| Summary: | Three new cyanobactins, trikoramides B (<b>1</b>)–D (<b>3</b>), have been isolated from the marine cyanobacterium, <i>Symploca hydnoides</i>, following a preliminary bioassay-guided isolation of the two most active polar fractions based on the brine shrimp toxicity assay. These new cyanobactins are new analogues of the previously reported cytotoxic trikoramide A (<b>4</b>) with differences mainly in the <i>C</i>-prenylated cyclotryptophan unit. Their planar structures were elucidated from their 1D and 2D NMR spectral data in combination with the HRMS/MS data. Marfey’s method, 2D NOESY NMR spectroscopic and ECD spectra analyses were used to determine the absolute stereochemistry of trikoramides B (<b>1</b>)–D (<b>3</b>). Trikoramides B (<b>1</b>) and D (<b>3</b>) exhibited cytotoxicity against MOLT-4 acute lymphoblastic leukemia cell line with IC<sub>50</sub> values of 5.2 µM and 4.7 µM, respectively. Compounds <b>1</b> and <b>3</b> were also evaluated for their quorum-sensing inhibitory assay based on the <i>Pseudomonas aeruginosa</i> PAO1 <i>lasB-gfp</i> and <i>rhlA-gfp</i> bioreporter strains. Although trikoramide B (<b>1</b>) exhibited weak quorum-sensing inhibitory activity, the Br-containing trikoramide D (<b>3</b>) exhibited moderate to significant dose-dependent quorum-sensing inhibitory activities against PAO1 <i>lasB-gpf</i> and <i>rhlA-gfp</i> bioreporter strains with IC<sub>50</sub> values of 19.6 µM and 7.3 µM, respectively.
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