Sequence Ion Structures and Dissociation Chemistry of Deprotonated Sucrose Anions

We investigate the tandem mass spectrometry of regiospecifically labeled, deprotonated sucrose analytes. We utilize density functional theory calculations to model the pertinent gas-phase fragmentation chemistry of the prevalent glycosidic bond cleavages (B₁-Y₁ and C₁-Z₁ reactions) and compare these predictions to infrared spectroscopy experiments on the resulting B₁ and C₁ product anions. For the C₁ anions, barriers to interconversion of the pyranose [α-glucose-H]⁻, C₁ anions to entropically favorable ring-open aldehyde-terminated forms were modest (41 kJ mol⁻¹) consistent with the observation of a band assigned to a carbonyl stretch at ~ 1680–1720 cm⁻¹. For the B₁ anions, our transition structure calculations predict the presence of both deprotonated 1,6-anhydroglucose and carbon 2-ketone ((4S,5S,6R)-4,5-dihydroxy-6-(hydroxymethyl)dihydro-2H-pyran-3(4H)-one) anion structures, with the latter predominating. This hypothesis is supported by our spectroscopic data which show diagnostic bands at 1600, 1674, and 1699 cm⁻¹ (deprotonated carbon 2-ketone structures), and at ~ 1541 cm⁻¹ (both types of structure) and RRKM rate calculations. The deprotonated carbon 2-ketone structures are also the lowest energy product B₁ anions.

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