Antibacterial Cyclic Lipopeptide Enamidonins with an Enamide-Linked Acyl Chain from a Streptomyces Species

Three cyclic lipopeptides, including one known (1) and two new (2 and 3) compounds, that possess the rare enamide linkage group were discovered from Streptomyces sp. KCB14A132, an actinobacterium isolated from a soil sample collected from Jeung Island, Korea. The NMR and MS-based characterization showed that they differed in the amino acid residues in the peptide backbone. Application of Marfey's analysis, GITC derivatization, and modified Mosher's method, as well as ECD measurements provided the absolute configurations of enamidonin (1) and those of new compounds enamidonins B and C (2 and 3). The two new enamidonin analogues were shown to exhibit antibacterial activity against Gram-positive bacteria including methicillin-resistant and quinolone-resistant Staphylococcus aureus. Furthermore, evaluation of the extraction conditions and a close inspection of the LC-MS chromatograms revealed that the N, N-acetonide unit of the enamidonin family was formed during the acetone extraction process. The chemically prepared deacetonide derivatives of enamidonins were found to lack antibacterial activity, demonstrating that the dimethylimidazolidinone residue is necessary for antibacterial activity.