Sequence determination of permethylated oligosaccharides by chemical ionization mass spectrometry

Chemical ionization mass spectra of several permethylated oligosaccharides were studied using isobutane and ammonia as reagent gases. In the molecular ion region, protonated molecule [MH]+ or ammonium adduct ion [M·NH4]+ were observed. Fragmentations were mainly restricted to the cleavages of the glycosidic bonds between the glycosidic oxygen and the anomeric carbon atoms. Resulting fragment ions were classified into oxonium type ions and their counterpart ions with hydrogen or methyl transfer, which were very useful for determining the sequence of the constituent saccharide units. These ions were confirmed by shift techniques with pertrideuteromethyl derivatives, deuterated reagent gases, i-C42H10 and N2H3, and 15NH3. Finally, these results were applied successfully to the structural characterization of the unknown trisaccharides, viridotriose A (6), B (7) and C (8).