UBC Theses and Dissertations
Studies on the determination of structural characteristics of three types of hemicelluloses Joseleau, Jean-Paul
Part I of this thesis describes the isolation and purification of the principal hemicellulose from the reed Arundo donax, Monocotyledon/ Gramineae. By use of the classical techniques, a structural investigation demonstrated that this hemicellulose is an arabino-4-0-methy1-glucuronoxylan with on average 10 arabino-furanose residues and 5 4-O-methyl-glucopyranuronic acid residues occurring as branch points with respectively α(l→3) and α (l→2) linkages on the β(1→4) linked backbone of xylopyranosyl residues. The results of periodate oxidation, methylation and osmometry agreed well in demonstrating an average degree of polymerization of approximately 80. Enzymic hydrolysis of the polysaccharide with an endoxylanase of known specificity for β (l→4) xylosidic bonds showed that the anomeric nature of the linkages between the xylose units was of the β configuration. P.m.r. spectroscopy of the permethylated polysaccharide confirmed the above result by showing a strong doublet at p.p.m. 4.24 δ(J 6.5 Hz) corresponding to a β linkage, and a peak at p.p.m. 5.53 β corresponding to α anomers. The structure of the polysaccharide was deduced to be of the following form : [diagram omitted]. In Part II, the isolation of the hemicelluloses from the wood of Redwood, Sequoia sempervirens, has been carried out by the use of different methods. Timell's method using the solubility differences of the hemicelluloses between potassium hydroxide and sodium hydroxide allowed the isolation of a glucomannan which was easily purified by barium hydroxide complexation. For the separation of xylan from galactoglucomannan, a delignification of the polysaccharide mixture was necessary. Purification of the acidic xylan was achieved by complexation with cetyltrimethylammonium salt,also allowing the recovery of a galactoglucomannan which was further purified by selective precipitations with barium hydroxide. The structure of the xylan was determined with emphasis on the methylation procedure. It is demonstrated that repeated Hakomori methylations on an acidic xylan can be performed without degradation of the main chain. The structural features of the xylan consist of a xylosyl backbone with an average degree of poly merization of about 55 and carrying on average one 4-0_-methyl glucuronic acid residue for four xylose residues and one arabinofuranooe residue every 20 xylose residues. A study of the localization of the uro nic acid substituents was carried out with the use of a specific xyla-nase and was shown not to be randomly distributed on the xylosyl main chain. The structures of the glucomannan and galactoglucomannan are also described. Both present the customary structure taht is a β(1→4) linked glucomannan with, in the latter case, some side chain galactose units (1→6) linked to the main chain. [diagram omitted] The results are compared to the data reported in similar hemicelluloses from coniferous woods and the literature for show that in the present wood the ratio of mannose to glucose is unusually high. The validity of the methods of isolation of these polysaccharides is discussed and it is demonstrated that there is a great heterogeneity concerning the composition of the galactoglucomannans present in this wood. In Part III, the cell wall carbohydrate composition of a one-month-old stem from Arundo donax was established for tissues at different stages of maturity. There is a relationship between the increase in xylose and glucose content, the decrease in arabinose and galactose content, together with the elongation of the fibres. Similar changes were also found at the level of one elongating internode. These variations are interpreted in terms of polysaccharide deposition in the cell wall of the fibres with maturatioa. Each internode appears as a physiological unit where the progressive fibrous character of the tissues corresponds to a characteristic cell wall polysaccharide composition. The isolation and purification of the arabino-4-0- methyl-glucuronoxylans, by stepwise alkaline extraction from tissues at three different stages of maturity, revealed that the main change in the hemi-cellulose's structure according to growth is in the increase of the degree od polymerization. As the plant matures the average chain length of the xylan increases from about 60 in the youngest internodes to about 150 in the older ones. The structural features of the xylan which is already present in the youngest tissues are the same regardless of the age of the tissues. More subtle differences, such as the modification of the substituents occurring on the xylan backbone, were found in the acetyl distribution which increases, and in the presence of 4-0-methyl substituents on the glucuronic acids which decreases as the plant ages. In all cases examined, the xylose and xylan content of the tissues reaches a maximum value in the intermediate stage of the maturation and then decreases. The isolation of an exo-xylanase in the corresponding tissues suggests that xylans are not only structural polysaccharides but also can act as reserve elements.
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