Argyrins, immunosuppressive cyclic peptides from myxobacteria. II. Structure elucidation and stereochemistry.

The structures of argyrins A∼H were elucidated by NMR spectroscopy, chemical degradation and X-ray analysis as cyclic octapeptides. Argyrins A and B, in addition to the common amino acids tryptophan, glycine, dehydroalanine and alanine or α-aminobutyric acid, sarcosine, contain 2-(1-aminoethyl)thiazol-4-caboxylic acid and the novel amino acid 4'-methoxytryptophan. In argyrins C and D the latter is replaced by 4'-methoxy 2'-methyl-tryptophan. According to NMR analysis the solution and crystal conformations of argyrins A and B are identical in CDCl 3 and slightly different in acetone-d 6 . Argyrins A and B are identical with the antibiotics A21459 A and B, whose structures are revised with respect to 4'-methoxytryptophan.