysaccharides are extensively branched and are readily soluble in water. Typical of certain tropical trees is a spontaneous formation of exudate gums, which are exuded as viscous fluids at sites of injury and after dehydration give hard, clear nodules rich in polysaccharides. These gums, for example, gum arabic, consist of highly branched, water-soluble polysaccharides. 

The amount of hemicelluloses of the dry weight of wood is usually between 20 and 30% (cf. Appendix). The composition and structure of the hemicelluloses in the softwoods differ in a characteristic way from those in the hardwoods. Considerable differences also exist in the hemicellulose content and composition between the stem, branches, roots, and bark. 

Other Polysaccharides Besides galactoglucomannans, arabinoglucuronoxylan and arabinogalactan, softwoods contain other polysaccharides, usually present in minor quantities. They are built up dominantly 

Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an 0-acetyl-4-0-methylglucuronoi3-D-xylan, sometimes called glucuronoxylan. Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans. 

Depending on the hardwood species, the xylan content varies within the limits of 15-30% of the dry wood. As can be seen from Fig. 3-15, the backbone consists of /3-D-xylopyranose units, linked by (1 — 4)-bonds. Most of the xylose residues contain an acetyl group at C-2 or C-3 (about seven 

Two different L-arabinans have been isolated from azuki beans (Phaseolus radiatus) by alkaline extraction and separated by gel chromatography. No other sugar residues were present and, from structural analysis, the polymers were composed of chains of 1,5-linked L-arabinofuranose residues with branch points at 0-2 and/or 0-3. These branches were composed of non-reducing terminal L-arabinofuranose residues and there were more (1 2)-linkages than (13)-linkages. The polysaccharides differed in their molecular weights and solubilities in aqueous ethanol. 

An endo-L-arabinanase and two a-L-arabinofuranosidascs are produced when Bacillus subtilis is grown in a medium containing sugar beet arabinan. The endo-L-arabinanase was homogeneous by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, had a molecular weight of 3.2x10, and a pH optimum of 6.0. It is able to release residues of L-arabinose from suspension-cultured sycamore cell walls. a-L-Arabinofuranosidase activity was determined 

Hirose, Chem. Pharm. Bull., 1978, 26, 1729. 

An L-arabino-D-galactan, in 2% yield, has been isolated from the fruits of Dillenia indica. From the results of partial acid hydrolysis and methylation analyses, when 2,6-di-O-methyl-D-galactose, 2,3,6-tri-O-methyl-D-galactose, 2,3,4,6-tetra-(9-methyl-D-galactose, and 2,3,5-tri-O-methyl-L-arabinose residues in the molar ratio 7 3 1 8 were identified, it was concluded that the polysaccharide was a (1- 4)-j3-D-galactan that contained L-arabinofuranose residues linked at 0-3 of some of the D-galactose residues in the main chain. 

The occurrence, isolation, chemistry, and physico-chemistry of the plant L-arabino-D-galactans and their protein conjugates have been reviewed. The structural relationship between those found in woods, gum exudates, and plant callus cells, and whole tissue is discussed and the nature of the glycoproteins compared with L-arabinose- and D-galactose-containing cell-wall glycoproteins. The possible biological role of these materials is also considered. 

Enzymic analyses of aqueous extracts of barley obtained at 25—100°C showed that there is an exponential relationship between the P-D-glucan content and the extraction temperature, which could, in part, reflect the increased solubility at higher temperatures of larger molecules containing more sequences of (1 3)-P-d- 

spectroscopy showed the presence of a- and P-D-glucosidic linkages in the molar ratio 4 1. 

The rate of synthesis of a P-D-glucan from UDP-D-glucose in slices of pea-epicotyl tissue was increased by preliminary, short-term treatment of the tissue with the cellulases obtained from auxin-treated peas. The enzymic treatment 

D-mannose linked to unidentifled hexose and 2-acetamido-2-deoxyhexose residues. 

The binding of four murine myeloma immunoglobulins with inulin and with a series of inulin-derived oligosaccharides has been examined. The binding sites on two of the immunoglobulins showed highest afllnity for a sequence of three D-fructofuranosyl residues, whereas it was highest with a sequence of four D-fructofuranosyl residues for the others. 

Light scattering and viscosity measurements have been performed on partially methylated derivatives of Konjac D-mannan in order to clarify the macromolecular and chemical properties of the native polysaccharide. The weight average molecular weight was found to be 1.0—-1.2 x 10 and it was suggested that the polysaccharide adopts a random coil configuration. The intrinsic viscosity is 

An extensive investigation of the polysaccharides found in cambial tissue of Populus tremuloides and Tilia americana has been carried out. The tissue from poplar contained pectic material (40%), arabinogalactan (12%), xylan (10%), xyloglucan (6%), glucomannan (1%), cellulose (21%), and protein (10%). The tissue from basswood had a very similar composition except that it contained more xyloglucan and less xylan. Except for the xylan and glucomannan content, the cambial tissue from these two woods had compositions very similar 

A number of sub-fractions have been isolated from the hemicellulose B fraction of the husks of Sorghum grain. Three of these sub-fractions were each found to contain residues of L-arabinose, D-xylose, D-galactose, D-glucose, and D-glucuronic acid and its 4-0-methyl ether. Structural studies confirmed that the three polysaccharides were xylans with the usual )8-l,4-linked main chain. There were considerable differences in the D.P. of these polysaccharides, the values obtained being 993, 2380, and 851. 

