Grasses xylans, structure

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... 

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... 

It is evident that xylans from different grasses have many structural features in common. The infrequent discovery of certain of those features, and the normal discovery of others, reflects the tendency, which changes with increasing knowledge, to seek to isolate xylans having features clearly established to be present in xylans previously iso-... 

The xylan of esparto grass has a linear structure, that is, it is poly(i8-(1 — 40-D-xylopyranose). It has a degree of polymerization of only about 70. All other xylans are branched. 

The precise location of the 0-acetyl substituents within the (arabino-)xylo-oligosaccharides has not been established yet. To our knowledge only few publications reported the presence of O-acetyl substituents in xylans from grasses (38,39) and is it for the first time that O-acetyl substituents are presented to occur in xylan-fragments obtained from cereals. A reason to explain the fact that the O-acetyl substituents are overseen in general can be that studies regarding the structural features of xylans normally include alkali-extractions to... 

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. 

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