The Constitution of a Methyl Glucuronoxylan From Kapok (Ceiba Pentandra)

The seed hairs of kapok (Ceibapentandra) on alkaline extraction have yielded a hemicellulose composed of xylose and uronic acid residues. Partial hydrolysis of the polysaccharide gave 2-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-xylopyranose, 4-O-methyl-D-glucuronic acid, and, probably, galacturonic acid. Hydrolysis of the fully methylated hemicellulose yielded a mixture of 2-O- and 3-O-methyl-D-xylose, 2,3-di-O-methyl-D-xylose, 2,3,4-tri-O-methyl-D-xylose, and 2-O-(2,3,4-tri-O-methyl-α-D-glucopyranosyluronic acid)-3-O-methyl-D-xylopyranose in a molar ratio of 1.1:38:1:6. The number-average D.P. of the native and the methylated polysaccharides was 177 and 124, respectively. On the basis of this and other evidence it is suggested that the average hemicellulose molecule contains approximately 180 1,4-linked β-D-xylopyranose residues, one eighth of which carry a single terminal side chain of 4-O-methyl-D-glucuronic acid, attached through an α-glycosidic bond to C 2 of the xylose. The xylan framework contains, on the average, slightly less than two branching points per macromolecule, most of them probably originating from C 3 . The number of acid side chains and branches is twice as large as that of the otherwise similar 4-O-methyl glucuronoxylan present in the seed hairs of milkweed floss.