Two series of acidic oligosaccharides were isolated from the partial acid hydrolysate of a xylan from larchwood. The first series contained only one uronic acid residue per molecule while the second series contained two uroiiic acid residues and between two and four D-xylose residues per molecule. The structures of the four oligosaccharides are shown, (2)—(5). It was noted that in each oligosaccharide the two uronic acid residues were attached to contiguous D-xylose residues. Since more of the second series than the first was isolated, it was suggested that a far higher proportion of contiguous side chains are 

The principal classes of polysaccharides found in the non-cellulosic matrix of Avena sativa coleoptile cell walls are glucuronoarabinoxylans and hemicellulosic j8-D-glucans. Pectin is a minor component. One of the xylans con- 

The jS-D-glucan synthase activity of Pisum sativum epicotyls was less in particulate fractions than in tissue slices, in the synthesis of alkali-insoluble jS-D-glucans. The o-glucans contained both J5-(1 - 3) and - 4) linkages but the ratio was not constant, the )S-(1 4) linkage being preferred at low 

The cell walls of barley endosperm have been shown to contain a microfibrillar phase embedded in an amorphous matrix. The microfibrillar components are 

Gruber, J. Mohannmed, and J. Schurz, Cellulose Chem. Technol., 1974, 8, 423. 

The rates of delignification of the cell walls of Lolium pererme and Trifolium pratense at different stages of maturity have been determined after treatment with sodium chlorite. If the lignin content was less than 10%, delignification 

Tomoda and N. Satoh, Chem, and Pharm. Bull. (Japan), 1974, 22, 2306. 

Shimahara, H. Suzuki, N. Sugiyama, and K. Nisizawa, Agric. and Biol. Chem. Japan), 

Abdel-Fattah, S. S. Mabrouk, M. Edress, and M. S Shaulkamy, Carbohydrate Res., 

A xylan extracted from the holocellulose present in the stalks of Nicotiana tabacum has been purified by ion-exchange chromatography. The results of methylation analysis, partial hydrolysis with acid, and enzymic hydrolysis indicated that the xylan is composed of a straight chain of approximately 100 /S-(l - 4)-linked D-xylopyranosyl residues. Curie point-g.l.c. of the xylan yielded 2-furaldehyde, which is probably formed via 3-deoxy-D- /ycero-pent-2-ulose. Another major product was identified as 3-hydroxy-2-penteno-1,5-lactone (21) by high-resolution m.s. this lactone has also been prepared by pyrolysis of a hexuronic acid-containing xylan from beech. Since the tobacco-stalk xylan contains no acidic components, the lactone (21) must arise by 

An arabinoxylan (mol. wt. 5 x 10 ) has been isolated by gel filtration and ion-exchange chromatography of aqueous extracts of wheat flour. This polysaccharide was able to agglutinate a number of normal cell types (for example. 

The quantities of non-cellulosic D-glucan and cellulose present in cell walls of the cortex tissue of apples vary little during ripening. Cell-plate formation in Zea mays seedlings and in tuber slices of Jerusalem artichokes involves a (1 3)- 

Eight oligosaccharides containing D-fructose have been isolated from the roots of asparagus Asparagus officinalis), Identification of the constituent monosaccharides, g.l.c. of their methylated derivatives, partial hydrolysis with acid, and enzymic studies showed that the oligosaccharides are members of a series of l (l- -D-fructofuranosyl) -sucroses of the 1-kestose and neokestose type. 

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D-galactose, C HiiOe. Crystallizes in the pyranose form m.p. 1I8-120 C (monohydrate), 165-5" C (anhydrous). An isomer of glucose which is fairly widely distributed in plants. It is a constituent of raffinose and slachyose, of hemicelluloses, of pectin, of gums and mucilages, and of some glycosides. In animals it forms half the lactose molecule and is the sugar found in the brain. Chemically it is very similar to glucose. It has the structure... 

C. The pentose sugar of straw, cotton-seed hulls and various hemicelluloses, and of some glycosides, including the primeverosides. It is not fermentable and behaves chemically as other sugars. 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

Table 1. Approximate Hemicellulose Content of Selected Vegetable Materials and Their Residues...

Raw material Cellulose, % Hemicellulose, % Lignin, % Pectic material, % Extractives, % Reference... 

Fig. 3. The changes in coniferous hemicellulose as a result of pulping processes and before bleaching.

Derivatives of hemicellulose components have properties similar to the ceUulosic equivalents but modified by the effects of thek lower molecular weight, more extensive branching, labile constituents, and more heterogeneous nature. Acetates, ethers, carboxymethylxylan (184), and xylan—poly(sodium acrylate) (185) have been prepared. 

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