O-acetyl groups

Rha, Ara and Gal are the neutral sugar components from all the fractions. Xyl is not present in Fla and is significantly present in the hemicellulose fractions, indicating that this monosaccharide is component of hemicellulosic polymers. Chemical composition of the water fractions were determined (Table V). High protein contents and the presence of O-acetyl-groups were observed in four aqueous fractions. Neutral sugar and uronic acid composition points to inclusion of these polymers in the class of pectic polysaccharides. 

There is strong cross-reaction between S11A and S18. The partial structures proposed for S18 (Refs. 1 and 38), 17 and 18, with glycerol phosphate and the O-acetyl group linked to positions not yet determined, are also remarkably similar to that of S11A. 

Baddiley and coworkers42 have studied the structure of S13, which is composed of D-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose, and ribitol residues, and phosphate groups in the molar proportions 2 1 1 1 1. O-Acetyl groups are also present. A neutral pentasaccharide was obtained by hydrolysis with alkali, followed by enzymic dephosphorylation. On mild, acid hydrolysis, this yielded two main products, a trisaccharide (19) and a disaccharide (20), the structures of which were determined by conventional methods. 

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. 

O-acetyl groups are readily removed when the heteroxylans are extracted with alkali (saponified), and the isolated product can therefore be misleadingly low in O-acetyl content. Other chemical groups may also be linked to the heteroxylans. There is now evidence to suggest that lignin is covalently attached to heteroxylans and other plant cell wall polysaccharides... 

Replacement of an O-acetyl group by an O-trifluoroacetyl group greatly increases the volatility of the sugar derivative, and this behavior was first utilized by Vilkas and associates,224 who studied the separation of pentoses, hexoses, aminoglycoses, methyl glycosides,... 

A lipophilic gel, namely, Sephadex LH-20, has found application in studies of partially acetylated dextrans. In describing a modified procedure for the replacement of the O-acetyl groups in such dextrans by O-methyl groups prior to acid hydrolysis and identification of the fragments (which indicates the distribution of the substituents in... 

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