The role of the mobile proton in fucose migration

Abstract

Fucose migration reactions represent a substantial challenge in the analysis of fucosylated glycan structures by mass spectrometry. In addition to the well-established observation of transposed fucose residues in glycan-dissociation product ions, recent experiments show that the rearrangement can also occur in intact glycan ions. These results suggest a low-energy barrier for migration of the fucose residue and broaden the relevance of fucose migration to include other types of mass spectrometry experiments, including ion mobility-mass spectrometry and ion spectroscopy. In this work, we utilize cold-ion infrared spectroscopy to provide further insight into glycan scrambling in intact glycan ions. Our results show that the mobility of the proton is a prerequisite for the migration reaction. For the prototypical fucosylated glycans Lewis x and blood group antigen H-2, the formation of adduct ions or the addition of functional groups with variable proton affinity yields significant differences in the infrared spectra. These changes correlate well with the promotion or inhibition of fucose migration through the presence or absence of a mobile proton.

Keywords: Carbohydrates; Fucose migration; Infrared spectroscopy; Internal residue loss; Mass spectrometry.

Fig. 1

Glycan rearrangement reactions consist of two independent, consecutive steps: fucose migration and then collision-induced dissociation (CID) with internal residue loss (IRL) products and glycosidic fragments. The energy barrier for a fucose migration is low and the product of a fucose migration reaction is stable. The IRL fragment is labeled with an asterisk. A, B, and C represent monosaccharides. Glycosidic linkages are indicated with bars

Fig. 2

Schematic view of the experimental setup and the attached Fritz Haber Institute free-electron laser (FHI-FEL). The setup consists of a nano-electrospray ionization (nano-ESI) source, a quadrupole mass filter and a helium-droplet source, which is on-axis with a coolable ion trap and the IR laser beam

Fig. 3

a Symbol nomenclature for glycan (SNFG, left panel) [42] and the Lewis y series (right panel) with the tetrasaccharide Le^y and the two trisaccharides BG-H2 and Le^x that are substructures of Le^y. b IR spectra of the parent ions BG-H2 (upper panel) and Le^x (lower panel) investigated as [M+NH₄]⁺ species (m/z 547) in the range of 1000–1200 cm⁻¹ and 1600–1710 cm⁻¹. The two IR spectra are nearly identical

Fig. 4

Stacked overlay of the IR spectra of the parent ions BG-H2 and Le^x investigated as [M+H]⁺ and [M+NH₄]⁺ species in the range of 1000–1710 cm⁻¹ (break between 1150 and 1640 cm⁻¹). The spectra of the protonated species are reproduced from Mucha et al. [38] and shown in the of 1020–1690 cm⁻¹. The four IR spectra are highly similar

Fig. 5

a Overlay of the IR spectra of [Le^x+NMe₃H]⁺ (black line), [Le^x+NEt₃H]⁺ (blue line) and [Le^x+NH₄]⁺ (blue filled) in the range of 1000–1710 cm⁻¹ (break between 1150 and 1640 cm⁻¹). The spectra of the alkylammonium adducts are very similar and differ from that of the ammonium adduct of Le^x. b Proton affinity of different adduct ions in kcal mol⁻¹ [43] compared to the proton affinity of the C=O group in a representative N-acetylglucosamine (gray line) [44]. Due to the local environment, the PA of the C=O group in Le^x can be slightly different

Fig. 6

a Chemical structures of the investigated, labeled trisaccharides. Bold red atoms indicate possible protonation sites. b IR spectra of the parent ions Le^x-AA (upper panel) and BG-H2-AA (lower panel) investigated as [M+H]⁺ species (m/z 651) in the range of 900–1800 cm⁻¹. The two IR spectra are distinguishable from each other, and the gray dashed lines indicate major differences. c IR spectra of Le^x-procainamide (upper panel) and BG-H2-procainamide (lower panel) investigated as [M+H]⁺ species (m/z 749) in the range of 1000–1800 cm⁻¹. For both isomeric sets, the two IR spectra are clearly distinguishable from each other, and the gray dashed lines indicate major differences