An amphipathic α-helix at a membrane interface: A structural study using a novel X-ray diffraction method

The amphipathic α-helix is a recurrent feature of membrane-active proteins, peptides, and toxins. Despite extensive biophysical studies, the structural details of its affinity for membrane interfaces remain rather vague. We report here the first results of an effort to obtain detailed structural information about α-helices in membranes by means of a novel X-ray diffraction method. Specifically, we determined the transbilayer position and orientation of an archetypal class A amphipathic helical peptide in oriented fluid-state dioleoylphosphatidylcholine (DOPC) bilayers. The peptide, Ac-18A-NH₂ (Ac-DWLKAFYDKVAEKLKEAF-NH₂), is a model for class A amphipathic helices of apolipoprotein A-I and other exchangeable lipoproteins. The diffraction method relies upon experimental determinations of absolute scattering-length density profiles along the bilayer normal and the transbilayer distribution of the DOPC double bonds by means of specific bromination, and molecular modeling of the perturbed lipid bilayer (derived using the transbilayer distribution of the double bonds) and the peptide. The diffraction results showed that Ac-18A-NH₂ was located in the bilayer interface and that its transbilayer distribution could be described by a Gaussian function with a 1/e-half-width of 4.5 (± 0.3) Å located 17.1 (± 0.3) Å from the bilayer center, close to the glycerol moiety. Molecular modeling suggested that Ac-18A-NH₂ is helical and oriented generally parallel with the bilayer plane. The helicity and orientation were confirmed by oriented circular dichroism measurements. The width of the Gaussian distribution, a measure of the diameter of the helix, indicated that the Ac-18A-NH₂, helix penetrated the hydrocarbon core to about the level of the DOPC double bonds. Bilayer perturbations caused by Ac-18A-NH₂ were surprisingly modest, consisting of a slight decrease in bilayer thickness with a concomitant shift of the double-bond distribution toward the bilayer center, as expected from a small increase in lipid-specific area caused by the peptide.