Extracted and depolymerized alginates from brown algae Sargassum vulgare of Lebanese origin: chemical, rheological, and antioxidant properties

Purified sodium alginate (PS alginate) was isolated from the brown seaweed Sargassum vulgare collected from the Lebanese Mediterranean coast and then depolymerized into homopolymeric polyguluronate (PolyG) and polymannuronate (PolyM) blocks by controlled acid hydrolysis. These fractions of PS alginate issued from S. vulgare were characterized in terms of composition and structure by SEC, elemental analysis, FTIR and H-1 and C-13 NMR spectroscopy. An alginate with a low content of protein (< 0.62 %) and a molecular weight of 110 200 g mol(-1) was identified as sole polysaccharide. Depolymerized PS alginate fractions, PolyG (32.6 %) and PolyM (22.3 %), were found to have close molecular weights, of 7500 and 6900 g mol(-1), respectively. From NMR analysis, values of F (G), F (M), M/G ratio, F (GG), F (MM), and F (GM) (or F (MG)) blocks were determined and compared with those of alginates from S. vulgare of Brazilian origin and other Sargassum species. Our PS alginate appeared different from the Brazilian S. vulgare alginate, with a lower M/G ratio (0.785 instead of 1.27), a predominance of the G blocks (F (G) and F (GG) > 0.5) instead of the M blocks, and it showed more similarity to the composition of some alginates extracted from other species of Sargassum. High G or M contents (a parts per thousand yen80 %) were measured from PolyG and PolyM blocks, respectively. The viscosity of the PS alginate and its fractions PolyG and PolyM was determined. PS alginate from S. vulgare of Lebanese origin showed a Newtonian flow behavior for concentration lower than 0.5 % in 0.1 M NaCl solution, while a shear-thinning pseudoplastic behavior is observed for concentration range between 0.75 and 10 %. Also, storage (G') and loss (GaEuro(3)) moduli were studied for two concentrations of PS alginate solutions (5 and 10 %). Antioxidant properties of the non-depolymerized and depolymerized alginates were evaluated by determining the scavenging ability of the stable radical DPPH (2,2-diphenyl-1 picrylhydrazyl). Clearly, the results demonstrated differences in radical scavenging efficacy between PolyG and PolyM fractions. The higher hydroxyl radical scavenging activity was observed for the PolyG fractions (similar to 92 % at 2 mg mL(-1)) and this activity was comparable with those of standard antioxidants. These PolyG fractions could be valuable in foods or pharmaceutical products as alternatives to synthetic antioxidants.