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Polysaccharides extracted from Ulva pertusa Kjellm (Chlorophyta) are a group of sulfated heteropolysaccharides, the ulvans. In this study, different molecular weight ulvans were prepared by H 2 O 2 degradation and their antioxidant activities investigated including superoxide and hydroxyl radical scavenging activity, reducing power and metal chelating ability. The molecular weights of natural and degraded ulvans were 151.7, 64.5, 58.0, and 28.2 kDa, respectively, as determined by high performance gel permeation chromatography. Among the four samples, U 3 (the lowest molecular weight sample) showed significant inhibitory effects on superoxide and hydroxyl radicals with IC 50 values of 22.1 μg mL −1 and 2.8 mg mL −1 ; its reducing power and metal chelating ability were also the strongest among the four samples. All the other samples also demonstrated strong activity against superoxide radicals. The results indicated that molecular weight had a significant effect on the antioxidant activity of ulvan with low molecular weight ulvan having stronger antioxidant activity.Abbreviations: U, polysaccharide extracted from U. pertusa (ulvan, Mw, 151.7 kDa); U 1 , degraded ulvan of molecular weight 64.5 kDa; U 2 , degraded ulvan of molecular weight 58.0 kDa; U 3 , degraded ulvan of molecular weight 28.2 kDa; NBT, nitro blue tetrazolium; PMS, phenazine methosulfate; NADH, nicotinamide adenine dinucleotidereduced; EDTA, ethylene diamine tetra-acetic acid; TCA, trichloroacetic acid; TBA, thiobarbituric acid; DR, deoxyribose; HPGPC, high performance gel permeation chromatography; GC, gas liquid chromatography.

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