Acetyl group xylan

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. 

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... 

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). 

In addition to enzymes hydrolyzing the glycosidic linkages in xylans, the requirement of esterases to remove esterified acids from xylans has recently been discovered. Acetyl xylan esterases remove O-acetyl groups from the C-2 and C-3 positions of xylose residues in both xylan and g looligomers. Feruioyl esterases liberate ferulic acid from arabinoxylans of monocotyledons. 

Woods contain a number of acyl (mainly acetyl) groups, and it is probable that these are associated with the xylan components of the hemicellulose fraction. Evidence for the original esterification of aspen-wood xylan comes from the observation that the isolated xylan (obtained by alkaline extraction) is readily cleaved by periodate, whereas only a small proportion of the D-xylose residues in the wood itself are oxidized under similar conditions.66 Hemicelluloses still containing acyl groups may be extracted from wood holocelluloses by means of dimethyl sulfoxide, and further quantities... 

Feedstock material and processed solids were analyzed for glucan, xylan, arabinan, and acetyl groups by quantitative acid hydrolysis according to standard methods (25). The acid-insoluble residue was considered as Klason lignin, after correction for the acid-insoluble ash (determined by igniting the contents at 575°C for 5 h). 

Depending on the hardwood species, the xylan content varies within the limits of 15-30% of the dry wood. As can be seen from Fig. 3-15, the backbone consists of /3-D-xylopyranose units, linked by (1 —> 4)-bonds. Most of the xylose residues contain an acetyl group at C-2 or C-3 (about seven... 

Aliphatic acetate (including acetyl groups in xylan)... 

The acetyl groups in hardwood xylans have been found to exhibit a remarkable stability under the relatively drastic conditions of acid sulfite cooking [258]. In steam treatments of birch wood, the acetyl group was more stable than the 4-0-methyl glucuronosyl unit [259]. 

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