Xylan-containing

Unlike the xylans already discussed, Rhodymenia palmata /3-D-xylan contains mixed (l- 3) and (l- 4) linkages. On reaction with a cellulase that had /S-D-xylanase activity,74 a series of (l- 4) and mixed-linkage xylosaccharides was produced that included (1 - 4)-0-xylo-biose, -triose, -tetraose, and a mixed-linkage trisaccharide, namely, 32-0-/3-D-xylosyl-xylobiose (17). endo-(l - 4)-/3-D-Xylanase has been shown62 to catalyze... 

Particulate preparations from com shoots readily incorporate 14C-labeled D-xylose from UDP-D-xylose-14C into a polysaccharide in which the D-xylose residues are combined by / -l,4-D-xylosyl bonds (31). It was shown that this polysaccharide, similar to natural plant xylan, contains a small proportion of L-arabinose units which have the furanose configuration. 

XII or XIII, it is reasonable to assume that the xylan backbone is unbranched. In the case, however, of xylans containing L-arabinofuranose residues, it is much more difficult to obtain direct evidence for the order of linkage of sugar residues. It is doubtful if it would be possible by present methods to detect a small proportion of L-arabinofuranose residues terminating main xylan chains (as in XIV) if these residues were also attached as side-chains (I), as commonly found. 

In the foregoing brief discussion, the chemistry of a few, carefully purified preparations of xylan from various sources has been considered. In general, these preparations have been found to have a high xylan content and to contain only small amounts of other carbohydrate units. Through studies of these substances, it has been established that xylans contain predominantly 4 1 /3-n-glycosidic bonds, with little branching. 

The xylan of hardwoods (O-acetyl-4-O-methylglucuronoxylan) consists of at least 70 p-xylopyranose residues (average degree of polymerization between 150 and 200) linked by P-l,4-glycosidic bonds (Fig. 11.4-4) 139. Every tenth xylose residue carries a 4-O-methylglucuronic acid attached to the C-2 of xylose 131. In addition, hardwood xylans are highly acetylated e. g. birchwood xylan contains more than 1 mol of acetic acid per 2 mols of xylose 140. Acetylation occurs usually at the C-3 rather than the C-2 position of xylose. Acetylation at both positions has also been reported 141, 142. The presence of these acetyl groups is responsible for the partial solubility of xylan in water 133. The alkali extraction of xylan leads to the deacetylation of this substrate 140. ... 

A chlorite holocellulose from white-birch wood, on extraction with methyl sulfoxide, gave a xylan containing 5.3 0-acetyl groups per ten xylose residues. In this case, the yield was only 5% of the original wood. The hemi-cellulose contained 58.1% of unsubstituted, 11.8% of 2-0-substituted, 24.0% of 3-0-substituted, and 6.1% of 2,3-di-O-substituted xylose residues. The 0-acetyl groups in native hardwood xylans are thus distributed between C-2 and C-3 of the xylose residues, with the majority located at C-3. It should, perhaps, be pointed out that, so far, only this species has been investigated, and moreover, only a minor part (15%) of the xylan portion of this wood has been studied. 

A relationship has been developed for calculating the specific rotations of xylans containing both 4-0-methyl-a-D-glucuronic and L-arabinofuranose side-chains, such as those occurring in softwood xylans. In a comparison with arabino-(4-0-methylglucurono)xylans from six species, the calculated and found values agreed weU for three. In two of the other three xylans, the polysaccharides were of doubtful purity. 

Thomber and Northcote have studied the changes in the chemical composition accompanying the differentiation of a cambial cell into xylem and phloem. In phloem from sycamore maple ( Acer pseudoplatanus), the xylan contains twice as many acid side-chains as the xylan in the xylem, whereas exactly the opposite occurs with Scots pine. The glucomannan first deposited in the phloem contains more D-mannose residues than that laid down in the xylem. 

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