Arabinoxylans isolation

In contrast to these findings, a glucuronoarabinoxylan isolated from oat coleoptiles did not bind to cellulose in vitro under reaction conditions that allowed other heteroxylans to bind.53 This oat heteroxylan had, however, a high percentage of arabinosyl side chains that would be likely to hinder binding sterically. A similar inability to bind to cellulose is exhibited by an arabinose-rich arabinoxylan isolated from cultured, barley-aleurone cell-walls.61... 

Goldschmid and Perlin,259 using an enzyme preparation that contained a D-xylanase, isolated from Streptomyces QMB 814, to degrade wheat-flour arabinoxylan, isolated a tetrasaccharide (AX3) which was shown by chemical methods to be 0-a-L-Ara/-(l —> 3)-0-[/3-l>Xylp-(l - 4)]-/3-D-Xylp-(l -> 4)-D-Xylp (4). 

Based upon these studies, Table III summarizes the partially deduced structure of the mono-, di-, and trisaccharides, obtained by the partial degradation of the original arabinoxylan. These structures may not be completely accurate, because of some uncertainty in the theory used to speculate the mode of linkage. Consequently, each one needs to be isolated to unambiguously verify its structure. These results do, however, provide some indication of the complicated structure of the side-chains of bran arabinoxylan. 

Of the related polysaccharides mentioned earlier (see p. 267), only arabinoxylans have been examined in any detail by enzymic methods. Goldschmid and Perlin153 studied the fine structure of wheat arabinoxylan by using the /3-D-xylanase from Streptomyces QMB 814. Their results indicated that arabinoxylan molecules are mainly constituted of highly branched regions in which isolated and paired L-arabinosyl (A) branches are separated by single D-xylosyl (X) residues, as shown in 12, but that, at unequal intervals (averaging... 

When this enzyme acts on (1— 5)-L-arabinan, arabino-oligosaccha-rides are produced in the initial stage of the reaction. The end products are L-arabinobiose andL-arabinose. When the strain F-l 1 enzyme acts on (1— -L-arabinan, the extent of decomposition is4 23.3%, whereas, that of beet L-arabinan is only 3.3%. This enzyme is inactive towards phenyl and p-nitrophenyl a-L-arabinofuranoside, arabinoxylan, arabinogalac-tan, and gum arabic. These results led to the conclusion that endo-L-arabinanase preferentially cleaves 5-linked arabinosyl residues. The action of endo-L-arabinanase on L-arabinan produces arabino-oligosaccharides, from which L-arabinotriose was isolated.27 When acting upon cell walls obtained by sycamore-cell cultivation, this enzyme releases L-arabinan.27... 

Rao and Muralikrishna (2004b) isolated a fraction of the arabinoxylan from ragi and determined its structure using a combination of methyla-tion, enzyme digestion, NMR, and MALDI-TOF-MS. They determined that the arabinoxylan was a (l-4)-l inked xylose backbone carrying arabinose at the C-3 position and an occasional glucuronic acid at the (l-3)-position. Xylose was also found in the side chain (Fig. 6.6). 

All the arabinoxylans, so far examined, are of this general type, and, where these structures have been distinguished, the evidence points to direct attachment of the L-arabinofuranose units to the backbone as in I. In one case, direct proof of this mode of linkage has been obtained by the isolation... 

In other cases, the evidence in favor of structure I is based upon the examination of a degraded polysaccharide isolated after controlled acid hydrolysis has resulted in removal of some or all of the acid-labile L-arabinofuranose residues without significant cleavage of D-xylopyranosyl linkages. Thus, arabinoxylans I and II would give rise to the corresponding degraded polysaccharides IV and V. Structures IV and V may be differentiated by... 

Several xylo-oligosaccharides of d.p. 2-7 have been isolated from enzymic hydrolyzates of terrestrial-plant D-xylans, arabinoxylans, arabino-4-O-methylglucuronoxylans, and 4-O-methylglucuronoxylans by the action of endo-xylanases,5,6,38-40,42-44,47,181,215-217,222,228 236,247-250, 254,256,258-261 an(j have been shown to constitute a series of /3-D-(l — 4)-linked D-xylo-oligosaccharides. 

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... 

Goldsehmid and Perlin have isolated, by enzymic hydrolysis of an arabinoxylan from wheat flour, a tetrasaccharide which was shown to be 0-a-L-arabinofuranosyl-(l—>3)-0-[i3-D-xylopyranosyl-(l— )]-0- 8-D-xylopy-ranosyl-(1 4)-D-xylose. The L-arabinofuranose side-chains in this polysaccharide are thus present in the a-L configuration, in agreement with the above evidence. 

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