Blackbody Infrared Radiative Dissociation of Protonated Oligosaccharides

The dissociation pathways, kinetics, and energetics of protonated oligosaccharides in the gas phase were investigated using blackbody infrared radiative dissociation (BIRD). Time-resolved BIRD measurements were performed on singly protonated ions of cellohexaose (Cel₆), which is composed of β-(1 → 4)-linked glucopyranose rings, and five malto-oligosaccharides (Mal_x, where x = 4–8), which are composed of α-(1 → 4)-linked glucopyranose units. At the temperatures investigated (85–160 °C), the oligosaccharides dissociate at the glycosidic linkages or by the loss of a water molecule to produce B- or Y-type ions. The Y ions dissociate to smaller Y or B ions, while the B ions yield exclusively smaller B ions. The sequential loss of water molecules from the smallest B ions (B₁ and B₂) also occurs. Rate constants for dissociation of the protonated oligosaccharides and the corresponding Arrhenius activation parameters (E_a and A) were determined. The E_a and A-factors measured for protonated Mal_x (x > 4) are indistinguishable within error (~19 kcal mol⁻¹, 10¹⁰ s⁻¹), which is consistent with the ions being in the rapid energy exchange limit. In contrast, the Arrhenius parameters for protonated Cel₆ (24 kcal mol⁻¹, 10¹² s⁻¹) are significantly larger. These results indicate that both the energy and entropy changes associated with the glycosidic bond cleavage are sensitive to the anomeric configuration. Based on the results of this study, it is proposed that formation of B and Y ions occurs through a common dissociation mechanism, with the position of the proton establishing whether a B or Y ion is formed upon glycosidic bond cleavage.