Separation of sulfated fucose-containing isomers found in polysaccharides from brown seaweed using cyclic ion mobility mass spectrometry

Abstract

Brown seaweeds and some marine invertebrates contain bioactive polysaccharides, including fucoidan, with antimicrobial, antibacterial, and antifungal properties. Improved characterization of their structures would be valuable. To our knowledge, fucoidan extracted from the macroalga Laminaria digitata grown in the North Atlantic has not been structurally determined. We show that both singly and doubly sulfated fucose units, and singly sulfated galactose species present within fucoidan exist as isomers. These were identified via analysis of the biopolymer using Cyclic Ion Mobility Mass Spectrometry (cIM-MS), which has not been used to determine isomeric species present in fucoidan-derived structures previously. Along with using cIM, tandem MS (MS/MS) provided unique fragmentation related to differing sulfate locations and glycosidic linkages. Connectivity positions between both sulfated fucose and galactose trimers and dimers were identified using Collisional Cross Section (CCS) modelling via a machine-learning approach (AllCCS2 prediction software) and drift times from cIM. Previous studies determined that the linkage between the sulfated galactose dimer ([Gal₂SO₃]⁻) was 1 → 4 due to the presence of m/z = 361. Herein, both fragmentation and drift time suggest glycosidic linkages between the monomer units at 1 → 4 and 1 → 3 positions. Ions of high m/z were identified, e.g. [Fuc₃(SO₃)₂-H₂O]²⁻, along with smaller, lighter ions such as a dehydrated sulfated fucose, [FucSO₃-H₂O]⁻. A major ion seen in this work appears at m/z = 357, which we hypothesize as a galactose dimer where one unit has ring-opened to yield a deprotonated glycolic acid ion. The presence of this species has not been reported in earlier structural investigations and speaks to the structural complexity of fucoidan present in Laminaria digitata harvested in Atlantic Canada.