Pentosans,— Arabinose

Besides stimulating the production and release of certain hydrolases, GA can also retard enzyme activity. Consistent with the observation that GA induces degradation of aleurone cell walls is the observation that GA inhibits the synthesis of pentosan (arabinose and xylose) components of the wall. Between 4 and 16 h after GA addition to isolated aleurone tissue there is a marked decline in pentosan biosynthesis, in part due to the reduced activity of a membrane-bound arabinosyl transferase, and perhaps also of xylosyl transferase [62]. Changes in such membrane-bound enzymes could reflect a shift in the role of membranes from the production of cell wall components to the production, or packaging of hydrolases. 

Generally, pentoses are polymerized (combined chemically into very large molecules) to form pentosans. Arabinose is a 5-carbon sugar found in large amounts in gums. When a number of arabinose molecules are linked together, the pentosan, araban, is formed. 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Both D-glucuronic acid and/or its 4-O-methyl ether and arabinose are also present in cereal xylans (4). Endospermic arabinoxylans of annual plants, often referred to as pentosans, are because of their branched structures more soluble in water and dilute alkali than xylans of lignocellulosic materials. They also have at least one, or even two, substituents per xylose residue (5). 

Hemicelluloses (cellulosans) is the family name of polysaccharides that includes pentosans (C3H804) , made up of the pentose units and hexosans (C6H,0O5) 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 ... 

A complete table giving, for different weights of phloroglucide, the corresponding weights of furfural and arabinose, araban, xylose, xylan, pentose and pentosan, has been compiled by Tollens and Krober. 

The other carbohydrates in cane juice are the soluble polysaccharides vaguely classified under the terms "hemi-celluloses, soluble gums and pectins. It is possible that some of these polysaccharides may enter the juice during the milling of the cane as the plant cell structure is destroyed. A gummy product has been isolated from cane fiber by alkali extraction followed by alcohol precipitation. Acid hydrolysis of this substance yielded crystalline D-xylose and L-arabinose.10 Such gums in Trinidad cane juices were isolated by alcohol precipitation at suitable hydrogen ion concentration and assayed for pentose content by the Tollens 2-fural-dehyde assay the results showed an apparent pentosan content of 0.04-0.07%u of the Brix solids. 

Many studies of nonenzymatic browning have been carried out using model systems of monosaccharides and amino acids. Given the ease of release of arabinose from the pentosan, there is the possibility that Maillard reactions could take place if the temperature is appropriate. 

From this type of schematic representation it would be expected that a more highly branched xylan chain would give more potential for reaction of the arabinose residues, either through crosslink formation or by Maillard-type reactions. Indirect evidence supporting this concept can be derived from pentosan analysis of the hemicellulose fraction of grasses and legumes. 

Apart from their intrinsic interest the methyl ethers of certain of the pentoses, especially those of L-arabinose and D-xylose, are of great importance in structural studies of the naturally occurring pentosans, and of plant gums and mucilages.1 This also applies to the methyl ethers of L-rhamnose and L-fucose. 

The only pentosans which have been studied to any great extent are the xylans. These can be obtained from all lignified cell membranes by extraction with 5% sodium hydroxide solution after lipids, pectic materials etc. have been removed. The conditions necessary to avoid degradation during isolation have been outlined.2 Early structural determinations indicated that arabinose was present as a non-reducing end-group in a branched molecule which had a chain-length of 18-20 D-xylo-furanose residues.27 It has been shown recently, however, that the xylan from esparto holocellulose can be purified free of arabinose after several re-precipitations of the copper-complex.28... 

L-arabinose and D-xylose in hemi-cellulose of sugar Cane, V, 279 Banana starch. See under Starch. Barium acid heparinate, III, 146 Barium 2-desoxy-D-gIuconate, III, 144 Barium D-gluconate, III, 144 Barium hypobromite, III, 163 Barium salts, in preparation of aldonic acids with NaCN, I, 23 Bark, tree, pentosan content, V, 271 Barley, alpha amylase of malted, V, 255-265... 

That not all of the xylans of wood are homogeneous polymers of anhydro-D-xylose units has been shown by the classical studies of O Dwyer (1923 to 1940) on hemicelluloses of American white oak. O Dwyer prepared a hemi-cellulose fraction from water-extracted, oakwood sawdust by extraction for two days with 4% aqueous sodium hydroxide solution. The polysaccharide material was obtained, after acidification, by the addition of ethanol. The product ([a]n —75° in 1 % sodium hydroxide), contained 70% of pentosan, and yielded n-xylose, n-maiinose, n-galactose, and L-arabinose on hydrolysis. 

This sugar is also an aldo-pentose and is stereo-isomeric with arabinose. It is known as wood sugar because it is obtained by the hydrolysis of wood gum, i.e.f of the pentosans present in this gum. It is crystalline and melts at 140°- 60°. It is optically active, being dextrorotatory. Its osazone melts at 160°. By reduction it yields a penta-hydroxy alcohol and by oxidation it yields tri-hydroxy glutaric acid. 

Pentosan. Pentosan analysis measures the amount of five-carbon sugars present in wood (xylose and arabinose residues). Although the hemicelluloses consist of a mixture of five- and six-carbon sugars see discussion of hemicelluloses), the pentosan analysis reports the xylan and arabinan content as if the five-carbon sugars were present as pure pentans. Pentoses are more abundant in hardwoods than softwoods the difference is due to a higher xylose content in hardwoods see Table XIII for examples). 

Pentosans are the total anhydroxylose and arabinose residues in wood. 

Pentosans are the total anhydroxylose and arabinose residues in wood, f Average of five trees, s Average of four trees. 

Attempts to estimate the xylan content of wood from pentosan analyses are fraught with difficulties of many kinds. In this method, which has previously been discussed in detail in this Series, arabinose and xylose residues are converted by strong hydrochloric acid into 2-furaldehyde, which is determined by one of several available methods. The values for pentosan and (4-0-methylglucurono) xylan in Table III show, with one exception, good agreement but this is probably fortuitous, since it is known that uronic acids, under the conditions used, give rise to one third of the theoretical yield of 2-furaldehyde. A previous claim that 4-0-methylgluc-... 

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