Small concentrations of alamethicin induce a cubic phase in bulk phosphatidylethanolamine mixtures

Under normal conditions, excess water dispersions of liquid crystalline 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE) are known to convert from a liquid crystalline lamellar (L(alpha)) to inverse hexagonal (H-II) phase at about 60 degrees C. The nonlamellar phase behavior of lipid systems is also known to depend on the monolayer spontaneous curvature. The single-channel activity of alamethicin in black lipid bilayer membranes has been shown to be dependent upon the lipid composition of the membrane. Since the monolayer spontaneous curvature properties (e.g., the monolayer spontaneous curvature, curvature coefficients and bilayer thickness) vary with lipid composition, the single-channel activity of alamethicin presumably also correlates with the monolayer spontaneous curvature properties, Accordingly, we reasoned that if alamethicin couples to the curvature properties of a lipid film, then the curvature properties must, in turn, be perturbed by the presence of alamethicin and that this perturbation should be observable in the lipid phase behavior. Here X-ray diffraction and NMR are used to show that the presence of as little as 1% alamethicin introduces a large region of cubic phase into the thermal phase diagram. This suggests that perturbation of the nonlamellar phase behavior of a lipid system may be a method to survey different channel-forming molecules for possible behavior that indicates that the ion channel is sensitive to the monolayer spontaneous curvature properties.