Xylan from esparto grass

Marchessault and his coworkers studied, in more detail, similar single crystals of a true xylan from esparto grass. As may be seen from the electron micrograph (see Fig. 5), the hexagonal platelets are similar to those obtained by Yundt, and form more-or-less perfect hexagons. The layer... 

A xylan from esparto grass 63, 64) is a singly branched chain of about 76 D-xylopyranose units linked jS-D-1— 4 in the main chain but with a 1—>3 link at the point of branching. From the cell wall of pear wood, another similarly single-branched xylan is obtained which has about 115 D-xylo-pyranose units, but it contains, in addition, a terminal D-glucopyranosyl-uronic acid unit at one point 65), The xylan from the red seaweed 57,58) Rhodymenia palmata seems to be composed only of D-xylopyranose units... 

True xylans are relatively rare. The xylan from esparto grass gives only D-xylose on hydrolysis, and it has been shown to be a chain of p-D- i - 4)-linked D-xylopyranose units. It has a DP of about 75 and has a single (i 3)-linked branch point. The xylan backbone resembles cellulose in structure. Chemically combined D-glucuronic acid is the biochemical precursor of the D-xylose units forming the xylan. 

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... 

Fig. 14.—Polymer Single Crystal from Esparto-grass Xylan. (Screw dislocations and lamellar texture are typical of these crystals, which contain folded polymer-chains. The insert shows the schematic diagram of the electron diffractogram that corresponds to the dry crystal form (see Section V,l, p. 460), and confirms that the molecular axis is normal to the lamellar plane).

Hemicelluloses are non-cellulosic low molecular weight polysaccharides that are found together with cellulose in plant tissues 1311. In the cell walls of land plants, xylan is the most common hemicellulosic polysaccharide, representing more than 30% of the dry weight 1321. Most xylans are heteropolysaccharides which are composed of 1,4-linked (3-D-xylopyranosyl residues 133, 134, 135). This backbone chain is substituted with acetyl, arabinosyl, and glucuronosyl residues 133. Homoxylans, on the other hand, consist of xylosyl residues exclusively and have been isolated from esparto grass 136, tobacco stalks 137, and guar seed husk 138. ... 

Xylan from wood has received somewhat less attention from carbohydrate chemists than has the xylan from such rich sources as wheat straw, corn cobs, and esparto grass. However, specific optical rotations of the xylans obtained from the aforementioned sources are of the same general order of magnitude as those of some xylans obtained from wood. Yundt crystallized xylans from paper birch and from barley straw as sphero-crystals, and compared their properties (see Table VII). 

From methylatioii data and molecular-weight determinations, a structure for esparto-grass xylan ([a] D —92°, in aqueous XaOH) has been suggested by Hirst and associates. The molecule consists of about 75 (3-D-xylopyranose units linked 4- 1, with a single branch point (3 —1) linkage. A similar structure has been proposed for a xylan isolated from... 

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