Extraction and Identification of Phlorotannins from the Brown Alga, Sargassum fusiforme (Harvey) Setchell

Abstract

Phlorotannins are a group of complex polymers of phloroglucinol (1,3,5-trihydroxybenzene), which are unique compounds from marine brown algae. In our present study, a procedure for extraction and enrichment of phlorotannins from S. fusiforme with highly antioxidant potentials was established. After comparison of different extraction methods, the optimal extraction conditions were established as follows. The freeze-dried seaweed powder was extracted with 30% ethanol-water solvent with a solid/liquid ratio of 1:5 at temperature of 25 °C for 30 min. After extraction, the phlorotannins were fractioned by different solvents, among which the ethyl acetate fraction exhibited both the highest total phlorotannin content (88.48 ± 0.30 mg PGE/100 mg extract) and the highest antioxidant activities. The extracts obtained from these locations were further purified and characterized using a modified UHPLC-QQQ-MS method. Compounds with 42 different molecular weights were detected and tentatively identified, among which the fuhalol-type phlorotannins were the dominant compounds, followed by phlorethols and fucophlorethols with diverse degree of polymerization. Eckol-type phlorotannins including some newly discovered carmalol derivatives were detected in Sargassum species for the first time. Our study not only described the complex phlorotannins composition in S. fusiforme, but also highlighted the challenges involved in structural elucidation of these compounds.

Keywords: Sargassum fusiforme; UHPLC-DAD-ESI-MS; brown alga; extraction method; identification; phlorotannins; polyphenol.

Figure 1

Evaluation of the effects of (a) ethanol concentration; (b) solid/liquid ratio; (c) extraction temperature; and (d) extraction time on the TPC in S. fusiforme. The letters on the scatters indicate statistically different groups (p < 0.05).

Figure 2

The (a) TPC and (b) DRSA of different solvent fractions of S. fusiforme and the comparison of (c) DRSA and (d) FRAP of ethyl acetate fraction with trolox and commercially available tea polyphenols with a purity of 90%. EE, ethanol extracts; EF, ethyl acetate fraction; BF, 1-butanol fractions; AR, the aqueous residue; Trolox, a synthetic antioxidant analogous to vitamin E; TP, the commercially available tea polyphenols with a purity of 90%. The letters on the bar indicate statistically different groups (p < 0.05).

Figure 3

The (a) UV chromatograms recorded at 280 nm and (b) TIC of the ethyl acetate fraction. Peaks marked with numbers were tentatively identified using UHPLC-QQQ-MS, which were shown in Table 1. Numbers with (+) were detected in positive mode; the others were detected in negative mode.

Figure 4

Structure of (a) molecular weight 794 (No. 5); (b) molecular weight 622 (No. 22); (c) molecular weight 746 (No. 23); and (d) molecular weight 652 (No. 36) tentatively identified in S. fusiforme extract and proposed fragmentation pathway.