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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 …