Field desorption mass spectrometry of oligosaccharides

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Abstract
Field desorption mass spectrometry was used to analyze carbohydrate polymers with 5-14 hexose units without prior derivatization. In all examples, the MW of the oligosaccharide could be determined by the abundant quasimolecular ions of the type MNa+, MH+, MNa22+ and MNa33+. Fragmentation at glycosidic linkages was observed in varying extents. The reduced oligosaccharide Man8GlcNAcH2, obtained from [human] IgM, gave quasimolecular ion signals MNa+ at m/z 1544, MH+ at m/z 1522, MNa22+ at m/z 784 and MNa33+ at m/z 530, all corresponding to its assumed MW of 1519.5. Mycobacterial [Mycobacterium smegmatis] methylmannose polysaccharides with the general structure ManxMeMany-OCH3 were also successfully analyzed. Man1MeMan13-OCH3, the largest homolog, gave the expected signal of the quasimolecular ion MNa+ at m/z 2506. The larger polysaccharides were analyzed by a KRATOS MS-50 mass spectrometer with a high-field magnet enabling full sensitivity to be maintained up to 3000 atomic mass units. Polysaccharides up to m/z 1978 were analyzed using a KRATOS MS-9 mass spectrometer operated at 4 kV. The signal-to-noise ratio, which becomes a serious problem in field desorption mass spectrometry at low accelerating voltages, and the low instrument sensitivity were improved considerably by adding scans with low total ion currents obtained over a longer desorption time. In this way, complete sequence information on methylmannose polysaccharides up to Man1MeMan9-OCH3 (MNa+ at m/z 1802) was obtained. Analysis of a presumed Man1MeMan7-OCH3, gave a spectrum consistent only with the structure Man2MeMan6-OCH3, revealing the existence of a methylmannose homolog with 2 unmethylated mannoses at the non-reducing end of the chain.