Xylan dimethyl

Dimethyl- and 3,5-dichlorophenylcarbamates of other polysaccharides197 such as curdlan (32), dextran (33), xylan (34), chitosan (35), galactosamine... 

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... 

The late discovery of acetyl xylan and feruloyl esterases has been partly due to the lack of suitable substrates. Xylans are often isolated by alkaline extraction, in which ester groups are saponified. Treatment of plant materials under mildly acidic conditions, as in steaming or aqueous-phase thermomechanical treatment, leaves most of the ester groups intact. These methods, however, partly hydrolyze xylan to shorter fragments (63,69). Polymeric acetylated xylan can be isolated from delignified materials by dimethyl sulfoxide extraction (70). The choice of substrate is especially important in studies of esterases for deacetylation of xylans. The use of small chromophoric substrates (p-nitrophenyl acetate, a-naphthyl acetate, and methylumbelliferyl acetate) analogously to the assays of disaccharidases may lead to the monitoring of esterases unable to deacetylate xylan (33, 63, 64). 

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. 

Woods contain a number of acyl (mainly acetyl) groups, and it is probable that these are associated with the xylan components of the hemicellulose fraction. Evidence for the original esterification of aspen-wood xylan comes from the observation that the isolated xylan (obtained by alkaline extraction) is readily cleaved by periodate, whereas only a small proportion of the D-xylose residues in the wood itself are oxidized under similar conditions.66 Hemicelluloses still containing acyl groups may be extracted from wood holocelluloses by means of dimethyl sulfoxide, and further quantities... 

Dimethyl-D-xylose has been isolated from the products of hydrolysis of the methylated seed mucilages of Plantago lanceolate,1 and P. ovata11 and from the algal xylan of Rhodymenia palmate,. ... 

It is apparent from the foregoing that a more specific method for the determination of xylan would be desirable. To this end, the determination of xylose, after acid hydrolysis of the polysaccharide material, has been attempted. Xylose may be oxidized to xylonic acid which can be precipitated with cadmium bromide as the double salt, but the precipitation is not quantitative. Xylose forms an insoluble, crystalline di-O-benzylidene dimethyl acetal which permits identification in the presence of other sugars, but the necessity for anhydrous reaction conditions precludes the adaptation of this method to ordinary analysis. AVise and Ratliff prepared this derivative of both d- and L-xylose, as well as analogous derivatives from other aromatic aldehydes, and concluded that, with either the di-O-benzylidene or the di-O-(p-isopropylbenzylidene) dimethyl acetal, an excellent, highly specific, qualitative test was available for d- or n-xylose. 

The relative reactivity of the hydroxyl groups in a (4-0-methylgluc-urono) xylan toward methylation with dimethyl sulfate and alkali has been studied by Croon and Timell. The hydroxyl group at C-3 is less reactive than that at C-2, both in the xylose and in the 4-0-methyl-n-glucuronic acid residues. These results are similar to those obtained previously with both an esparto xylan and cellulose. ... 

Xylan Fractional precipitation Dimethyl suUbxide/ethanol 1887... 

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