Linkages acetolysis

Definite proof that the interchain linkage is 6 —> 1-a-D has been obtained by Wolfrom and coworkers,91 who have isolated crystalline /3-isomaltose [6-0-(/3-D-glucopyranosyl)-D-glucose] octaacetate (in 1 % yield) from the mild acetolysis of waxy maize starch and have presented evidence that... 

Depolymerization of the permethylated carbohydrate is achieved by hydrolysis with acid. Under these conditions, the amino sugar residues are N-deacetylated, and the aminohexosidic linkages become resistant to hydrolysis. Stellner and coworkers29 showed that, when the acid degradation is conducted in 95% acetic acid, the amino sugar residues are also liberated, and can be analyzed by the methylation technique.29 Therefore, acetolysis followed by acid hydrolysis is now commonly used, as it allows the analysis both of hexose and hexosamine residues. 

Glycosidic linkages may also be cleaved in other solvents, such as methanol. In this case, water is rigorously excluded, so that the elements of a molecule of methanol are added across the glycosidic linkage to afford the methyl glycosides. Methanolysis is usually catalyzed by the addition of dry hydrogen chloride to the methanol. Other solvents, such as acetic anhydride-acetic acid (acetolysis) and formic acid (formolysis), are occasionally used for special purposes. 

Formolysis and acetolysis are not common methods for cleavage of glycosidic linkages. They do have some unique applications, however. For instance, methylated polysaccharides are not generally soluble in hot water, and consequently, hydrolysis is best preceded by formolysis under these circumstances. For example, 5 mg of methylated polysaccharide is dissolved in 3 mL of 90% formic acid, and the solution is kept for 2 h at 100°. The formic acid is removed by evaporation at 40°. The residue is dissolved in 1 mL of 250 mM sulfuric acid and the solution is heated for 12 h at 100°, cooled, the acid neutralized with barium carbonate, the... 

Acids hydrolyze (1 — 4)-linkages faster than (1 — 6)-linkages, whereas acetolysis cleaves (1 —> 6)-linkages faster than non-(l — 6)-linkages.M M ... 

The differences in the rates of hydrolysis of various linkage types by a particular glycosidase can be used to provide information about this aspect of structure. Jack-bean a-D-mannosidase cleaves a-(l- 2) and a-(l- 6) linkages much faster than a-(l -> 3). Oligosaccharides, obtained by endo-N-acetyl-/J-D-glucosaminidase hydrolysis of ovalbumin, were subjected to acetolysis, which selectively cleaved the a-(l - 6) bonds. A tetrasaccharide isolated after this treatment was then incubated with jack-bean o-d-... 

One difficulty we encountered in the approach described above was liberation of the 3-carbon fragment from the auxiliary. The glycosidic linkage resisted acid hydrolysis and a rather severe acetolysis followed by basic peroxide oxidation to degrade the carbohydrate moiety had to be performed. A second approach to the use of glucose as an auxiliary is illustrated in scheme 11. 

A branched mannan extracted from the cells of the yeast Tricho-sporon aculeatum also contains a-D-(l- 2)- and a-D-(l— 6)-link-ages (see Table III). Partial acetolysis gives rise to a (I— 2)-linked mannohexaose this represents a maximum of five consecutive a-D-(I- 2)-linkages in the mannan. 

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