Trimethylsilyl ethers monosaccharides

The separation of disaccharides and higher oligomers is not essentially different from the separation of monosaccharides, except that the volatility rapidly decreases with increasing molecular weight. Oligosaccharides may be transformed into volatile derivatives, commonly the trimethylsilyl ethers, either directly or after reduction. Other derivatives, such as trifluoroacetates, have been used, but the acetates have low volatility. Oligosaccharides are not usually converted into their methyl glycosides prior to trimethylsilylation. Detailed examples are listed in Table VII (see p. 130). 

The use of gas chromatography requires volatile compounds, of course, and derivatives must be made of the hydrolyzed monosaccharides. Many investigators have used methylation or acetylation to produce these derivatives (10,16). However, both of these methods require more steps than are desirable with a small sample. In addition, the reaction time required for quantitative conversion to the derivative may be a problem. The use of trimethylsilyl ethers makes the derivatization exceedingly simple and relatively fast. Formation of these derivatives in the absence of water has been shown to occur without anomerization and to proceed quantitatively (30,31). The reaction mixture is chromatographed directly (Figure 9). 

Figure 9. Tracing of an actual chromatograph of the trimethylsilyl ethers of those monosaccharides liberated in the hydrolysis of the gum of Acacia Senegal.

In 1965, Sawardeker and Sloneker (S2) reported on the effectiveness of a liquid phase of Carbowax 20 M for the GLC resolution of trimethylsilyl ethers of mixtures of monosaccharides. Figure 27 shows the chromatogram they obtained under these conditions. 

After methanolysis of glycoproteins and 0-trimethylsilylation of the monosaccharides, re-A -acetylation of amino-sugars has been performed directly on gas liquid chromatographic columns, by injecting acetic anhydride together with the sample.Quantitative A -acetylation occurs whilst the O-trimethylsilyl ethers remain intact. 

The analysis of carbohydrates continues to be of considerable importance in the biological sciences. The diversity of structure and function of carbohydrates in organisms contributes to the difficulty of analysis of these materials. There exist a number of techniques for the analysis of carbohydrates, but no single technique has universal applicability. For the identification of carbohydrates, trimethylsilyl ethers are widely used in GC/MS. Veness and Evans " showed that GC/FTIR can also be used, with some advantages, for this purpose. A selection of 42 monosaccharides and related compounds were examined and, in each case, unique spectra were obtained for the differing compounds and their isomeric forms, allowing unambiguous identification. These results indicate that GC/FTIR of trimethylsilyl ethers of monosaccharides is a useful analytical technique for their identification. The resultant spectra are unique, easy to interpret, and stereo-isomeric forms are readily differentiated. 

Veness, R.G. Evans, C.S. Identification of monosaccharides and related compounds by gas chromatography-Fourier transform infrared spectroscopy of their trimethylsilyl ethers. J. Chromatogr. A, 1996, 721,165. 

In a useful variant of the classical methylation procedure for linkage analysis in the structural determination of oligo- and poly-sacccharides, the fully methylated oligomer is treated with triethylsilane and boron trifluoride etherate or triethylsilane and trimethylsilyl trifluoromethanesulfonate. This procedure produces partially methylated anhydroalditols. These are acetylated and analyzed by GLC-MS.222 223 This reductive-cleavage method makes possible simultaneous determination of identity, ratio, linkage position, and especially the ring size for each monosaccharide component. 

Monosaccharides were measured by adding 0.2 ml of the enzyme supernatant and 0.2 ml of a standard solution of inositol in a 2-ml vial. The solution was freeze dried and the simple sugars measured as their silyl ethers by GCL using DMF as the solvent and Sylon BTZ (Supelco, Bellefonte, PA) (Ai,0-Bis(trimethylsilyl)acetamide, Trimethylsilylimidazole, Trimethylchlorosilane, 3 2 3) as the derivatizing reagent. Phenolic acids were measured by GLC as their silyl ethers using A,0,bis(trimethylsilyl) trifloroacetamide (BSTFA) as previously described [23]. 

Synthesis and Reactions.- The high sensitivity of Raman spectroscopy to the stereochemistry of monosaccharide acetates is referred to in Chapter 2, and the reduction of propionates to propyl ethers by EtjSiH in the presence of both trimethylsilyl triflate and boron nifluoride etherate is covered in Chapters 5 and 18. 

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