Hemicelluloses structural features

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

While the hemicelluloses obtained from the germ, aleuron, and caryopsis coat cell walls all showed a similar monosaccharide composition, this was not the case for the endosperm tissue. Thus, a major difference in the structure of hemicellulosic polysaccharides exists between the preparations obtained from the endosperm cell walls and those from the cell walls of the other parts of the grain, i.e., rice bran. (Rice bran consists of the caryopsis coat, aleuron layer and germ.) Comparison of the detailed structural features of the hemicellulosic polysaccharides of endosperm and bran cell walls will be discussed in the following sections. 

Table 3-5 summarizes the main structural features of the hemicelluloses appearing in both softwoods and hardwoods. 

The various procedures used prior to the intentional extraction of hemicelluloses may affect quantitative or structural conclusions about the hemicelluloses as they occur in Nature. It must be uncertain whether structural features are destroyed or modified by the use of... 

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. 

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. 

A pentasaccharide and a trisaccharide have been isolated from an enzymic hydrolyzate of hemicellulosic material from oat coleoptiles.224 Methylation analysis of their structures indicated that these compounds may derive from j8-D-(l — 4)-linked D-glucans having terminal D-xylopyranosyl groups on some 0-6 atoms. Xylo-glucans having these structural features are present in the primary cell-walls of dicotyledons.22411... 

Hemicelluloses are a family of four basic types of polysaccharides, composed of two or more monosaccharide residues. All have structural features similar to cellulose in that they have their main chains that are -(1 4) linked, with the exception of the arabinoglactans that are /3-(l—>3) linked. The main chains are homopolysaccharides composed of a single monosaccharide residue, but they are highly branched by one or two different kinds of monosaccharides that are linked for the most part to give single monosaccharide branches. 

R 126 M. A. Kabel, H. A. Schols and A.G.J. Voragen, Identification of Structural Features of Various (O-Acetylated) Xylo-Oligosaccharides from Xylan-Rich Agricultural By-Products A Review , p. 108 R 127 P.T. Larsson, Interaction between Cellulose I and Hemicelluloses Studied by Spectral Fitting of CP/MAS C-NMR Spectra , p. 254 Vol. 867, 2004... 

Annual plants produce a rich selection of hemicelluloses, often with quite different structures compared with those found in woods, although of course the basic chemical features are always those of polysaccharides. It follows that specific applications are associated with these different structural features, notably as food additives. The presence of charged monomer units is one of the most exploited characteristics, because of the ensuing rheological sensitivity to physical parameters. The properties and appbcations of plant and seaweed hemicelluloses are dealt with in Chapters 13 and 24, the latter dealing with the specific features related to polyelectrolytes. 

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. 

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