The Chemical Defensive Compounds And Structure-Activity Relationships Of Four Corals From The South China Sea

Since corals of marine invertebrates are lack of efficient physical protection ability, they can only survive relying on chemical defensive strategy by their secondary metabolites. The secondary metabolites of corals have exhibited an astonishing array of chemical defensive activities such as antifouling, allelopathy, antifeeding as well as antimicrobial activity. The prominent antifouling ability of corals indicated that their secondary metabolites may play an important role in inhibiting the attachment of other organisms. In this paper, the antifouling substances and their structure-activity relationships of four corals from Chinese tropical sea area were investigated under the guidance of chemical defensive effects, especially antifouling activity against the larval settlement of barnacle Balanus amphitrite, combining with modern technology of marine natural products. The results should provided basic data for research and development of marine antifouling agents.The chemical defensive substances of four corals including Subergorgia rubra, Sarcophyton infundibuliforme, Scleronephthya sp., Dendronephthya sp. and a coral-derived fungus Nigrospora oryzae were isolated by column chromatography on Si gel, Sephadex LH-20 and preparative HPLC, and identified by spectroscopy of NMR, MS, IR and UV. From these organisms, 82 compounds (1–82) were obtained, including 21 new compounds and 2 metabolites isolated from natural source for the first time. Six structural types were found, including 50 steroids, 16 diterpenoids, 5 alkaloids, 7 lipids, 1 triterpenoid and 1 indole compound. The gorgonian S. rubra were chemically studied for the first time. A series of new 9,10-secosteroids were obtained, and their biogenetic pathway were deduced. These compounds provided foundation for the research on structure-activity relationships.With an emphasis on antifouling activity against the larval settlement of barnacle B. amphitrite, the antifouling activity of the compounds were screened resulting in finding of 8 bioactive compounds. The structure-activity relationships were analyzed, and the founctional groups of some bioactive compounds were confirmed. The cembrane diterpenoids generally had potent activity against the larval settlement of barnacle B. Amphitrite, one of which (40) inhibited larval settlement of B. amphitrite with a IC50 value of 1.25–2.5μg/mL. The structure-activity relationship analysis suggested that the occurrence of hydroxyl group at C-13 resulted in the rapid descend of antifouling activity. The absence of the hydroxyl group at C-13 and presence of the hydroxyl group at C-14 significantly contributed to antifouling activity, while the acetylation of the hydroxyl group at C-14 exhibited minor impact to antifouling activity. The lactone and dipeptide alkaloid compounds isolated from the coral-derived fungus also had strong chemical defensive activities. The correlation between coral-derived fungus and host was illustrated preliminary. In addition, the epidioxyergosterol compound 27 showed strong inhibition against larval settlement of B. amphitrite with IC50 of 0.85μg/mL, and displayed potential as antifouling agent.From the corals that survive relying on chemical defensive mechanisms colleted from the South China Sea, bioactive secondary metabolites were isolated under the guidance of chemical defensive effects. Antifouling active compounds found by our research provided bioresource and compounds source for the research and development of efficient, non-toxic, and natural marine antifouling agents.