Determination of singlet oxygen quenching and antioxidant activity of Bieckols isolated from the brown alga Eisenia bicyclis

In this study, we determined 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activities as well as the reducing power for screening of antioxidant activity of bieckols in Eisenia bicyclis (Kjellamn) Setchell (E. bicyclis). The compounds 6,6′-bieckol, 6,8′-bieckol and 8,8′-bieckol are representative members of the phlorotannins family. The isolated bieckols displayed markedly strong DPPH and ABTS radical scavenging effects when compared to those of positive controls (BHA and Trolox). These bieckols were also found to have significant reducing power. The isolated bieckols (6,6′-bieckol, 6,8′-bieckol and 8,8′-bieckol) were found to effectively suppress the detrimental effects of ¹O₂ on type-II photosensitization. The concentrations required to exert a 50 % quenching effect on ¹O₂ (QC₅₀) were found to be 30.7, 35.7 and 49.4 μM for 6,6′-bieckol, 6,8′-bieckol and 8,8′-bieckol, respectively. Interestingly, all these bieckols were found to be superior to histidine (5.9 mM), a well-known ¹O₂ quencher. These results suggested that brown algae phlorotannins, and bieckols in particular, may play an important role in protecting marine organisms against sunlight damage by eliminating ¹O₂. This knowledge may contribute to the development of natural bioactive products with potential applications in reducing photo-produced oxidative damage involving reactive oxygen species in living organisms.