Separation of Sulfated Fucose-containing Isomers found in L.digitata using Cyclic Ion Mobility Mass Spectrometry

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Brown seaweeds and some marine invertebrates contain bioactive polysaccharides including fucoidan. These biopolymers have antimicrobial, antibacterial, and antifungal properties, and improved characterization of their structures would be valuable as it can correlated with their bioactivity. To our knowledge, fucoidan extracted from the macroalga Laminaria digitata has not been structurally determined. In this paper, 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) also provided unique fragmentation related to differing sulfate locations and glycosidic linkages. Along with sulfate location, connectivity positions between both sulfated fucose and galactose trimers and dimers respectively, were identified using Collisional Cross Section (CCS) modelling and drift times from cIM. Previous studies determined that linkage between the sulfated galactose dimer ([Gal2SO3]-) were linked (1→4) due to the presence of m/z = 361. However, differences in both fragmentation and drift time suggest changes in glycosidic linkages between the monomer units [(1→4) versus ((1→3) linkage]. Ions of high m/z were identified such as [Fuc3(SO3)2-H2O]2-, along with smaller, lighter ions such as a dehydrated singly sulfated fucose monomer ([FucSO3-H2O]-) with m/z = 225. Interestingly, 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 into fucoidan and speaks to the structural complexity of fucoidan present in Laminaria digitata.

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