Xylans structural features

Since further elaboration of the evidence for the common structural features of xylans is unnecessary, our attention may be turned to differences in fine structure arising from the proportions and modes of linkage of other sugar residues, the presence or absence of branching in the backbone of xylose residues, and variations in molecular size. 

Some of the main structural features of the xylans isolated from the Gramineae are outlined in Table I. 

Source Chain length11 DPn, methodb Nature of xylan chain Other structural features Refer- ences... 

Some of the structural features of wood xylans are summarized in Table II. 

P. Mandal, C. A. Pujol, E. B. Damonte, T. Ghosh, and B. Ray, Xylans from Scinaia hater. Structural features, sulfation and anti-HSV activity, Int. J. Biol. Macromol., 46 (2010) 173-178. 

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. 

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... 

Identification of Structural Features of Various (O-Acetylated) Xylo-Oligosaccharides from Xylan-Rich Agricultural By-Products ... 

In this publication the rese ch performed within the framework of the PhD-research concerning the characterisation of complex xylo-oligosaccharides from xylan rich by-products (2) is reviewed. The separation and structural features of differently substituted xylo-oligosaccharides obtained from hydrothermally treated wheat bran, brewery s spent grain, com cobs and Eucalyptus wood is described. 

Structural features of xylans orriginally present in by-products... 

Taking into account the structural features of the xylans originally present the composition of the hydrolysates obtained after hydrothermal treatment of the four by-products is studied (5,9). The results of the composition of hydrolysates of each of the four by-products obtained at similar process conditions are summarised in Table I (5,9). 

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... 

HPAEC, RP-HPLC, RP-HPLC-MS, MS, RP-HPLC-NMR and NMR spectroscopy showed to be very useful for the separation and characterisation of the detailed structures of the substituted XOS. Furthermore, the use of these methods resulted in the recognition of some structural features, which have not been reported before to our knowledge. 0-acetyl substituents were found to occur not only in hardwood xylans as shown in literature, but also in xylan-fragments obtained from cereals (brewery s spent grain). Additionally, 2-0- or 3-0-acetyl substituted terminal xylosyl residues were analysed by NMR to occur in XOS present in the Eucalyptus wood hydrolysates. The complete structural characterisation of several O-acetylated XOS obtained from Eucalyptus wood hydrolysates was established by using (RP-HPLC-)NMR. 

In conclusion, the appearance of the micrometer-sized structures formed by the xylans seems to be related to the morphology of the cellulose substrate rather than the degree of crystallinity. The crystal structure of the cellulose substrate (cellulose I or II) is not unimportant but its influence is indirect, through the absence of fibril-like surface features that can induce the formation of xylan structures on the regenerated substrates (cellulose II). On a nanometer scale, the xylan layer looks similar on all of the cellulose substrates, supporting the conclusion that the cellulose surfaces studied are different on the micro scale but quite similar on the nano and molecular levels. 

Recent studies have revealed certain interesting features in the structure of a birch xylan (Fig. 3-16). The unit next to the reducing xylose end group is D-galacturonic acid, linked to a L-rhamnose unit through the C-2 position. The rhamnose unit, in turn, is connected through its C-3 position to thje xylan chain. 

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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