Energetics of cross-ring cleavages and their relevance to the linkage determination of oligosaccharides

The decomposition of oligosaccharide anions is investigated using the low energy collisional activation and Fourier transform mass spectrometry. The dissociation of the anion, specifically the monosaccharide rings, provides information on the types of linkage that form the glycosidic bonds. The fragment ions observed under liquid secondary ion mass spectrometry ionization and low energy collisional activation conditions show that the deprotonation of the anomeric position is necessary for cleaving the saccharide ring. For this reason, fragmentation occurs readily on the reducing end of the molecule. When the reducing ring is derivatized, or linked so that deprotonation of the anomeric ring is not possible, cross-ring fragmentation is not observed. A mechanism for cross ring cleavage is proposed based on experimental results. Molecular orbital calculations are performed to predict the activation barrier of each step and the overall dissociation process.