Separation of disaccharide epimers, anomers and connectivity isomers by high resolution differential ion mobility mass spectrometry

Glycans are ubiquitous, structurally diverse molecules that have specific and general roles involving metabolism, structure, and cell-to-cell signalling. Functional specificity depends strongly on the complexity of structures that polysaccharides can adopt based on their subunit composition, length, extent of branching, glycosidic bond connectivity and anomeric configuration. However, a rapid and comprehensive characterization of glycan isomers can be challenging owing to limitations associated with their separation. Here, ten composition, anomeric and connectivity disaccharide isomers were separated and detected using high-resolution differential ion mobility-mass spectrometry (DMS-MS, also known as FAIMS). Focus was primarily directed to compositional isomers corresponding to epimers that differ by the axial or equatorial position of a single hydroxyl group. DMS resolving power was enhanced 14-fold primarily by increasing the fraction of helium in the ion carrier gas and lowering the flow rate. At relatively high disaccharide concentrations, DMS-MS of each disaccharide resulted in complex and unique multi-peak spectra with up to ten fully and partially resolved peaks for β-1,4-mannobiose (Man-1,4β-Man), which can be attributed to the DMS separation and subsequent dissociation of ionic non-covalently bound oligomers into monomer ions. Each DMS spectrum has at least one differentiating peak that is not in the other spectra, indicating that DMS can be used to fully or partially resolve composition, configuration and connectivity isomers. At relatively low disaccharide concentrations, disaccharide epimers can also be readily separated by DMS. The integration of high-resolution, ambient pressure DMS with complementary reduced-pressure ion mobility and MS-based glycomics and glycoproteomics workflows may be useful for improving the characterization of glycans and glycosylated biomolecules.