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The constitution of the hemicellulose of apple wood

University of British Columbia

The hemicellulose isolated from apple wood (var. Golden Transparent) by alkaline extraction has been shown to contain a 4-0-methylglucuronoxylan. Hydrolysis of the hemicellulose yielded neutral sugars and uronic acids. Paper chromatographic examination of the neutral sugars showed D-xylose to be the major component with small amounts of other sugars corresponding to rhamnose, arabinose and galactose also being present. Two other sugars with Rf values greater than that of xylose were found but the identity of these components has not been established. An aldobiouronic acid was isolated and characterized as the crystalline acetate of 2-0-(4-0-methyl-⍺-D-glucopyranosyluronic acid)-D-xylose. In order to determine the mode of union of the component sugars the polysaccharide was methylated and then hydrolyzed to give 2,3,4-tri-O-methyl-D-xylose, 2,3 di-O-methy1-D-xylose, 2-0-(2,3-di-O-methyl-D-xylopyranosyl)-2,3-di-O-methyl-D-xylose, a dimethylated lyxose, 2-0- and 3-0-methyl-D-xylose, 2-0-(2,3,4-tri-O-methyl-D-glucopyranosyluronic acid)-D-xylose, and probably a trimethylated rhamnose. (The dimethylated lyxose isolated is not an integral constituent of the native polysaccharide since no lyxose was obtained in the acid hydrolyzate. It is thought to arise by epimerization of 2,3-di-0-methyl-D-xylose.) Quantitative analysis of the methylated hemicellulose has shown the tri-, di- and monomethyl pentose and 2,3,4-tri-O-methyl-D- ⍺- glucuronic acid to be present in the mole ratios of 1:97:21:19, respectively. The methyl ester of the methylated aldobiouronic acid was reduced with lithium aluminum hydride and the resulting neutral disaccharide hydrolyzed. The cleavage products were identified as 3-0-methyl-D-xylose and 2,3,4-tri-0-methyl -D-glucose indicating the uronic acid portion of the molecule to be linked through its reducing end to position 2 of a xylose moiety. Results obtained in this work show that the carbohydrate polymer isolated from apple wood consists of a backbone of approximately 119 anhydro-D-xylopyranose linked by 1,4-β-glycosidic bonds. The side chains are composed of single units of 4-0-methyl-D-glucuronic acid which occur at every sixth xylose residue. A rhamnose unit may perhaps be present as a side chain too, although it is not known whether this sugar is an integral constituent of the glucuronoxylan. The general features of the hemicellulose are very similar to glucuronoxylans isolated from other hardwoods and especially resemble white elm and cherry wood in its structure and high uronic acid content. We are grateful to Dr. C. T. Bishop who presented this work at the 43rd conference of The Chemical Institute of Canada, Ottawa, June I960.

Text

2012-02-09

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http://hdl.handle.net/2429/40613

Chemistry

University of British Columbia

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