Fragmentation reactions in the mass spectrometry analysis of neutral oligosaccharides.

A method is described to obtain multicollision dissociation threshold (MCDT) values. These values provide relative reaction thresholds for dissociation in the three major gas-phase fragmentation reactions of oligosaccharides complexed to alkali metal ions. The quasimolecular ions are produced using matrix-assisted laser desorption/ionization Fourier transform mass spectrometry. The MCDTs for alkali metal ion dissociation and glycosidic bond and cross-ring cleavages were resolved from the kinetic energy dependence of collision-induced dissociation (CID) products. The relative strengths of alkali metal ion binding to N,N'-diacetylchitobiose (chitobiose) and N,N',N"-triacetylchitotriose (chitotriose) were probed using sustained off-resonance irradiation (SORI) CID. Experiments to evaluate MCDT values and the method for obtaining them were performed by studying alkali metal ion coordinated crown ethers. Molecular dynamic simulations were also performed to provide insight into the alkali metal ion binding of chitin-based oligosaccharides. The relative dissociation thresholds of glycosidic bond cleavages and cross-ring cleavages were determined for various alkali metal ion coordinated oligosaccharides. The activation barriers of glycosidic bond cleavages were found to depend on the size of the alkali metal ion. Cross-ring cleavages were found to be independent of the alkali metal ion but dependent on linkage type. The results suggest that glycosidic bond cleavages are charge-induced while cross-ring cleavages are charge-remote processes.