Hydrolysis of methylated polysaccharides

The problems inherent in the hydrolysis of methylated polysaccharides are similar to those outlined, but, in addition, the possibility of occurrence of demethylation must be considered. This matter has been studied by Croon and coworkers,102 who found that hydrogen chloride, either in water or methanol, causes a significant amount of demethylation. Formolysis in 98% formic acid caused considerable degradation, whereas 90% formic acid or sulfuric acid gave acceptable results. The hydrolysis of a methylated dextran with 90% formic acid has been described in detail.103 The methanolysis of a methylated... 

A, Hydrolysis of naturally-occurring polysaccharide B, inethylation of aldobiouronio acid isolated after graded hydrolysis of polysaccharide C, hydrolysis of methylated polysaccharide D, methylation of partially methylated aldobiouronic acid isolated by method C. 

With the exception of 4-methylxylose all the methyl ethers that are derivable theoretically from D-xylopyranose or D-xylofuranose are known. From the products of hydrolysis of methylated polysaccharides 2-methyl-, 3-methyl-, 2,3-dimethyl-, 2,4-dimethyl-, 3,4-dimethyl- and 2,3,4-tri-methyl-D-xylose have been separated. No D-xylofuranose derivatives have been isolated from a natural source. Unknown at the present time are the 4-methyl-, 4,5-dimethyl-, 2,4,5- and 3,4,5-trimethyl- and... 

Since L-arabinose is a constituent unit of many natural products a number of methylated derivatives of this sugar have been isolated by the hydrolysis of methylated polysaccharides. Thus methylated peanut (Arachis hypogea) araban and sugar beet araban give trimethyl-L-arabofuranose, 2,3-dimethyl-L-arabinose and 2-methyl-L-arabinose in equal proportions.84 From methylated gum arabic trimethyl-L-arabo-furanose and 2,5-dimethyl-L-arabinose have been isolated.85 These same compounds have also been isolated from methylated cherry gum86 and methylated damson gum,87 whereas methylated mesquite gum88 affords 3,5-dimethyl- and 2,3,5-trimethyl-L-arabinose on hydrolysis. 

As mentioned in the introduction, trimethyl-L-arabofuranose is frequently found among the products of hydrolysis of methylated polysaccharides. The sugar was first isolated by the methylation of methyl L-arabofuranosides (a/3 mixture) with silver oxide and methyl iodide.46 Oxidation of the free sugar with nitric acid gave a dimethoxyhydroxy-glutaric acid. 

Of the methyl ethers of L-fucose, only the 3-methyl, 2,3-dimethyl and 2,3,4-trimethyl ethers have been isolated so far by the hydrolysis of methylated polysaccharides. 2-Methyl- and 3,4-dimethyl-L-fucose have been prepared synthetically. 

Several different methods are used for the hydrolysis of methylated polysaccharides. As these ethers are generally insoluble in hot water (and, sometimes, even in cold water), it is usually necessary to use a non-aqueous or only partially aqueous medium for the initial hydrolysis. Treatment with methanohc hydrogen chloride (methanolysis) and subsequent hydrolysis of the methyl glycosides formed is widely employed. Another method involves heating with concentrated or aqueous formic acid (formo-... 

The method of hydrolysis should, of course, be efficient enough to de-polymerize the polysaccharide to the monomeric state, but care should be taken to choose conditions that will give the least possible further degradation of the resultant monomers. Uronic acid residues appear in the hy-drolyzate mainly as methylated aldobiouronic acids, because of the great resistance of glycosiduronic linkages to hydrolysis. The hydrolysis of methylated polysaccharides is, under normal conditions, always accompanied by slight demethylation and, in some cases, this may make the results difficult to interpret. 

This compound was isolated after permethylation of the mixture resulting from the hydrolysis of methylated polysaccharides containing 2-amino-2-deoxy-D-glucose, ... 

Most early publications on bacterial polysaccharides were concerned with impure products and poorly-described organisms. Many more recent papers are of limited value also, due to low yields, lack of characterization of products and arbitrary interpretations of data. Low yields of methylated polysaccharides may be due to degradation of the bacterial polysaccharide during methylation, or to degradation of the hydrolytic products of the methylated polysaccharide (to form methyl levulinate, etc.46). The great importance of (a) complete methylation of polysaccharide products prior to structural determination by hydrolysis and (6) quantitative identification of the hydrolytic products, has been emphasized previously. Other difficulties in end group analysis have been discussed recently.7... 

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... 

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. 

In 1951, Whistler and Durso reported68 the isolation of epimelibiose by partial, acid hydrolysis of the polysaccharide guaran, followed by charcoal-column fractionation of the products. ThA material was obtained in anhydrous crystalline form from a mixture of methyl and butyl alcohols it had m.p. 201°, [a]26D + 120.9° — +124.6° in 36 hr. (c 2, in water). The yield was 2.2% of the weight of guaran used. Epimelibiose has also been produced72" in crystalline form following the ammonia-catalyzed alkaline isomerization of melibiose. 

Mild, acid hydrolysis of European-larch arabinogalactan results in the formation of 3-0-/3-L-arabinopyranosyl-L-arabinose (XXXIX). 26 Although this disaccharide has been isolated as an acid-reversion product from L-arab-inose,131 it is accompanied under these conditions by two other arabinose-containing disaccharides, neither of which was detected in the larch polysaccharide hydrolyzate. Since hydrolysis of methylated -galactan gave approximately equimolar proportions of 2,3,4-tri-O-methyl-L-arabinose and 2,5-di-O-methyl-n-arabinose, traces of 2,3,5-tri-O-methyl-L-arabinose, and no mono-O-methyl-L-arabinose, it is clear that the L-arabinose units must arise from an arabinogalactan rather than from an arabinan, and that the majority of these must be present in 3-0-/3-L-arabinopyranosyl-L-arabino-furanose side-chains (XLV) linked, in some way as yet unknown, to the framework of D-galactose units (XLI). 

Many polysaccharides contain L-rhamnose as a constituent, and various methylated L-rhamnoses have therefore been isolated by the hydrolysis of methylated gums and mucilages. For example, trimethyl-L-rhamnopyranose has been isolated from methylated gum arabic86 and from the methylated polysaccharide from the somatic portion of the cells of Mycobacterium tuberculosis,26 and in both these polysaccharides L-rham-nopyranose residues evidently occupy terminal positions in the molecule. 

This sugar has not been prepared synthetically. It is, however, a frequent constituent of the hydrolysis products of methylated polysaccharides containing galactose radicals, for example plant gums26 and the so-called galactogen of the albumin glands of the snail Helix pomatia.27,28... 

Hydrolysis Produces of Methylated Polysaccharide from CibbereUa fi fikuroP ... 

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