Chemical Diversity And Biological Evaluation Of Sulfurcontaining Macrolides And Other Secondary Metabolites From The Endophytic Fungi Of Marine Plants

Sulfide derivatives are widely found in marine microorganisms,while marine fungi are one of most studied communities among the marine microorganisms.Most of the marine-derived fungi are symbiotic with their hosts and can be separated and obtained from the hosts.Among them,marine plants-derived fungi play an important role in marine microorganisms,especially,endophytic fungi from mangroves and alga are the most important ones.In this thesis,different species of endophytic fungi from algal and mangrove plants were isolated,and,based on the results from chemical and biological screening,three strains Cladosporium cladosporioides MA-299,Penicillium variabile EN-394,and Trichoderma longibrachiatum EN-586 were selected for scale-up fermentation and chemical investigation.After fermentation,a variety of chromatographic techniques were used to isolate and purify the metabolites,and various spectral methods were used to identify the structure of the isolated compounds.As a result,a total of 57 compounds were isolated and identified,and 22 of them are new compounds.The structure types of the isolated compounds covered from sulfur-containing macrolides to tetraene and triene lactones,steroids,terpenes and polyketones,and some of these compounds were tested for antimicrobial activity,enzymatic inhibitory activity,antitumor,antioxidation,and other biological activities.Sulfur source was added to fermentation medium for C.cladosporioides MA-299to promote the synthesis of sulfur-containing macrolides.Briefly,32 compounds were isolated and identified from the metobolites of C.cladosporioides MA-299,among them,13 compounds were reported for the first time,including seven sulfur-containing macrolides(CC1*–CC4*,CC6*–CC8*)and six non-sulfur lactones(CC9*–CC11*,CC14*–CC16*).The crystal structures of sulfur-containing 12-membered macrolides(CC1*and CC7*)were described for the first time,resulting in the structure revision of pandangolide 3,in which the sulfur side chain was corrected from C-3 to C-2.CC9*is the first reported macrolide with bicyclo 5/9 ring system,and its structure as well as absolute configuration was determined by X-ray crystallographic analysis.For P.variabile EN-394,17 compounds were isolated and identified from the metabolites of the fungal strain,including two ene lactones(PV1*and PV2*),one steroid(PV7*),and two polyketones(PV10*and PV11*)discovered for the first time.The absolute configuration of PV7*was determined by X-ray crystallographic analysis.Eight metabolites were isolated and identified from the fungal strain T.longibrachiatum EN-586,with four new compounds including a terpenoid(TL1*)and three polyketones(TL4*–TL6*).The identified compounds were tested for antimicrobial activities,enzymatic inhibitory activities,antitumor activities,and antioxidation activities.The results revealed that the antimicrobial activities of sulfur-containing macrolides,against aquatic bacteria and plant pathogenic fungi,were generally better than those of non-sulfur-containing compounds(CC1*–CC8*vs CC10*–CC17),indicating that sulfur substitution plays a key role in the antimicrobial activity of macrolides.CC9*,a nonsulfur-containing macrolide,also exhibited comparable bioactivities,possibly due to the presence of the double bond and the formation of bicyclic system via connection of C-2 and C-6.The triene lactone PV6 showed obvious inhibitory activity against Edwardsiella ictarda,while PV7*has significant inhibitory activities against a variety of aquatic pathogenic bacteria,such as Edwardsiella ictarda,Vibrio anguillarum,Pseudomonas aeruginosa,and plant pathogenic fungi,such as Physalospora piricola Nose,Bipolaris sorokiniana,Fusarium oxysporum f.sp.momodicae,and Helminthosporium maydis,with MIC of 1.0–4.0 g/mL.In addition,compounds CC1*,CC5,CC6*,CC12,and CC14*showed inhibitory activity against acetylcholinesterase,with IC₅₀ of 4.69,7.51,1.06,1.79,and 40.26?M,respectively,whereas PV2*and PV3exhibited obvious scavenging activity of DPPH(1,1-diphenyl-2-picrylhydrazyl)radicals,with IC₅₀ values of 19.35 and 12.15?g/mL,respectively.In summary,some sulfur-containing macrolides,ene lactones and other kinds of compounds with novel structures and significant bioactivities were identified from three strains of endophytic fungi that were obtained from marine algae and mangrove plants.The results from this work enriched the chemical diversity of marine fungal metabolites and provided basis for discovery of leading compounds.