Decorated 1 -4-Diequatorially Linked Polysaccharides - the Plant Hemicelluloses

The detailed molecular structure of the hemicelluloses varies from species to species and even from part to part of the same plant, but a common feature is that the main chain is decorated with side groups, some of which can contain a carboxylic acid group. The decoration stops the hemicellulose molecules aggregating undecorated xylan or mannan are nearly as insoluble as cellulose. 

The basic conformational motif of these molecules appears to be the two-fold helix, maintained by intra-chain 05 H03, familiar from cellulose. Electron diffraction studies on single crystals of mannan I revealed an antiparallel arrangement in an orthorhombic unit cell containing four equivalent mannoses. The polymer chain adopted a two-fold helix with (jo=-81°, F = -161° jy 79°) and 161°. In addition to the intra-chain 05 H03 hydrogen 

Because of the importance of this polysaccharide, the various substitutions of the backbone glucan have their own nomenclature  

X Backbone glucose with a-D-Xylp-(1 6) branch L Backbone glucose with p-D-Galp-(1 2)-a-D-Xylp-(1 6) branch F Backbone glucose with a-L-Fucp-(1 2)-(3-D-Galp-(l- 2)-a-D-Xylp-(l- 6) branch 

J Backbone glucose with a-L-Galp-(1 2)-(3-D-Galp-(1 2)-a-D-Xylp-(l- 6) branch 

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