Polysaccharides trimethylsilylation

Amylopectins.— A TMS derivative of amylopectin has been prepared using hexamethyldisilazane in DMF as the silylating reagent. The extent of tri-methylsilylation, which ranged from d.s. 0.9 to 2.2, depends on the structure and solubility of the polysaccharide. Trimethylsilylated amylopectin was found to be completely soluble in such organic solvents as toluene, benzene, and chloroform. 

Fig. 5 Synthesis of polylactide-grafted polysaccharide (Dex-g-PLA) by trimethylsilyl-protection method...

Zollfrank (2001) reacted beech wood with trimethylsilyl chloride under anhydrous conditions in tetrahydrofuran in the presence of pyridine. Pyridine was present to scavenge the HCl produced during the reaction, thus limiting polysaccharide degradation. After reaction at room temperature for 12 hours, the wood was extracted with methanol to remove the pyridinium chloride by-product. SEM of the modified wood indicated that there was no evidence of cell wall degradation. 

Although this article is primarily concerned with gas-liquid chromatography of simple carbohydrates, it is worth noting that certain polysaccharides have been trimethylsilylated. Thus, chlorotrimethyl-silane in pyridine yielded fully trimethylsilylated amylose, cellulose, and pullulan, but failed with dextran and amylopectin.196 Cellulose, amylose, and polyvinyl alcohol have also been trimethylsilylated in molten N-(trimethylsilyl)acetamide.197 As examples of a new class of... 

In analyzing polysaccharide hydrolyzates, some workers have assumed that, because mutarotation in pyridine is slow and the tri-methylsilylation reaction is fast, no change in the composition occurs during trimethylsilylation. Others have equilibrated the monosaccharides in pyridine, before trimethylsilylation.172,255 In nonaqueous solvents, the mutarotation may be catalyzed by lithium perchlorate172 or, more conveniently, by 2-pyridinol,256,257 which is volatile, but does not interfere with subsequent analyses. 

The separation and identification of disaccharides is often an important step in the elucidation of the structure of a natural polysaccharide, and Percival484 has published useful data on the O-trimethyl-silyl derivatives of a variety of disaccharides and their reduction products. In some instances, the trimethylsilyl ethers of the disaccharide alditols have lower retention times than those of the disaccharide derivatives. The per-O-trimethylsilyl derivatives of gentiobi-itol and maltitol were encountered in studies on the structure of Pneumococcus Type II capsular polysaccharide.4843... 

In model experiments,205 it was found that concentration of an aqueous solution of the aldonic acids in the presence of hydrochloric acid gives exclusively the 1,4-lactones, which, on subsequent trimethyl-silylation, give only one peak on the chromatogram. This method was successfully employed for the separation of D-galacturonic, D-glucuronic, and D-mannuronic acids. Of the 1,4-lactones examined, only the trimethylsilyl ether of D-mannono-1,4-lactone was obtained in crystalline form. However, all of the derivatives showed characteristic differences in their infrared spectra in the range of 1500 to 600 cm"1. When this method was applied to the determination of uronic acids in a variety of polysaccharides,205 it was impossible to find any hydrolytic conditions under which the uronic acids were quantitatively released and then reduced, a problem experienced by other workers.20 The method was, however, successful in affording a qualitative, microscale procedure for the identification of hexuronic acids (which otherwise are difficult to detect). 

The separation of saccharinic acids as their O-trimethylsilyl derivatives has also been used by Feather and Harris512 in a study on decomposition of sugars and in studies on bacterial polysaccharides.434... 

The characterization and quantitative determination of uronic acid components in polysaccharides faces the problem of complete release of the uronic acids without accompanying decomposition. This is a difficult task because of the acid resistance of the glycosiduronic bond. From a comparison of several methods, it was shown that methanolysis combined with trifluoroacetic acid hydrolysis is the best for the liberation of uronic acids.204 The identification can be performed by gas chromatography of the trimethylsilyl derivatives.205... 

M. Matsuo, R. Takano, K. Kamei-Hayashi, and S. Hara, A novel regioselective desulfation of polysaccharide sulfates Specific 6-O-desulfation with N, 0-bis(trimethylsilyl)acetamide, Carbohydr. Res.,2A (1993) 209-215. 

After their isolation by chromatographic techniques (anion-exchange chromatography, size exclusion, etc.), different analytical methodologies have been used to identify and quantify the polysaccharides in wine the most commonly used being the traditional methylation analysis followed by GC-MS (Doco and Bril-louet 1993). Polysaccharides have also been determined after solvolysis with anhydrous methanol containing HCl by GC-MS of their per-G-trimethylsilylated methyl glycosides (Vidal et al. 2003). Other techniques such as Fourier transform infrared spectroscopy (FTIR) have been more recently proposed (Coimbra et al. 2002,2005 Boulet et al. 2007). 

Other techniques have begun to be used more often in structural studies. Immunological precipitation has been used to identify structural attributes of gums by their similarity to virus sheath polysaccharides (8). Gas chromatographic studies have made use of methylated (23), acetyl-ated (16), formylated, trifluoroacetylated, and trimethylsilylated derivatives (19,24). 

In studies of polysaccharides structure, the alditol acetate procedure remains the most widely used GLC procedure. The advent of high-resolution glass capillary columns has allowed very efficient separations. Recent applications of these columns to alditol acetate separations have been described (55-57). The alditol acetate procedure requires reduction of the sugars with sodium borohydride. After removal of boric acid, the sample is acetylated by conventional means. Various polar stationary phases have been used in GLC separation of alditol acetates, in both packed and capillary columns. A low-polarity phase was used in a report ( ) which demonstrated the separation of trimethylsilylated alditols, and the neutral sugars in a hemicellulose sample were resolved. 

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