Xylan acetylation

Scientific name/Common name Glucan Xylan Galactan Arabinan Man nan Uronic anhydride Acetyl Lignin Ash... 

Note. The name ending in -an refers to the unsubstituted polysaccharide. Thus xylan occurs in nature in unacetylated and partially acetylated forms. Xylan designates the unacetylated material, and xylan acetate an acetylated derivative. 

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). 

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

This enzyme was shown to be specific for xylan oligomers and small acetylated synthetic substrates. Many characteristics have been published recently about this type of enzyme, purified from Trichoderma reesei, and A. oryzae [6], and a different A. n/ger preparation[7]... 

Acetyl values from fully acetylated xylan preparations indicate a D. P. of about 100 or greater. The theoretical acetyl value for a fully acetylated xylan molecule of infinite size is 39.8%. As the molecule decreases in length, its acetyl content increases because of the increasing proportion of triacetyl-bearing end units. Assuming an accuracy of 0.1% for the acetyl determination, a noticeable deviation from theory would be apparent in acetylated xylan of less than D. P. 100. 

In contrast to alkaline pulping, the acetyl groups of the hemicellu-loses are relatively stable at low pH, as are the glucuronic acid residues of the xylans. The main effect of high temperature and low pH is the hydrolysis of the glycosidic linkages in the polysaccharide... 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

En me Treatment of LCCs. Purified Bacillus circulans xylanase (specific activity, 110 units/mg protein), purified Coriolus versicolor )9-mannanase (specific activity 179 units/mg protein), purified Thermoascus aurianticum /8-glucosidase (specific activity, 150 units/mg protein), and purified Schizophyllum commune acetyl xylan esterase (AXE specific activity, 180 units/mg protein) were used. Unit activity for xylanase and )9-mannanase was defined as the amount of enzyme catalyzing the release of 1 /xmol reducing power per minute at 50°C and pH 6.0 using oat spelts... 

Xylans are heteropolysaccharides which are depolymerized by j8-l,4-D-endoxylanases. Due to the abundance and variety of substituents in native xylans, different accessory enzymes are also needed for the total hydrolysis of xylan. The knowledge of a-glucuronidase, a-arabinosidase, and acetyl xylan- and feruloyl esterases has increased considerably in recent years. In addition to acting in synergism with endoxylanases and )8-xylosidase for the complete hydrolysis of xylan, some of these accessory enzymes are also capable of changing the structure of polymeric xylans. 

Hardwood xylans and xylans of annual plants may contain up to 7% O-bound acetyl groups. Seven out of ten xylose residues in native hardwood xylan are acetylated on C-2 and/or C-3 (10). Because of the possible migration of O-acetyl groups between 2- and 3-positions during and after isolation of hemicellulose components, it is difficult to determine their original distribution in nature (11). The ratios reported for 2-, 3-, and 2,3-positions of acetyl groups in birch xylan have been 2 4 1 (3) and 2 2 1 (10) and in bracatinga xylan 3 3 1 (12). 

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