Compositional, structural, and functional characterization of fucoidan extracted from Sargassum polycystum collected from Saint Martin's Island, Bangladesh

Fucoidan is a sulphated polysaccharide with diversified biological activities commonly found in brown seaweeds. In our present study, fucoidan was extracted from Sargassum polycystum collected from Saint Martin's Island in three different extraction methods. The composition of the fucoidans was determined by measuring the total carbohydrate, sulfate, uronic acid, fucose, and protein contents. Fourier-transform infrared spectroscopy (FT-IR) and Proton nuclear magnetic resonance (1H NMR) were studied to determine the structural groups of extracted fucoidans. Functional activities were analyzed by antioxidant activity and anti-diabetic activity assay. Fucoidan yield was obtained highest in the acid extraction method (0.061 ± 0.007 g/g), followed by the water extraction method (0.058 ± 0.005 g/g) and salt extraction method (0.041 ± 0.005 g/g). Water extracted fucoidan showed the highest carbohydrate content (62.97 %), uronic acid content (6.46 %), and protein content (2.62 %). Sulphate content (28.56 %) was highest in salt extracted fucoidan, whereas fucose content (24.0 %) was highest in water extracted fucoidan. FT-IR and 1H NMR analysis confirm several characteristic picks for the extracted fucoidans. Extracted fucoidans presented comparable antioxidant activity, but salt-extracted fucoidan presented the highest DPPH radical scavenging assay (86.85 ± 0.74 %, IC50 = 0.03 mg/mL), ABTS radical scavenging assay (65.13 ± 0.92 %, IC50 = 1.94 mg/mL), Phosphomolybdenum assay (4.356 ± 0.11) and Nitric Oxide (NO) radical scavenging assay (57.08 ± 0.52 %, IC50 = 3.73 mg/mL) in dose-dependency manner. Further anti-diabetic activity assay resulted in the highest α-amylase inhibition activity (62.82 ± 1.36 %, IC50 = 2.30 mg/mL) and α-glucosidase inhibition activity (55.73 ± 0.84 %, IC50 = 3.71 mg/mL) by salt-extracted fucoidan. These functional potentials of seaweed may add significant value to the development of functional food and pharmaceutical materials.