Dissection of Fragmentation Pathways in Protonated N-Acetylhexosamines

Structural characterization of carbohydrates by mass spectrometry necessitates a detailed understanding of their gas phase behavior, particularly for protonated carbohydrates that can undergo complex structural rearrangements during fragmentation. Here we utilize tandem mass spectrometry, isotopic labeling, gas-phase hydrogen/deuterium exchange, and ion mobility measurements to characterize structures of the various product ions of protonated N-acetylhexosamines. Following the facile loss of the reducing end hydroxyl group, we identify two primary fragmentation pathways. Detailed mapping of each step in the fragmentation pathway provides new insight into the mechanisms that drive collision-induced dissociation of protonated carbohydrates. Several of the smaller fragment ions are mixtures of structural isomers, and the relative distributions of these structures reveals information about the stereochemistry of the precursor molecule.