Structural analysis methylation techniques

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

The analysis of monosaccharide mixtures as the permethylated derivatives was proposed early in the application of gas-liquid chromatography to carbohydrates, but the method has now been superseded by more convenient procedures.230,231 There are, however, situations in which this method is useful, such as during a structural study of a polysaccharide by the methylation technique. The mixture of partially methylated monosaccharides obtained by methanolysis may then be fully methylated, and the proportions of the various monosaccharides determined. This approach has been used, for example, in studies on a galactomannan392 and on tamarind-kernel polysaccharide.393 Such an analysis also constitutes a useful check to ensure that no significant change in the composition of the polysaccharide occurred during methylation. 

However, two other applications of mass spectrometry to carbohydrate chemistry seem the most important and promising. These are the identification of partially methylated monosaccharides, and structural analysis of di- and oligo-saccharides (and penetration of this new technique into polysaccharide chemistry). 

Mass spectrometry provides a rapid, sensitive method for the structural analysis of oligosaccharides and, especially in combination with g.l.c., offers a valuable technique for the analysis of oligosaccharides from natural sources and from polysaccharide hydrolyzates. Methyl ethers " or trimethylsilyl ethers are suitable derivatives for the g.l.c. separation of oligosaccharides. 

Rauvala, H., Finne, J., Krusius, T., Karkkainen, J. and Jamefelt, J. (1981). Methylation Techniques in the Structural Analysis of Glycoproteins and Glycolipids . Advances in Carbohydrate Chemistry and Biochemistry, 38, 389-416. 

Trapping can involve either nitroxides followed by. separation and characterization or tlie use of nitroso compounds and subsequent structural analysis by ESR. As an example of the former, the trapping of the radicals from the reaction of t-butoxy radicals and methyl methacrylate (MMA) by l,l,3,3-tetramethylisoindolinyl-2-oxy (1) is shown (Scheme 1). Alkyoxyamines were isolated by conventional techniques and their pathways deduced. The methyl radical, formed by P-scission of the t-butoxy radical, is trapped as the methoxyamine, which in turn can add a further monomer unit in a thermally activated step growth addition to form (2). As an example of the latter, the radicals from the same reaction are now trapped by 2-methyl-2-nitrosopropane as tlie corresponding nitroxyl radicals. 

In 1983 Holker, Simpson, and O Brien reported the isolation of the diversonolic esters (972, 973) (Fig. 13.15), which they obtained from Penicillium diversum, along with lichexanthone (840a) and several other known compounds (634). The authors performed a structural analysis for these new compounds based on methyl-ation, proton-NMR analysis, chelate ferric effects, and other spectroscopic techniques, which led to the assignment of the structures of these new compounds as 970 and 971. These substances appear reminiscent of hemisecalOTiic acids (ergochrome monomers), but the structures originally proposed were later found to be incorrect (see below). 

American Viscose was the dominant rayon company at the time and well-puriBed dissolving grade wood cellulose was constantly improved thanks to fundamental research on methods of carbohydrate analysis. Purves had originally studied at St. Andrews University where Haworth, Hirst, Purdy and Irvine pioneered the methylation techniques which were essential for chemical structure determination. 

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