Glycolipids are compounds in which sugars are linked to glycerides by glycosidic bonds, which with different structures, such as monogalactosyl diacylcerol (MGDG) (Sun et al., 2021), monogalactosyl monoacylglycerol (MGMG) (Sanina et al., 2012), digalactosyl diacylglycerol (DGDG) (Plouguerné et al., 2020), digalactosyl monoacylglycerol (DGMG) (Sun et al., 2021; Xu et al., 1992), sulfoquinovosyl diacylglycerol (SQDG) (Plouguerné et al., 2020), sulfoquinovosyl monoacylglycerol (SQMG) (Da Costa et al., 2015), and etc (Melo et al., 2015; Sanina et al., 2000), have been isolated from various marine macroalgae. In this work, the glycolipids from Bangia fusco-purpurea, Gelidium amansii, Gloiopeltis furcata, Gracilaria lemaneiformis, Gracilaria sp., Palmaria palmata, Porphyra yezoensis and Scagassum sp. were investigated. Among marine macroalgae, Bangia fusco-purpurea, Gracilaria lemaneiformis, Gracilaria sp. and Porphyra yezoensis contained higher content of glycolipids. Therefore, the extraction process of their glycolipids was conducted through single factor experiments (Table 1) and response surface experiments (Table 2), including some factors: the solid-to-liquid ratio, extraction temperature, time, ultrasonic power, and/or volume fraction of methanol. From single factor experiments (Fig. 1 and Fig. 2), the optimum extraction conditions of the glycolipids from Bangia fusco-purpurea, Gracilaria lemaneiformis, Gracilaria sp. and Porphyra yezoensis were obtained (Table 3). The extraction of Bangia fusco-purpurea extract with higher content of glycolipids was further optimized (Fig. 4). The suitable extraction conditions of the glycolipids from Bangia fusco-purpurea was: 1:27 g/mL, 49 ℃, 98 min and 500 W, which had not been reported previously.
The existence of hydrophilic sugar group and lipophilic acyl group determines that they are amphoteric compounds. And this special amphiphilic makes they have special physiological activities. For example, glycoglycerolipids from Sargassum vulgare have good antifouling activity (Plouguerné et al., 2020). A galactoglycerolipid isolated from Lobophora variegata show significant allelopathic activity against the coral Montastraea cavernosa and the sponge Agelas clathrodes (Slattery and Lesser, 2014). Glycoglycerolipids from macroalgae present antivirus activity (Mattos et al., 2011; Wang et al., 2007), antibacterial activity (Arunkumar et al., 2005), antialgal activity (Sun et al., 2021; Sun et al., 2019), and other activities (Banskota et al., 2014; Tsai and Pan, 2012; Xiao, 2021). Among these researches mentioned-above, moisture-absorption and moisture-retention activities of glycolipids from marine macroalgae have received less attention. The researches point out that the moisture-absorption and moisture-retention mechanism of seaweed polysaccharides is (1) seaweed polysaccharides contain a large number of hydrophilic groups, such as hydroxyl and carboxyl, which can be combined with water molecules in the form of hydrogen bonds (Xu et al., 2011); (2) Algal polysaccharide molecular chain and water molecules can be crosslinked and wound in space to form a network structure (Ilekuttige et al., 2019). According to the composition of glycolipids, they should also have the moisture-absorption and moisture-retention activities. However, there was no study about the moisture-absorption and moisture-retention activities of glycolipids from marine macroalgae at present. In Fig. 3, the extracts from Bangia fusco-purpurea, Gracilaria lemaneiformis, Gracilaria sp. and Porphyra yezoensis showed certain moisture absorption and moisturizing activity. And H33, which purified from Bangia fusco-purpurea showed also the moisturizing activity close to that of the extract from Bangia fusco-purpurea, and that of glycerol, glycerin and sodium alginate in the first 42 hours. It is clear that glycolipids from Bangia fusco-purpurea have better moisturizing activity.
Up to now, the isolation, purification and identification of glycolipids from marine macroalgae were rarely reported (Mattos et al., 2011; Plouguerné et al., 2014; Sun et al., 2021). In view of the great application potential of glycolipids with multi activity in the medicine, food and other fields, more glycolipids from marine macroalgae need to be isolated and purified. In the present study, the extracts of Bangia fusco-purpurea, Gelidium amansii, Gloiopeltis furcata, Gracilaria lemaneiformis, Gracilaria sp., Palmaria palmata, Porphyra yezoensis and Scagassum sp. were selected as raw material for isolating glycolipids using liquid-liquid extraction, and preliminarily analyzed by TLC (Fig. 5) and HPLC compared with standard MGDG, SQDG and DGDG. Results showed that SQDG seems to be ubiquitous in those marine macroalgae (Table 4). The glycolipids of Bangia fusco-purpurea were further purified to obtain two glycolipids with arabinopyranosyloxy, hexadecanyl-1-O-α-D-arabinopyranosyloxy (1→4)-α-D-arabinopyranosyloxy (1→4)-α-D-arabinopyranoside and docosanyl-1-O-α-D-arabinopyranosyloxy-(1→4)-3-O-acetyl -α-D-arabinopyranosyloxy-(1→4)-α-D-arabinopyranoside (Fig. 6, Fig. 7 and Fig. 8). Those glycolipids isolated from marine macroalgae mainly contained galactose, isorhamose and glucose (Plouguerné et al., 2020; Sun et al., 2021; Wu et al., 2009). It has not been reported that the glycolipids containing arabinose have been isolated from marine macroalgae. This type of glycolipid was found only in a few corals, such as Sinularia cervicornis (He et al., 2002) and Sinularia firma (Kumar and Lakshmi, 2006). Cervicoside, an arabinomonoacyl glycolipid, exhibited cytotoxicity against human SKMG-4, Hep-G2 and CNE2 cell in vitro (He et al., 2002). H33, an arabinomonoacyl glycolipid, showed moisturizing activity (Fig. 3). Surprisingly, a disaccharide (β-Gal-(1–3)-β-Xyl) was also obtained from Bangia fusco-purpurea in this study. It is reported that β-Gal-(1–3)-β-Xyl Used to detect intestinal lactase activity. Unfortunately, the SQDG determined by TLC and HPLC was not obtained from this macroalgae.
Marine carbohydrates are usually biodegradable and biocompatible, and their biological characteristics are helpful to discover new bioactive substances with special pharmacological characteristics. Glycolipids from marine macroalgae are also a type of marine carbohydrate, obviously, they haven't received enough attention. In Ascophyllum nodosum (Le Tutour et al., 1998), Agarophyton Vermiculophyllum (Honda et al., 2019), Fucus vesiculosus (Le Tutour et al., 1998), Fucus serratus (Le Tutour et al., 1998), and Palmaria palmata (Lopes et al., 2019), MGDG, DGDG and SQDG were detected (Honda et al., 2019; Le Tutour et al., 1998; Lopes et al., 2019). SQDG and DGDG were determined in Chondrus crispus through hydrophilic interaction liquid chromatography-electrospray ionization mass spectrometry (Melo et al., 2015). However, those glycolipids did not be isolated and purification. In future work, it is necessary to screen and isolated glycolipids from more seaweeds.