Orientation of cecropin A helices in phospholipid bilayers determined by solid-state NMR spectroscopy

The orientation of the insect antibiotic peptide cecropin A (CecA) in the phospholipid bilayer membrane was determined using N-15 solid-state NMR spectroscopy. Two peptide samples, each specifically labeled with N-15 at Val(11) or Ala(27), were synthesized by solid phase techniques. The peptides were incorporated into phospholipid bilayers, prepared from a mixture of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol, and oriented on glass slides. The N-15 chemical shift solid-state NMR spectra from these uniaxially oriented samples display a single N-15 chemical shift frequency for each labeled residue. Both frequencies are near the upfield end of the N-15 chemical shift powder pattern, as expected for an alpha-helix with its long axis in the plane of the membrane and the NH bonds perpendicular to the direction of. the magnetic field. These results support a mechanism of action in which CecA binds to and covers the membrane surface, thereby causing a general destabilization and leakiness of the lipid bilayer membrane. The data are discussed in relation to a proposed mechanism of membrane lysis and bacterial killing via an ion channel activity of CecA.