Fucose migration: where to ?

Fucose is a ubiquitous monosaccharide linked to the core of major classes of glycans and the identification of fucose location is one critical bottleneck in glycomics. This is due to the remarkable inclination of the fucosyl residue to migrate to neighboring throughout mass spectrometric analysis. Such molecular rearrangements first reported in the 90’s and called “internal residue loss” can be mistaken for diagnostic fragments and lead to false assignment of the position of fucose on the glycan core. While a variety of misleading fucosylated fragments has been observed for glycan standards, the exact molecular structure of fucosylated products after rearrangement remains elusive and their formation unpredictable. This constitutes a major obstacle to the sequencing of fucosylated glycans. Using Infrared ion spectrosopy, we resolve the molecular structure of fucosylated fragments of four Lewis and Blood Group H antigen trisaccharides. Our findings suggest that fucose migration, which was reported decades ago, results in fragment ions, which can be fully characterized. Additionally we report a new type of fucose migration, which does not feature any internal residue loss and therefore had not been previously detected by mass spectrometry: it consists of a local type of migration, where the fucose remains on the initial residue with a change of regiochemistry. Therefore, such glycan fragments previously regarded as diagnostic carry previously undetected molecular rearrangements. Infrared ion spectroscopy enables molecular characterization of glycan fragments and this knowledge is essential to the interpretation of glycomics data, as well as the understanding of the processes underlying Mass Spectrometry analysis.