Arabinoxylan chains

The presence of feruloyl and p-coumaroyl acids linked via L-arabinose residues has been verified in several studies of xylan (6-8). The amounts of these components, however, are rather small. Every 15th arabinose unit in barley straw arabinoxylan is estimated to be esterified with ferulic acid, and every 31st with p-coumaric acid (7). These hydroxycinnamic acids are bound to C-5 of the arabinose residue (7, 8). It has been suggested that oxidative dimerization of ferulic acid residues crosslinks the arabinoxylan chains and renders them insoluble as a result of the diferuloyl bridges (9). 

Certain cereal grains, especiaUy wheat and rye, contain hemicelluloselike arabinoxylans [9040-27-1], commonly caUed pentosans. Wheat flour pentosans are divided into two types water-soluble and water-insoluble arabinoxylans, which respectively constitute - 1.1 1.6% and 0.4—0.7% of the total flour. These polysaccharides have functional roles in dough development and baking performance. The water-soluble wheat-flour arabinoxylans consist of a (1 — 4)-linked chain of P-D-xylopyranosyl units substituted at 0-2 and/or 0-3 with single-unit a-L-arabinofuranosyl units. Preparations from each source consist of a family of molecules of various molecular weights and xyl ara ratios. 

In contrast to AGX, the GAX consists of an arabinoxylan backbone, which contains about ten times fewer uronic acid side chains than a-L-Ara/ ones, and has some Xylp residues doubly substituted with these sugars. GAX are located in the non-endospermic tissues of cereal grains such as in wheat, corn, and rice bran. The degree and pattern of substitution of GAX appears to vary with the source from which they are extracted. These differences are reflected in the ratio of Ara to Xyl, the content of MeGlcA, and the presence... 

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. 

In describing and interpreting some of the more important properties of plant galactomannans, comparisons will be made with structurally similar polysaccharides, including the closely related glucomannans and galactoglucomannans, and those based on (1 — 4)-/3-D-xylan main-chains (for example, the arabinoxylans) and (1 — 4)-/3-D-glucan main-chains [for example, the amyloids and sodium 0-(carboxymethyl)cellulose]. 

Similar structural problems exist for the distribution of side chains in galactoglucomannans, xyloglucans, arabinoxylans, and 4-0-methylglucuronoxylans, and even for the distribution of acetyl groups in glucomannans and for related features in many other polysaccharides. 

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... 

Terminal L-arabinosidic linkages in L-arabinose conjugates are also hydrolyzed by the enzyme. The enzyme of R. fiava releases L-arabinose from the polysaccharide of the water shield (Brasenia schreberi J. F. Gmel)38 and from the cotyledon of Tora bean (Phaseolus vulgaris).39 Some 70 to 80% of the side chains of the arabinoxylan in rice cell-wall are composed of L-arabinose. When the a-L-arabinofuranosidase from R. 

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). 

Flaxseed mucilage, associated with hull of flaxseed, is a gum-like material and composed of acidic and neutral polysaccharides. The neutral fraction of flaxseed mainly contains xylose (62.8%), whereas the acidic fraction of flaxseed is comprised mainly of rhamnose (54.5%), followed by galactose (23.4%) (Cui et al., 1994a). A study by Warrand et al. (2005) found that the neutral monosaccharides were a mixture of three major families of polymers, arabinoxylans with a constant A/X ratio of 0.24, and various amount of galactose and fucose residues in the side chains. Acidic hydrolysis yields xylose, galactose, arabinose, rhamnose, galacturonic acid, fucose, and glucose (BeMiller, 1973 Erskine and Jones, 1957). 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

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