Permethylation and Peracetylation

Peracetylation provides a quick and reversible derivatization that is effective in improving the sensitivity of mass-spectrometric detection of carbohydrates for measuring the mass of intact derivatized molecules over their underivatized native counterparts.142 144 

Permethylated oligosaccharides also provide increased ion signals over their underivatized counterparts, allowing the sensitive analysis of minor components.142,145,146 This method provides a smaller mass increase and a greater volatility than the peracetylation method,147 however the use of peracetylation 

In 1984, a procedure for permethylation of carbohydrates using iodomethane, catalyzed by sodium hydroxide139 was introduced that was shown to result in higher yields of permethylated products in a shorter reaction time than the Hakomori permethylation. It is this method of permethylation that has become routinely used. 

The choice between peracetylation and permethlyation is dependent on the experiment being performed. When dealing with samples in complex, salty matrices, acid-catalyzed peracetylation proves to be the right choice107,145 while permethylation tends not to be as effective under such conditions.143 

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A. Dell, Preparation and desorption mass spectrometry of permethyl and peracetyl derivatives of oligosaccharide, Meth. Enzymol., 193 (1990) 647-660. 

Mass Spectrometry (MS). MS is one of the key techniques used in structure determination of carbohydrates and analyses via electron impact (E.I.) and chemical ionization (C.I.) methods are performed routinely on low molecular weight permethylated or peracetylated carbohydrates. Recently, MS procedures have found wider application in the structure elucidation of less volatile higher molecular weight oligosaccharides as a result of instrumental developments (in particular, desorption methods of ionization, based on fast atom bombardment (FAB) (93), field desorption (FD), laser desorption (LD), plasma desorption (PD), and secondary ion (SI) mass spectrometry) and improvements in derivatization techniques. For example, a series of malto-oligosaccharides, starch and other glycans have been examined with LD FD-MS (94,95) whilst FAB techniques have been employed for studies of cello- and malto-oligosaccharides (96) and branched cyclo-dextrins (97). 

The ESI/MS/MS fragmentation spectrum of permethylated LNF-II is displayed in Figure 8.48. [243] This spectrum is somewhat more complicated than the one from the per-acetylated derivative, but allows additional structural information to be obtained. As for the peracetylated derivative, this spectrum contains the ions characteristic of the sequence observed at m/z 638 (B2, trisaccharidic Hex-[Fuc]-GlcNAc-Me 8) and 842 (B3, Hex-[Fuc]-GlcNAc-Hex-Me 11), which allow the branched structure and the sequence on the reducing side to be established. 

Direct derivatization of reducing sugars by permethylation, peracetyl-ation, or per( trimethylsilyl)ation gives a mixture of glycosides. These derivatives are suitable for g.l.c. analysis, although, for complex mixtures of sugars, the multiplicity of peaks caused by the derivatization may complicate elucidation of the results. The mass spectra of glycosides have been extensively studied, especially as peracetylated or permethyl-ated derivatives. In this article, these studies will be only briefly summarized. Recent developments and applications to studies of natural carbohydrates will, however, be discussed. 

Peracetylated glycosides are fragmented in a manner analogous to permethylated glycosides. A particular feature is the appearance of the triacetoxonium (2) and diacetoxonium (3) ions. The degradative path-... 

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