Sodium-cationized carbohydrate gas-phase fragmentation chemistry: influence of glycosidic linkage position

We investigate the gas-phase structures and fragmentation chemistry of two isomeric sodium-cationized carbohydrates using combined tandem mass spectrometry, hydrogen/deuterium exchange experiments, and computational methods. Our model systems are the glucose-based disaccharide analytes cellobiose (β-d-glucopyranosyl-(1 → 4)-d-glucose) and gentiobiose (β-d-glucopyranosyl-(1 → 6)-d-glucose). These analytes show substantially different tandem mass spectra. We characterize the rate-determining barriers to both the glycosidic and structurally-informative cross-ring bond cleavages. Sodiated cellobiose produces abundant Y₁ and B₁ peaks. Our deuterium labelling and computational chemistry approach provides evidence for 1,6-anhydroglucose B₁ ion structures rather than the 1,2-anhydroglucose and oxacarbenium ion structures proposed elsewhere. Unlike those earlier proposals, this finding is consistent with the experimentally observed B_n/Y_m branching ratios. In contrast to cellobiose, sodiated gentiobiose primarily fragments by cross-ring cleavage to form various A₂ ion types. Fragmentation is facilitated by ring-opening at the reducing end which enables losses of C_nH_2nO_n oligomers. Deuterium labelling and theory enables rationalization of these processes. Theory and experiment also support the importance of consecutive fragmentation processes at higher collision energies.