Xylans methyl ethers

Most structural work on xylan has been done on that from esparto grass and the principal attack made by way of the methyl ether. Xylan can be methylated by heating with methyl iodide and silver oxide,92-93 but complete etherification is difficult and considerable degradation probably occurs. On the other hand, complete etherification is attained by methylation in two operations with potassium hydroxide and dimethyl sulfate to give a dimethylxylan in almost quantitative yield70 showing [< ]22d — 92° in chloroform. Methylation with potassium hydroxide appears to proceed more readily than with sodium hydroxide.70-92... 

Xylans are the major hemicelluloses of many plant materials, where they often contribute to the rigidity of plant cell walls. Most xylans are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked j8-D-xylo-pyranose units. The degree and t pe of substitution of the backbone is dependent on the plant origin of a xylan. In addition to xylose, xylans may contain L-arabinose, D-glucuronic acid or its 4-O-methyl ether, and acetic, p-coumaric, and ferulic acids. 

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). 

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

Non-endospermic xylans have substituent groups of D-glucopyrano-syluronic acid, or its 4-methyl ether, or both. Individual molecules may lack such residues, but, almost certainly, other xylan molecules in the total hemicelluloses from each grass have both groups.137,157 Uronic acid groups may escape detection or identification in hydrolyzates of xylans studied by paper chromatography, and in hydrolyzates and methanolyzates of methylated xylans studied by g. l.c. after conversion of the products into volatile derivatives. 

In plants, the best known examples of methyl ethers are in xylans, many forms of which contain them. The majority occur as 4-OMe-glucuronic acid, at non-reducing terminals. Methyl ethers of xylose are also known (as 2 or... 

The enzyme responsible for the transfer was shown to have a of 6 X 10 M for 5-adenosylmethionine, which does not denote a particularly high affinity of binding. It had a pH optimum of 6.8, showed no requirement for divalent metal ions and was weakly inhibited by divalent nickel and zinc ions. It was therefore a different enzyme from that which forms the methyl ether groups in 4-Omethyl glucuronic acid of xylans. 

In annual plants, the main hemicelluloses are represented hy xylans (Fig. 13.3), which are more heterogeneous than the xylans from wood tissues [17]. They contain both glucuronic acid and/or its 4-(9-methyl-ether and arab-inose attached to C2 or C3 of the xylose units. Both xylan and glucomannans can be partly acetylated [18]. 

The molecular structures of water-soluble and -insoluble hemicelluloses from bamboo leaves, and of a hemicellulosic D-xylan of the former, have been examined. Attached to the main D-xylan chain of all the polysaccharides are the side-chains (7)—(10) and single residues of L-arabinofuranose and D-glucopyranosyluronic acid and its 4-0-methyl ether. The polysaccharides differ in the degree of substitution of the main D-xylan chain by these mono-, di-, and tri-saccharides. 

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. 

Arabinoxylans of various cereals differ not only in how the xylan chains are substituted, but also have different arabinose contents or ratios of these two sugars. The ratio of arabinose and xylose in wheat arabinoxylans varies from 0.50 to 0.71 and in less substituted rye arabinoxylans it ranges from 0.48 to 0.55. D-Glucuronic acid and its 4-0-methyl ether are found mainly in... 

As with most of the synthetic polysaccharides, complete methylation189 of the polyxylose was difficult. The fully methylated polymer was soluble in petroleum ether (30-60°) containing 6% of chloroform. Hydrolysis of the methylated xylan gave tri-, di-, and mono-O-methyl-D-xyloses, together with D-xylose, in the molar ratio of 31 33 19 5. The tri-O-methyl-D-xylose fraction contained 38% of 2,3,5-tri-O-methyl-D-xylose, the remainder being the 2,3,4-trimethyl ether. The dimethyl ethers included the 2,5-,... 

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

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