Three forms of soluble phenolics, free phenolics (FP), esterified-bound phenolics (EP) and glycoside-bound phenolics (GP), were extracted from Sargassum pallidum, and their chemical composition was identified by ultra-high performance liquid chromatography-quadrupole-time of flight-tandem mass spectrometry (UPLC-QTOF-MS/MS). Their antioxidant and α-glucosidase inhibitory activities in vitro were evaluated. Results demonstrated that the soluble phenolics in S. pallidum mainly existed in a bound form, GP and EP fractions contained the highest contents of total phenolics (68.30 mg/g) and flavonoids (83.49 mg/g), respectively. A total of 22 compounds were identified in the three phenolic fractions, including three simple phenolics, four flavonoids and two phenolic acids. Biological activity tests indicated GP had the highest cellular antioxidant activity (CAA value = 18.18 μmol/g), 1,1-dipheny1-2-picrylhydrazyl (DPPH) radical scavenging activity (IC₅₀ = 84.6 μg/mL) and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation scavenging activity (IC₅₀ = 127.01 μg/mL), while EP had the strongest oxygen radical absorbance capacity (5322.85 μmol/g) and α-glucosidase inhibitory activity with IC₅₀ value of 27.09 μg/mL. The hypoglycemic activity of EP was significantly higher than that of acarbose. Inhibition kinetics analysis showed that the inhibition mode of α-glucosidase by the three phenolic fractions was a mixed type. In conclusion, bound phenolics from Sargassum pallidum have excellent antioxidant and α-glucosidase inhibitory activities, which will provide a scientific basis for the development of natural antioxidants and hypoglycemic foods.

In this study, the structural characters, antioxidant activities and bile acid-binding ability of sea buckthorn polysaccharides (HRPs) obtained by the commonly used hot water (HRP-W), pressurized hot water (HRP-H), ultrasonic (HRP-U), acid (HRP-C) and alkali (HRP-A) assisted extraction methods were investigated. The results demonstrated that extraction methods had significant effects on extraction yield, monosaccharide composition, molecular weight, particle size, triple-helical structure, and surface morphology of HRPs except for the major linkage bands. Thermogravimetric analysis showed that HRP-U with filamentous reticular microstructure exhibited better thermal stability. The HRP-A with the lowest molecular weight and highest arabinose content possessed the best antioxidant activities. Moreover, the rheological analysis indicated that HRPs with higher galacturonic acid content and molecular weight showed higher viscosity and stronger crosslinking network (HRP-C, HRP-W and HRP-U), which exhibited stronger bile acid binding capacity. The present findings provide scientific evidence in the preparation technology of sea buckthorn polysaccharides with good antioxidant and bile acid binding capacity which are related to the structure affected by the extraction methods.