Polysaccharides partial acid hydrolysis studies

Structural studies on the oligosaccharide derivatives obtained by partial, acid hydrolysis of fully methylated polysaccharides often furnish valuable information on the positions at which the oligosaccharides were linked in the original polysaccharide. When the folly methylated Klebsiella O group 9 lipopolysaccharide,27 which contains D-galactopyranose and D-galactoforanose residues, was subjected to mild, acid hydrolysis, and the product reduced with lithium aluminum deuteride and remethylated with trideuteriomethyl iodide, a good yield of the disaccharide derivative 10 was obtained. 

Silylation reactions on polysaccharides with chlorosilanes and silazanes were attempted more than 50 years ago resulting in hydrophobic silyl ethers with both increased thermal stability and solubility in organic solvents [376]. The silylation reaction for the protection of hydroxyl groups in mono- and polysaccharides exhibits many advantages, e.g. fast silylation, solubility of silylether in organic solvents suitable for subsequent derivatisation, stability of the resulting silylether under basic conditions but easy deprotection of the silyl moieties by acid hydrolysis or nucleophilic agents like fluoride and cyanide ions [377]. The partial and complete silylation of dextran was studied in detail by Ydens and Nouvel [215-217]. 

The occurrence in many polyglycosiduronic acids of relatively resistant linkages, usually those between uronic acid residues and adjacent residues, has resulted in the isolation of several aldobiouronic acids on graded hydrolysis of acidic polysaccharides. The isolation of such aldobiouronic acids, and subsequent conversion to their fully methylated derivatives, has become a standard procedure in structural studies on polyglycosiduronic acids, especially those of plant gums and mucilages.1 In other cases, partially methylated aldobiouronic acids have been isolated from the hydrolyzates of methylated polysaccharides. The sources and methods of isolation of the methyl ethers of aldobiouronic acids so far examined are given in Table I. Some properties of derivatives are recorded in Table Y. 

The structures of the disaccharides and higher saccharides (obtained by partial hydrolysis of polysaccharides) can be examined by periodate or lead tetraacetate oxidation. Ahlborg studied the periodate oxidation of disaccharides and starch dextrins in 0.5 N sulfuric acid. Since the intermediate formate esters were hydrolyzed, these conditions were favorable for overoxidation. The (1 — 4)- and (1 —> 6)-linkages in the starch dextrins were each found to be associated with a characteristic consumption of oxidant. Neumtiller and Vasseur studied periodate oxidation in solutions of various acidities, from strongly acid to neutral, and showed that there were differences between different types of disaccharides. 

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