The association of xylan with cellulose in certain structural celluloses

Abstract

The cellulose of most plants and woods differs from that of cotton in containing associated polysaccharides (cellulosans) which are tenaciously retained and which must be considered as an integral part of the cellulose aggregate. Heat-drying produces some change in the properties of both components of such a cellulose, which is manifest in an increased availability to extracting and hydrolysing agents. The effect of heat-treatment may be observed repeatedly on the same sample and must involve breakage of the cellulose chains, though the xylan fraction is affected to a much greater extent. Preparations of the water-soluble material produced as a result of heat-treatment are mixtures which can be partially separated to give a portion of higher xylan content. Some oxidation undoubtedly occurs, and uronic groupings are present. The xylan may be removed from celluloses by treatment with either acid or alkali, but a concurrent loss of hexosan material takes place in all cases. In acid hydrolysis there is no apparent break in the continuity of the reaction. The reducing value of dilute acid extracts indicates that the material removed is not completely hydrolysed to reducing sugars. Continued boiling with alkali removes hexosan at a greater rate than xylan, and in effecting the same total loss a higher concn. of alkali in a short period extracts more xylan than a lower concn. for a longer period. Plant celluloses show considerable differences of behavior towards hydrolytic and extracting agents, and reveal distinct individualities. By solution and re-precipitation of cellulose, the organized molecular structure may be destroyed, with the result that the xylan, which was initially extracted only to a small extent by water and dilute alkali, becomes almost completely soluble. No equivalent change in the properties of the hexosan material occurs. These observations are in accord with the view that the cellulosan fraction of the cellulosic aggregate of plant materials and woods is oriented, and participates in the micellae, being retained by secondary valency forces identical with those which obtain between parallel cellulose chains in pure cotton cellulose.