Fragmentation partially methylated sugars

By analysis112 of the fragmentation patterns of partially methylated carbohydrates that have subsequently been deuteriomethylated, the disposition of the original methyl groups can be determined. This procedure may have a significant advantage over other methods of identification of partially methylated sugars, and the method has been discussed in a previous article in this Series.116... 

Partially methylated sugars can be analyzed as trimethylsilyl derivatives by m.s. By inspection of ions of the H-, J-, F-, and K-series, the methoxyl substitution-pattern could be determined. The spectra were complicated by the appearance of a T-series of fragments derived by loss of a methyl radical from radical-ions having one or more trimethylsilyl groups. Similarly, partially (hydroxy ethyl) ated sugars can be analyzed as their trimethylsilyl derivatives. ... 

The mixture of partially methylated sugars obtained on hydrolysis of fully methylated polysaccharides is routinely analyzed in the authors laboratory, as alditol acetates, by g.l.c.-m.s. For this reason, the mode of fragmentation of these derivatives has been extensively studied " by using the deuterium-labelling technique. The results of the studies are summarized next. 

The use of alditol acetates for the separation of mixtures of partially methylated sugars not only simplifies the g.l.c. but also the mass spectra, because such compounds contain carbon to carbon bonds of only three types those between (a) carbon atoms that each carries a methoxyl group, or (b) each bears an acetoxyl group, or (c) adjacent carbon atoms having one substituent of each type. Mass spectrometry of partially methylated alditol acetates thus gives rise to relatively few primary fragments that are characteristic of the pattern of substitution. This technique was introduced to carbohydrate chemistry by Lindberg and his associates, and has been employed extensively by them in subsequent structural studies, some of which are discussed next others are listed in Table XXXI (see p. 103). 

A GC/MS method, commonly called linkage analysis or methylation analysis, has also proven to be a successful approach to determining interresidue linkage and branch points [14]. It involves complete alkylation of all free hydroxyl groups. The fully methylated oligosaccharides are hydrolyzed into monomers, which are further reduced and acetylated to provide partially methylated alditol acetates (PAMAs). Acetylation takes place at those positions that were newly exposed by hydrolysis and reduction. The GC retention times and fragmentation patterns can reveal the mode of attachment in the native sugar. 

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