Xylans structures

Macromolecular Symposia, Vol. 232,1, (February 2005), pp. (1-12), ISSN 1521-3900 Ebringerova, A. Heinze, T. (2000). Xylan and xylan derivatives - Biopolymers with valuable properties, 1 - Naturally occurring xylans structures, procedures and properties.. Macromolecular Rapid Communications, Vol. 21, 9, 0une 2000), pp. (542-556), ISSN 1022-1336... 

The floss silk from Chorisia speciosa furnished a polysaccharide with a main chain of (1 -> 4) linked P-Xylp substituted at 0-2 by 5 % of uronic acid. The xylan structure also was interposed with a-Rhap units in small amounts. The defatted seeds furnished on aqueous extraction a major fraction, ((9-acetyl, 10 % and protein, 45 %) wich was hydrolysed and analysed by p.c. and GLC, showing Rha (20 %), Ara (16 %), Gal (64 %) and also uronic acids (45 %). Partial hydrolysis gave rise to a polysaccharide free of arabinose, with 46 % of uronic acids. Methylation analysis (GLC -MS) indicated a chain of (1 4) - linked Gal/ (42 % of 2,3,6-Me3-Gal). 

Pure xylanases can be useful in l ic research. They fmd application in analytical procedures designed to quantify specific xylan constituents 10) and to study xylan structure. They can be used to preparation of defined xylooligosacchaiides, eidia by hydrolysis of xylan 11-14) x by transglycosylation at high substrate concentrations 15,16). These products may serve as assay substrates and model compounds to study the mechanism of action of xylanolytic enzymes. Finally, xylanolytic eiuymes play an imptutant role in complex enzyme preparations used to the release of plant protoplasts. 

Due to the extreme variety of xylan structures, it is obvious that many kinds of enzymes are needed for their complete hydrolysis in nature. Xylanases (EC 3.2.1.8.) are the polysaccharide hydrolases responsible for the attack of the polymer backbone itself. The total hydrolysis or modification of heteroxylans requires in addition several different exo-glycosidases and esterases. The present knowledge of these enzymes is reviewed in this paper. 

Figure 11.4-6. (A) Action of xylanolytic enzymes on an hypothetical xylan structure. (B) Action of...

Xylans, the principle components of hardwood cellulose, are found to a lesser degree in softwoods. Two representatives of the xylans are glucu-ronoxylan, a major constituent of hardwoods, and arabinoglucuronoxylan, a lesser component of softwoods. Xylan structure is characterized by a backbone of D-xylose monomers linked by (1 —> 4) linkages. From this xylose backbone branch 4-O-methyl-a-D-glucopyranose and arabinose side chains, as well as acetyl groups. 

Ebringerova, A., Heinze, T, (2000). Xylan and xylan derivatives Biopolymers with valuable properties, 1. Naturally occurring xylans structures, procedures and properties. 

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. 

The reaction of beech wood xylan in N2O4/DMF with SO2 or SO3 yields sulfuric acid half esters of low DS (0.17-0.55) compared with the sulfation of cellulose due to the absence of reactive primary hydroxyl groups in the xylan structure (10). Moreover, algal cell wall microfibril homoxylan (P-l->3-xyIan) was sulfated with S03-pyridine under mild conditions and investigated with regard to the sulfation regioselectivity by means of different NMR techniques... 

---