FIGURE 1.

Shape structure concept of lipid polymorphism. Lipids with a small polar head have a molecular shape that resembles a truncated cone. They induce a negative curvature strain and favor the organization of membranes into inverted micelles (HII phases) or cubic (bicontinuous) structures. Lipids with a bulky polar head and only one acyl chain have a molecular shape similar to an inverted cone and induce a positive curvature strain in membranes. They favor the formation of tubular (HI) or spheric micelles. Lipids that have similar cross-sectional areas for the polar head and hydrophobic region look like cylinders. They form lamellar phases, with no curvature strain.