Scientists used NMR to map the 3‑D shape of a short piece of the human antimicrobial peptide LL‑37 when it sits on tiny fat‑like micelles, showing it forms a helix that lies on the surface and that changing its shape alters how it interacts with membranes.

Antimicrobial peptides are universal host defense membrane-targeting molecules in a variety of life forms. Structure elucidation provides important insight into the mechanism of action. Here we present the three-dimensional structure of a membrane peptide in complex with dioctanoyl phosphatidylglycerol (D8PG) micelles determined by solution NMR spectroscopy. The model peptide, derived from the key antibacterial region of human LL-37, adopted an amphipathic helical structure based on 182 NOE-generated distance restraints and 34 chemical shift-derived angle restraints. Using the same NOESY experiment, it is also possible to delineate in detail the location of this peptide in lipid micelles via one-dimensional slice analysis of the intermolecular NOE cross peaks between the peptide and lipid. Hydrophobic aromatic side chains gave medium to strong NOE cross peaks, backbone amide protons and interfacial arginine side chain HN protons showed weak cross peaks, and arginine side chains on the hydrophilic face yielded no cross peaks with D8PG. Such a peptide-lipid intermolecular NOE pattern indicates a surface location of the amphipathic helix on the lipid micelle. In contrast, the epsilon HN protons of the three arginine side chains showed more or less similar intermolecular NOE cross peaks with lipid acyl chains when the helical structure was disrupted by selective d-amino acid incorporation, providing the basis for the selective toxic effect of the peptide against bacteria but not human cells. The differences in the intermolecular NOE patterns indicate that these peptides interact with model membranes in different mechanisms. Major NMR experiments for detecting protein-lipid NOE cross peaks are discussed.