Polytheonamide B is by far the largest non-ribosomal peptide known at present, and displays extraordinary cytotoxicity (EC 50= 68 pg ml− 1, mouse leukaemia P388 cells). Its 48 amino-acid residues include a variety of non-proteinogenic d-and l-amino acids, and the absolute stereochemistry of these amino acids alternate in sequence. These structural features induce the formation of a stable β-strand-type structure, giving rise to an overall ...
