Xylan partial hydrolysis

The floss silk from Chorisia speciosa furnished a polysaccharide with a main chain of (1 -> 4) linked P-Xylp substituted at 0-2 by 5 % of uronic acid. The xylan structure also was interposed with a-Rhap units in small amounts. The defatted seeds furnished on aqueous extraction a major fraction, ((9-acetyl, 10 % and protein, 45 %) wich was hydrolysed and analysed by p.c. and GLC, showing Rha (20 %), Ara (16 %), Gal (64 %) and also uronic acids (45 %). Partial hydrolysis gave rise to a polysaccharide free of arabinose, with 46 % of uronic acids. Methylation analysis (GLC -MS) indicated a chain of (1 4) - linked Gal/ (42 % of 2,3,6-Me3-Gal). 

The three cellulases decomposed about 25-45% of the cellulose accompanied by solubilization of about 40-70% of the mannan and, by partial hydrolysis, of about 20% of the xylan present in the untreated sprucewood holocellulose. Based on the degradation products (cf. Table III, Columns 13-15, and Table II), the catalytic actions of the three cellulases—all isolated from Trichoderma viride—are similar or identical. The lower absolute degradation values obtained with cellobiohydrolase C might merely be a result of enzyme concentration. 

Wood xylans are characterized by the presence of residues of 4-O-methyl-D-glucuronic acid linked to position 2 of D-xylose in the main xylan chains (XXVI), and, accordingly, the aldobiouronic acid, 0-(4-0-methyl-a-D-glu-cosyluronic acid)-(l —> 2)-D-xylose (IX), is commonly isolated after the graded hydrolysis of such xylans. Partial acid hydrolysis of Monterey pine wood69 furnishes this compound together with the isomeric aldobiouronic acid, 0-(4-0-methyl-a-n-glucosyluronic acid)-(l—>3)-D-xylose, as a minor product, but the role of the (1 — 3)-linked aldobiouronic acid unit in the... 

The presence of uronic acid groups significantly reduces the hydrolysis rate of glycosidic linkages (Table 2). Thus, high yields of aldobiuronic acid dimers were obtained on partial hydrolysis of xylans [18, 19, 242]. The hydrolysis rate of different hardwood xylans was closely related to their uronic acid content [257]. A higher stability of softwood xylan as compared to... 

This method has been applied by Aspinall and Nicolson to the determination of the nature of the linkages in the highly branched arabino-xylans. 2 Rye-flour arabinoxylan was oxidized, during 4 days at 65°, with oxygen in the presence of Adams catalyst and sodium hydrogen carbonate. A total of 4% of uronic acid residues was introduced into the polysaccharide. After partial hydrolysis, the disaccharide 0-(L-arabinofurano-syluronic acid)-(l- 3)-D-xylose was isolated. As a result, it was determined... 

The methods used for structural analysis of (4-0-methylglucurono)xylans have also been applied to the arabinogalactans from larch wood. They include methylation, partial hydrolysis, and Smith degradation. In addition, Barry degradation has found extensive use. Because of the few, detailed, structural investigations so far reported, and because of the complicated nature of the polysaccharides, each of the four species studied will be discussed separately. 

There is some evidence that a small, perhaps variable, fraction of the acid side-chains in softwood xylans lacks the 4-0-methyl group. Roudier, in his extensive investigations on the uronic acids present in maritime pine, " isolated and identified 2-0(-a-D-glucopyranosyluronic acid)-D-xylose after partial hydrolysis of the wood. Hydrolysis of the xylan gave a small proportion of the same aldobiouronic acid. As previously mentioned (see Part I, p. 273), D-glucurono-6,3-lactone has been isolated from the wood of black spruce. " It has become evident that, despite the mild conditions often used, demethylation of the 4-0-methyl-D-glu-curonic acid residues may occur during hydrolysis of the above xylans. The presence or absence of D-glucuronic acid side-chains in hardwood or softwood xylans has, therefore, yet to be ascertained. 

The distribution of the acid side-chains in softwood xylans is not known. No oligosaccharides containing more than one 4-0-methylglucuronic acid residue were liberated on partial hydrolysis of a hemlock xylan, suggesting that few of the acid groups occur on contiguous D-xylose residues. 

All of the major polysaccharides in softwoods are now known these are cellulose, galactoglucomannans, arabino-(4-0-methylglucurono)xylan, ara-binogalactan, galactan, pectic material, and starch. It is clear, however, that other minor polysaccharide constituents, still unknown, must also be present. These polysaccharides have not yet been isolated, but their presence may be inferred from the formation of di- and tri-saccharides of unknown origin on partial hydrolysis of wood or crude hemicellulose preparations. 

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... 

While much of the work reported so far on alkyglycosides has dealt with hexose sugars, there are a few reports on the synthesis of pentose-based alkyglycosides that are effective next-generation surfactants. Starting from xylan, alkyl-/3-xyloside and -yS-D-xylobioside were produced by partial hydrolysis and simultaneous transglycosylation to various alcohols in a reaction catalysed by acetone powder of Aureobasidium pullulans cells with xylanase activity [77], The yield of the xyloside product increased while that of the xylobioside decreased with time and with an increase in concentration of the cells. The product yields were significantly improved and even alkyl-yS-xylotrioside was formed when the reaction was performed in supercritical fluids [78]. 

The extent of hydrolysis of D-xylans by /S-D-xylosidase should denote the extent of substitution, but variation in the action patterns of enzymes may be reflected in the various results that have been obtained using enzymes from different sources. /S-D-Xylosidase hydrolyzed a soluble D-xylan from Shirakamba wood68 to the extent of 39%, and rice-straw D-xylan released D-xylose,69 but other D-xylans underwent no significant hydrolysis.70 /3-d-Xylosidase has been employed in partially determining the structures of... 

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