New oxazoline-containing tetrapeptides from a marine-derived Streptomyces sp., strain CNX-225

Marine actinomycetes are emerging as a significant resource for the discovery of novel bioactive metabolites.  Members of the complex genus Streptomyces have been recognized to be adapted to growth in marine systems and to produce compounds with diverse structures and important biological activities.  In our continued investigations of marine-derived Streptomyces strains, we investigated strain CNX-225 and isolated three new oxazoline-containing tetrapeptides, chloroxazoic acids A, B and C (CHL A-C).  Although these peptides were found to be exceedingly rare, two related structures (JBIR-34, JBIR-35) have been recently reported [1].  Marine strain CNX-225 was cultured in saltwater-based media at 27 °C on a rotary shaker.  After 6 days, XAD-7 resin was added and metabolites were extracted from the resin with acetone.  The crude extract was partitioned into ethyl acetate, followed by purification using RP C₁₈ flash column chromatography and RP C₁₈ semi-preparative HPLC, resulting in the isolation of three new compounds, CHL A (13 mg) and two minor acids CHL B (3 mg) and CHL C (1.5 mg) from 10 L of culture broth.  Interpretation of ¹H-, ¹³C- and 2D-NMR data, coupled with HR-LCMS and X-ray crystallographic data, supported the presence of 4-methyloxazoline and chloroindole moieties in all isolated compounds, and allowed the assignment of the full amino acid residues.  In initial testing these tetrapeptides showed no cytotoxic activity against HCT-116 colon cancer cells, thus we are examining these compounds in a diversity of additional targeted and whole cell bioassays.

[1] Motohashi, K.; Takagi, M.; Shin-ya, K. J. Nat. Prod. 2010, 73, 226-228