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The hemicelluloses of Douglas fir Abdurahman, Nizam

Abstract

The Douglas fir holocellulose was prepared by Wise’s(8) modification of Jayme's sodium chlorite procedure(9). The hemicelluloses were extracted using 24% potassium hydroxide followed by 17% sodium hydroxide containing 5% boric acid. Meier(31) reported that barium hydroxide can be used to purify polysaccharide mixtures. The mode of action presumably being the formation of an insoluble complex with the cis-hydroxyl groups of the mannose or galactose integers. By using this method of purification Timell(30) isolated a galactoglucomannan from a 24% potassium hydroxide extract of eastern hemlock holocellulose. Following Timell's scheme the attempt to isolate a galactoglucomannan from Douglas fir did not proceed with the facility suggested. Two purifications by Meier's procedure failed to remove the xylose containing polysaccharide. Further batches of freshly prepared holocellulose were extracted and repeated attempts at the isolation and purification of a galactoglucomannan are at present being pursued. The addition of barium hydroxide not only removed the galactoglucomannan fraction but also purified the xylan which remained in solution. Four barium hydroxide treatments followed by two purifications by means of Fehling's solution gave an arabino-4-0-methyl-D-glucuronoxylan freed from galactose. Previous attempts at the purification of this type of polysaccharide from softwoods have failed to remove the galactose residues. A glucomannan fraction was extracted using 17.5% sodium hydroxide containing 5% boric acid. Four fractionations by the barium hydroxide procedure failed to remove the xylose residues. Two further fractionations via the copper complex removed the last traces of xylose but the fraction still contained about 8% galactose. This is not unusual for it has been reported that the majority of gymnosperm glucomannans contain at least 4% galactose. The glucose to mannose ratio is approximately 1:3.5 which is in the order of a true glucomannan. It appears that the possibility should be seriously considered that the galactose residues present in the so called "glucomannans" are actually integral parts of these polysaccharides. Additional experimental data in the near future will undoubtedly serve to solve the problem.

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