Antibiotics A21459 A and B, New Inhibitors of Bacterial Protein Synthesis. II. Structure Elucidation.

The structures of the antibiotics, active against a few Gram-negative bacteria and Clostridium difficile, were determined on the basis of physicochemical analyses on the intact molecules and on the acid hydrolysate of A21459 A. FAB-MS and 1H and 13C NMR investigations identified the amino acid units and determined their sequence. Antibiotics A21459 A and B are homodetic cyclic peptides constituted by eight amino acid units. They are glycine, methoxytryptophan, tryptophan, cysteine, alanine, sarcosine, dehydroalanine, and alpha-aminobutyric acid for A21459 A (alanine for A21459 B). Cysteine and alanine condensed to form a thiazole moiety, according to the biosynthesis of thiazole containing antibiotics.