Structural Features in Hemicelluloses

By 1960, much was known of the hemicelluloses, and, in particular, of the xylans, from grasses and woods.8 Xylans had been isolated from 

Some of the structural features account for galactose tanits139-141 143 145 in the parent hemicellulosic materials. Both the D and the L enantiomer are present,139-145 but it is often assumed that the galactose in land plants is the D enantiomer. 

Non-endospermic xylans from grasses are frequently stated to contain low proportions of non-xylosidic sugar units, whereas endosper-mic xylans, which are commonly isolated without deliberate fractionation of the readily extracted hemicellulosic material, are stated to be highly substituted by L-arabinofuranosyl groups. It is beyond dispute that xylans having low proportions of non-xylosidic units are present in, and commonly isolated from, the molecular populations of non-endospermic, hemicellulosic materials. 

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- 

With discretion, the structural information on features in xylans from other, higher land-plants can be used to assist, as in the past, in the search for similar features in the hemicellulose of plantstuffs under examination—no matter how often they have been previously examined. If new facts suggest new interpretations, earlier studies should be re-examined to determine whether the newer interpretation accords with facts established earlier. It is a misfortune of hemicellulose chemistry that it is rarely possible to disprove a reported fact, as, unless total hemicelluloses30,31 are isolated from plants of the same variety or cultivar grown under very similar conditions to the plants studied earlier, the hemicellulosic material under examination may not contain molecules having detectable amounts of the structural features noted earlier. 

---