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Some structural features of sapota achras gum Kabir, Mohammad Shahjahan


Sapote gum polysaccharide was examined for homogeneity by ion-exchange chromatography on diethylaminoethyl (DEAE)-cellulose and by fractional precipitation of its propionate derivative. The polysaccharide showed essential homogeneity and was therefore suitable for structural analysis. The polysaccharide showed a low negative specific rotation and contained residues of L-arabinose, D-xylose, D-glucuronic and 4-O-methyl-D-glucuronic acids. A new method was developed for the simultaneous estimation of 4-O-methyl-D-glucuronic acid and other uronic acids by gas-liquid chromatography. Uronic acids in polysaccharides may be reduced by reaction with lithium borohydride in tetrahydrofuran. After hydrolysis and reduction of the monosaccharides the acetylated alditols may be separated by gas-liquid chromatography on a column of butanediol succinate. The method permits the simultaneous estimation of 4-O-methyl-D-glucuronic acid, D-glucuronic acid and D-galacturonic acids. It was found by this method that L-arabinose, D-xylose, D-glucuronic and 4-O-methyl-D-glucuronic acids were present in the ratio of 1:2.8:0.48:0.52. An examination of the O-methyl-derivative of the gum yielded 2,3,5- (+++) and 2,3,4- (+)- tri-O-methyl-L-arabinose, 2,3,4- (+)- tri-O-, 2,3- (+)- di-O-, 3-0- (+++)-methyl-D-xylose, 3,4- (++)- di-O-methyl-D-glucuronic and 2,3,4— (++)- tri-O-methyl-D-glucuronic acid. The methylation data indicates the high degree of branching in the polysaccharide. The presence of 3,4-di-O-methyl-D-glucuronic acid also indicates that a part of the total uronic acids is branched at C-2. Autohydrolysis of the gum yielded a series of oligouronic acids and a lesser branched polysaccharide. The polysaccharide after methylation, reduction and hydrolysis afforded 2,3,4-tri-O-methyl, 2,3-di-0-methyl, 3-O-methyl-D-xylose and 2,3,4~tri-O-methyl-D-glucose in a molar ratio of 2: 1.3: 1: 2 indicating that the degraded polysaccharide has a xylose backbone. Sapote gum polysaccharide after successive periodate oxidation, borohydride reduction and mild acid hydrolysis yielded a series of homologous glycosides, (xylosyl) n=1,2,3 glycerol, which were fully identified by methylation and periodate oxidation. The occurrence of these glycosides signifies a random branching in the polysaccharide. Periodate oxidation and subsequent complete acid hydrolysis of the carboxyl reduced polysaccharide afforded ethylene glycol, glycerol, 2-O-methyl-D-erythritol, D-xylose and 4-O-methyl-D-glucose whose molar ratios were determined by gas-liquid chromatography (g.l.c.). The presence of 4-O-methyl-D-glucose indicates that a major part.of 4-O- methyl-D-glucuronic acid is branched at C-2 (a conclusion supported by the methylation data) and 2-O-methyl-D-erythritol arises from a minor part of 4-O-methyl-D-glucuronic acid with no substituent at C-2. Partial acetolysis of the carboxyl-reduced polysaccharide afforded a new series of oligosaccharides [formula omitted] and [formula omitted]. The results confirm the presence of the oligouronic acids in the polysaccharide. A new method for the selective cleavage of glycosiduronic acids, Hofmann. degradation, was applied to sapote gum in order to study the resulting degraded polysaccharide and to find out the nature of the substituent on 4-O-methyl-D-glucuronic acid. The degraded polysaccharide after methylation and hydrolysis afforded 2,3,4-tri-O-methyl-, 2,3-di-O-methyl-, and mono-O-methyl-D-xylose in a molar ratio of 3: 11: 2. An oligosaccharide originally attached at C-2 of 4-O-methyl-D-glucuronic acid was isolated. Digestion of the polysaccharide with arabinofuranosidase yielded free arabinose indicating that a major part of the terminal non-reducing arabinose units are in the furanose form and linked with xylose in [symbol omitted] configuration. Some enzymes with known ‘xylanase’ activity did not degrade the gum because of the high degree of branching of the polysaccharide. The structural evidence suggests that sapote gum polysaccharide possesses a highly branched xylan framework to which are attached L-arabinose units, D-glucuronic acid and 4-O-methyl-D-glucuronic acid containing side chains.

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