Methylation glycolipids

Several methylated sugars have been identified in hydrolyzates of LPS, cell-wall polysaccharides, and extracellular polysaccharides. A considerable number of these have been found in the LPS from photosynthetic prokaryotes. Two polysaccharides from Mycobacterium species, a glucan" and a mannan" are remarkable in that they contain high percentages of methylated sugars. Glycolipids from Mycobacterium species are also rich in methylated sugars, some of which have not been found elsewhere, but this is beyond the scope of the present article. 

In about 1936, sialic acid was discovered by Blix, who found it to be a component of submaxillary-gland proteins, and who described many of its properties. However, little notice was taken of this work at the time it was published. In 1941, Klenk, who was working on glycolipids of the brain, described a compound, later shown to be a methyl glycoside of sialic acid, that had been obtained by treatment of a lipid fraction with 5% methanolic hydrogen chloride at 105°. In 1954, Klenk and Faillard reported the first isolation of pure N-acetyl-neuraminic acid from animal sources. 

METHYLATION TECHNIQUES IN THE STRUCTURAL ANALYSIS OF GLYCOPROTEINS AND GLYCOLIPIDS... 

Methylation analysis (and its different applications) is one of the most widely used methods in the structural characterization of carbohydrate chains (reviewed in refs, 5-8). In the present article, the progress achieved, and the problems encountered, in the analysis of animal glycolipids and glycoproteins are discussed, although many of the topics also concern the analysis of carbohydrate chains in general. Emphasis is laid on the efforts made to find ways to lessen the amounts of sample needed for the analysis, because only small amounts of glycan samples isolated from biological sources are often available. 

Methanolysis of polysaccharides as an alternative to hydrolysis has been discussed in Section II (see p. 14). In brief, this method of depolymerization may be more convenient with glycolipids, because the fatty acids are simultaneously obtained as their methyl esters, and aminodeoxyhexoside bonds are more readily methanolyzed than hydrolyzed. An additional advantage is the increased stability of... 

The occurrence of 3-deoxy-D-manno-octulosonic acid in lipopoly-saccharides has prompted its synthesis,537 together with the D-gal-octo158 and the D-gluco analogs.157 Although gas-liquid chromatography was successfully used to analyze the products of these syntheses, it has been reported that methanolysis of a bacterial endotoxin lipopolysaccharide failed to yield 3-deoxy-D-manno-octulosonic acid, presumably because of the lability of the latter to acid.381 However, Kasai and Nowotny have reported four peaks for the O-trimethylsilyl derivatives of 3-deoxy-D-manno-octulosonic acid obtained by methanolysis of the glycolipid from a Salmonella minnesota mutant.538 Reduced 3-deoxy-D-manno-octulosonic acid and its methyl ester have also been analyzed successfully as their acetates.339,539... 

As a rule, qualitative and quantitative determinations of fatty acids are performed by g.l.c. of their methyl esters, obtained by treating the glycolipid with HC1 in methanol.160 To raise the volatility of the methyl esters of a-hydroxy acids, they are methylated161 or trimethylsilylated.162... 

---