Bottom-Up Elucidation of Glycosidic Bond Stereochemistry.

The lack of robust, high-throughput, and sensitive analytical strategies that can conclusively map the structure of glycans has significantly hampered progress in fundamental and applied aspects of glycoscience. Resolution of the anomeric α/β glycan linkage within oligosaccharides remains a particular challenge. Here, we show that "memory" of anomeric configuration is retained following gas-phase glycosidic bond fragmentation during tandem mass spectrometry (MS2). These findings allow for integration of MS2 with ion mobility spectrometry (IM-MS2) and lead to a strategy to distinguish α- and β-linkages within natural underivatized carbohydrates. We have applied this fragment-based hyphenated MS technology to oligosaccharide standards and to de novo sequencing of purified plant metabolite glycoconjugates, showing that the anomeric signature is also observable in fragments derived from larger glycans. The discovery of the unexpected anomeric memory effect is further supported by IR-MS action spectroscopy and ab initio calculations. Quantum mechanical calculations provide candidate geometries for the distinct anomeric fragment ions, in turn shedding light on gas-phase dissociation mechanisms of glycosidic linkages.