Esparto xylan

Hydrolysis of xylan produces principally D-xylose.71 By hydrolysis of straw xylan in 3% nitric acid, Heuser and Jayme72 obtained crystalline D-xylose in 85% yield. Using a similar procedure, Hampton, Haworth and Hirst70 obtained a yield of 93% crystalline D-xylose from esparto xylan. A practically identical yield was indicated by reducing sugar determination made on the hydrolysis products of corn seedling xylan.68... 

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... 

Considering the close resemblance in general structure and food-reserve function of these polysaccharides, a certain similarity in properties might be expected, and this similarity will be discussed in later Sections. Also, the parent polysaccharides cellulose, mannan, and esparto xylan are all intrinsically water-insoluble the progressive introduction of the monosaccharide and disaccharide stubs, if... 

The only pentosans which have been studied to any great extent are the xylans. These can be obtained from all lignified cell membranes by extraction with 5% sodium hydroxide solution after lipids, pectic materials etc. have been removed. The conditions necessary to avoid degradation during isolation have been outlined.2 Early structural determinations indicated that arabinose was present as a non-reducing end-group in a branched molecule which had a chain-length of 18-20 D-xylo-furanose residues.27 It has been shown recently, however, that the xylan from esparto holocellulose can be purified free of arabinose after several re-precipitations of the copper-complex.28... 

Many pioneer structural investigations were carried out in other groups of polysaccharides, notably on inulin, on the xylan from esparto, on the mannan from yeast and on a series of bacterial polysaccharides amongst the latter were included somatic and lipoid-bound polysaccharides from M. tuberculosis. Noteworthy also was the work on the dextran produced by strains of Leuconostoc, which is showing grqat promise as a blood plasma substitute. 

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... 

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. ... 

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... 

---