Effect of PIP2 on Bilayer Structure and Phase Behavior of DOPC: An X‐ray Scattering Study

Phosphatidylinositol 4,5‐bis‐phosphate (PIP₂) is an important lipid in regulation of several cellular processes, particularly membrane fusion. We use X‐ray diffraction from solid‐supported multilamellar 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC)/PIP₂ samples to study changes in bilayer structure and the lyotropic phase behavior induced by physiologically relevant concentrations of PIP₂. Electron‐density profiles reconstructed from X‐ray reflectivity measurements indicate that PIP₂ strongly affects structural parameters such as lipid head‐group width, bilayer thickness, and lamellar repeat spacing of DOPC bilayer stacks. In addition, at lower degrees of hydration, a few molar per cent of PIP₂ facilitates stalk‐phase formation and also leads to formation of a hexagonal phase, which is not observed in pure DOPC. These results indicate that the role of PIP₂ in membrane fusion could be, in part, due to its effect on the properties of the lipid bilayer matrix. Furthermore, coexistence of two lamellar phases with different lattice constants is observed in single‐component PIP₂ samples.