Stalk formation as a function of lipid composition studied by X-ray reflectivity

• We have used X-ray reflectivity to study the formation of stalks in lipid bilayers.
• There is a critical osmotic pressure at which the stalk phase appears.
• The work to reach the stalk phase by dehydration is determined.
• The transition is strongly influenced by membrane composition.

We have investigated the structure and interaction of solid-supported multilamellar phospholipid bilayers in view of stalk formation as model systems for membrane fusion. The multi-component bilayers were composed of ternary and quaternary mixtures, containing phosphatidylcholines, phosphatidylethanolamines, sphingomyelin, cholesterol, diacylglycerol, and phosphatidylinositol. Analysis of the obtained electron density profiles and the pressure–distance curves reveals systematic changes in structure and hydration repulsion. The osmotic pressure needed to induce stalk formation at the transition from the fluid lamellar to the rhombohedral phase indicates how membrane fusion properties are modified by bilayer composition.

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