1Graduate School of Fisheries Sciences, 2Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
Using reversed-phase high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS), we have determined the molecular species of digalactosyldiacylglycerols (DGDG) in the brown alga Sargassum yezoense, which is widely distributed in the coast of the Sea of Japan and rich in polyunsaturated fatty acids but not used effectively. The diacylglycerols released from DGDG by periodate oxidation followed by hydrazinolysis were converted to their 3,5-dinitrophenylurethanes (DNPU) and subjected to HPLC on a highly efficient ODS column (250×4 mm i.d., 4 μm particle size) using acetonitrile as the mobile phase, which gave clear resolution of individual molecular species. Reversed-phase HPLC in conjunction with negative ESI-MS gave weak [RCOO]- ions in addition to prominent deprotonated molecules [M-H]-, by which individual molecular species could be identified. The most predominant molecular species was 20:5n-3/18:4n-3 (46.8% of total DGDG), followed by 20:5n-3/18:3n-3 (11.1%). The DNPU derivatives were also analyzed by chiral-phase HPLC on (R)- and (S)-1-(1-naphthyl)ethylamine polymers, which gave complete resolution of enantiomeric 1,2- and 2,3-diacyl-sn-glycerols. The results clearly showed that the glycerol moieties in all the DGDG molecules had S-configuration(1,2-diacyl-sn-glycerols). The present study demonstrates that S. yezoense is a good source of polyunsaturated fatty acid-rich DGDG.