AbstractBacteriocin AS‐48 is a 70‐residue cyclic polypeptide fromEnterococcus faecalisthat shows a broad antimicrobial spectrum against both Gram‐positive and Gram‐negative bacteria. The structure of bacteriocin AS‐48 consists of a globular arrangement of five helices with a high positive electrostatic potential in the region comprising helix 4, the turn linking helix 4 and 5, and theN‐terminus of helix 5. This region has been considered to participate in its biological activity and in particular in membrane permeation. To understand the mechanism of the antibacterial activity of AS‐48 and to discriminate the several mechanisms proposed, a simplified bacteriocin was designed consisting of 21 residues and containing the high positively charged region. A disulfide bridge was introduced at an appropriate position to stabilize the peptide and to conserve the helix‐turn‐helix arrangement in the parent molecule. According to1H and13C NMR data, the designed simplified bacteriocin fragment adopts a significant population of a native‐like helical hairpin conformation in aqueous solution, which is further stabilized in 30% TFE. The designed peptide does not show any antibacterial activity, though it is shown to compete with the intact native bacteriocin AS‐48. These results suggest that the mechanism of membrane disruption by bacteriocin is not as simple as being driven by a deposition of positively charged molecules on the plane of the bacterial membrane. Some other regions of the protein must be present such as, for instance, hydrophobic regions so as to enhance the accumulation of the peptide and favour membrane permeation. Copyright © 2004 European Peptide Society an