Cyclic Peptides from a Ruegeria Strain of Bacteria Associated with the Sponge Suberites domuncula

Two new cyclic peptides, cyclo-(glycyl-L-seryl-L-prolyl-L-glutamyl) and cyclo-(glycyl-L-prolyl-L-glutamyl), have been isolated from the cell extract of a Ruegeria strain associated with cell cultures of Suberites domuncula. Three other cyclopeptides have been isolated for the first time from a natural source. Additionally, a new diastereoisomer of a known compound is reported. The structures of isolated compounds have been elucidated by means of spectroscopic data (1D-, 2D-NMR, HRESIMS) and chiral HPLC analysis. The new cyclopeptides exhibited moderate antimicrobial activity against Bacillus subtilis.