Novel Tandem Mass Spectrometry (MS/MS) Approaches for Structural Characterization of Glycans.

Abstract

Glycosylation is a prevalent post-translational modification playing significant roles in cellular interactions and changing with the onset of cancer. Detailed glycan structural information is crucial to further understand disease and to find vaccine candidates. Although several analytical techniques are used for glycan structural analysis, mass spectrometry occupies an important position due to its high sensitivity and selectivity. Tandem mass spectrometry (MS/MS) cleaves chemical bonds and structural information is elucidated from unique molecular fragments and specific fragmentation pathways. In this dissertation, novel MS/MS approaches involving ion-electron and ion-ion reactions are presented for improved glycan structural characterization compared with conventional collision activated dissociation (CAD). Divalent metal-assisted electron capture dissociation (ECD) and electron transfer dissociation (ETD) are directly compared. Metal adduction increases charge to enable ECD/ETD which require multiply charged precursor ions and also influences fragmentation. For magnesium adduction, extensive glycosidic and cross-ring cleavages, many unique to either ECD or ETD of underivatized glycans compared with CAD of the same precursor ions, or with ETD of permethylated glycans, are generated in both ECD and ETD. However, the number of structurally informative fragments and fragmentation efficiency are both higher in ECD. This discrepancy between ECD and ETD, likely related to the different pressures during these reactions, is even more significant for cobalt and calcium adduction. Trivalent metals are also used for the first time in glycan structural analysis, producing higher charge states and inducing enhanced ECD/ETD fragmentation compared with divalent metal adduction. Electron induced dissociation (EID), which is compatible with singly charged ions, is applied to both glycan cations and anions. Positive and negative ion EID yield complementary fragmentation patterns for singly protonated or deprotonated glycans compared with CAD. Reducing end derivatization with aromatic tags facilitates electronic excitation and improves EID fragmentation. Fucose migration in derivatized glycans is significantly reduced in positive ion EID compared with CAD and completely avoided in negative ion EID and CAD. Both metal-assisted ECD/ETD and aromatic tagging/negative ion EID are demonstrated to allow glycan isomer differentiation. The newly developed techniques can be used for structural characterization of, e.g., surface glycans on cancer stem cells.