Primary structure, conformation in aqueous solution, and intestinal immunomodulating activity of fucoidan from two brown seaweed species Sargassum crassifolium and Padina australis

•Structures of fucoidans from S. crassifolium and P. australis were determined.•Molecular models of the fucoidans were built based on chemical structure.•Intestinal immunological activity via Peyer’s patch cells of the fucoidans was estimated.•A better understanding on structure-activity relationship of fucoidan was proposed.

We studied the structure of fucoidans extracted from two brown seaweed species, Sargassum crassifolium and Padina australis, and their intestinal immunomodulating activity via Peyer’s patch cells of C3H/HeJ mice. ESI–MS analysis indicated that the dominant structure of both fucoidans has a backbone of α-(1 → 4)-linked and α-(1 → 3)-linked l-fucose residues and sulfate groups are attached at the C-2 and C-4 positions; branches of fucoidan from S. crassifolium are galactose residues with (1 → 4)- linkage and branching points are at C-4 of fucose, while fucoidan from P. australis, branches are sulfated galactose-fucose disaccharides and sulfated galactose monosaccharides attached to the main chain through (1 → 3)- or (1 → 4)- linkages. According to small angle X-ray scattering (SAXS) measurements, the two fucoidans have a branched structure. We simulated them with molecular models based on our proposed primary structure. These fucoidan samples have the ability to stimulate intestinal immunological activity via Peyer’s patch cells.