Probing fragmentation mechanisms of deprotonated isomaltotriose: Charge-remote or charge-directed?

Unimolecular dissociation mechanisms of polysaccharide ions are typically separated into three groups: cross-ring cleavages (forming A/X ions), and those resulting from cleaving different sides of the glycosidic bond (forming B/Y ions) and (C/Z ions). In this study, the relative energetics of the first steps of the cross-ring cleavage and both glycosidic bond cleavage channels for isomaltotriose, [glc(α1-6)glc(α1-6)glc], as well as a minor water loss channel were explored using density functional theory (DFT) calculations at the B3LYP/6-31+g(d) level of theory. RRKM approximations of the competing rate constants indicate that charge-remote reactions have energy barriers and activation entropies that deny them competition with charge-directed reactions in this example. [ABSTRACT FROM AUTHOR]