Bistratamides M and N, Oxazole-Thiazole Containing Cyclic Hexapeptides Isolated from Lissoclinum bistratum Interaction of Zinc (II) with Bistratamide K.

Two novel oxazole-thiazole containing cyclic hexapeptides, bistratamides M () and N () have been isolated from the marine ascidian collected in Raja Ampat (Papua Bar, Indonesia). The planar structure of and was assigned on the basis of extensive 1D and 2D NMR spectroscopy and mass spectrometry. The absolute configuration of the amino acid residues in and was determined by the application of the Marfey's and advanced Marfey's methods after ozonolysis followed by acid-catalyzed hydrolysis. The interaction between zinc (II) and the naturally known bistratamide K (), a cyclic hexapeptide isolated from a different specimen of , was monitored by ¹H and C NMR. The results obtained are consistent with the proposal that these peptides are biosynthesized for binding to metal ions. Compounds and display moderate cytotoxicity against four human tumor cell lines with GI values in the micromolar range.