Disposition of an Amphipathic

Alpha-Helix on the Bilayer

 

X-ray diffraction measurements on DOPC multilayers containing the ideally amphipathic 18-residue alpha-helical peptide have permitted to determine the precise location of the helix within the bilayer. Just as for the principal structural groups of the lipid, the thermal motion of the bilayer and the helix cause the trans-bilayer profile of the helix to be Gaussian. The axis of the 18Aa helix, which is parallel to the membrane surface, is located between the mean positions of the glycerol and carbonyl groups. (If the hydrocarbon core were a simple bulk slab of oleoyl chains, the peak of the carbonyl group would define the surface of the slab. The surface of the helix penetrates to about the level of the double-bonds without causing major perturbations in the structure of the bilayer. In terms of the polarity profile amphipathic helix resides at approximately the mid-point of the steep decent of polarity that arises as the headgroup region gives way to the hydrocarbon region.