Cyclic tetrapeptides from marine bacteria associated with the seaweed Diginea sp. and the sponge Halisarca ectofibrosa

A Pseudomonas sp. was cultured which was associated with the Japanese seaweed Diginea sp. Crude extracts prepared from this bacterial culture were found to inhibit the growth of other marine bacterial strains. From this bacterial culture, two new peptides cyclo-[phenylalanyl-prolyl-leucyl-prolyl] (3) and cyclo-[isoleucyl-prolyl-leucyl-alanyl] (4) have been isolated together with two known peptides (1) and (2). The crude extract from a culture of Pseudoalteromonas sp. associated with the Thai sponge Halisarca ectofibrosa was found to inhibit the growth of Bacillus subtilis and Vibrio anguillarum. Isolation studies yielded a fraction containing two peptides that were identified as cyclo-[phenylalanyl-leucyl]2 (5) and cyclo-[leucyl-isoleucyl]2 (6) by means of LC-MS and 2D NMR data. Absolute stereochemistry was confirmed by the synthesis of cyclo-[l-phenylalanyl-l-leucyl]2. Peptides (1)–(3) were also isolated from this bacterial strain. None of the individual peptides isolated in this study showed antibiotic activity.