The primary structure of alamethicin.

Alamethicin, an antibiotic that can transport cations and induce action potentials in synthetic membranes, is shown to be a cyclic peptide with 18 residues including 7-alpha-aminoisobutyric acid residues, two glutamine residues and one free carboxyl group. The composition indicates microheterogeneity. Alamethicin itself and many peptides derived from it are immune to enzymic digestion, but specific partial acid cleavages have allowed determination of the complete sequence. Diborane reduction has shown that the alpha-carboxyl group of glutamine-18 is free, but the ring is formed by a peptide bond between the imino group of proline-1 and the gamma-carboxyl group of glutamic acid-17. The structure is contrasted with that of other cation-transporting antibiotics. Model building allows a structure that could stack to form a tunnel with a lipophilic exterior and hydrophilic interior and flexible internal arms formed by the pendant C-terminal glutamine residue.