Abstract
Two novel phlorotannins with a molecular weight of 974, temporarily named 974-A and 974-B, were isolated from the polyphenol powder prepared from the edible marine brown alga Ecklonia kurome Okamura, and their chemical structures were determined by spectroscopic method. The isolated yield of the total of 974-A and 974-B was approximately 4% (w/w) from the polyphenol powder. In 974-A, the carbon at the C2′ position in the A ring of phlorofucofuroeckol-A forms a C–C bond with the carbon at the C2″ position of the C ring of triphloretol-B, while in 974-B, phlorofucofuroeckol-B and triphloretol-B form a C–C bond in the same manner as in 974-A. These structures were supported by high resolution-MS/MS data. To evaluate the antioxidant activities, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay and intracellular radical scavenging assay, using 2′,7′-dichlorofluorescin diacetate (DCFH-DA), were performed for 974-A, 974-B, and four known phlorotannins. The results of the DPPH assay showed that the IC50 values of 974-A, 974-B, phlorofucofuroeckol-A, and dieckol were significantly smaller than those of phlorofucofuroeckol-B, phloroglucinol, α-tocopherol, and ascorbic acid. Furthermore, the DCFH-DA assay suggested that 974-A, 974-B, and dieckol reduce intracellular reactive oxygen species most strongly among the tested compounds.
Highlights
Phlorotannins, the oligomers and polymers of phloroglucinol (1,3,5-trihydroxy benzene, Figure 1, 7), widely occur among marine organisms, especially in brown algae
Based on the means of linkage, phlorotannins can be classified into four subclasses: phlorotannins with an ether linkage, with a phenyl linkage, with an ether and a phenyl linkage, and with a dibenzodioxin linkage [1]
The crude polyphenol powder prepared from the edible marine brown alga Ecklonia kurome Okamura was reported to have inhibitory activities against α-amylase and α-glucosidase in vitro, and positive effects on oral carbohydrate tolerance test in vivo in genetically diabetic KK-Ay mice [10]
Summary
Phlorotannins, the oligomers and polymers of phloroglucinol (1,3,5-trihydroxy benzene, Figure 1, 7), widely occur among marine organisms, especially in brown algae. The crude polyphenol powder prepared from the edible marine brown alga Ecklonia kurome Okamura was reported to have inhibitory activities against α-amylase and α-glucosidase in vitro, and positive effects on oral carbohydrate tolerance test in vivo in genetically diabetic KK-Ay mice [10]. This alga was previously reported to contain abundant phlorotannin derivatives with dibenzodioxin linkages, such as eckol, dieckol (Figure 1, 8), 8,8′-bieckol and phlorofucofuroeckol-A (PFF-A, 3) by Fukuyama et al [11,12,13]. Regarding this background, we attempted to isolate and identify these major phlorotannins from the crude polyphenol powder from E. kurome for biological work. An unknown molecular ion ([M − H]−) at m/z 973 was detected using electrospray ionization mass spectrometry (ESI-MS) in high intensity
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