Observation of a cis amide isomer within a linear peptide

Bombolitins are a series of peptides isolated from bumble bee venom. They exhibit biological profiles within membrane environments similar to those of mastoparan, melittin, and crabrolin, in spite of the low sequence homology. All of these peptides are related in the amphiphilic nature of the a-helices that they can form. During the NMR investigation of bombolitin I in the presence of SDS micelles as a mimic for a biological membrane, we observed the presence of a cis amide bond between the first and second residues, Ile1 and Lys2. To our knowledge, this is the first report of a cis amide within a linear peptide without a proline.