Formation of unilamellar dipalmitoylphosphatidylcholine vesicles promoted by Ca2+ ions: A small‐angle neutron scattering study

Dipalmitoylphosphatidylcholine (DPPC) was hydrated in 0.2-60 mM solution of CaCl2 in heavy water and thor- oughly homogenized by freezing-thawing process. Small-angle neutron scattering (SANS) shows formation of unilamellar vesicles in the range 1-60 mM of CaCl2. From the Kratky-Porod plot ln(I(Q)Q 2 )v s.Q 2 of SANS intensity I(Q) in the range of scattering vectors Q corresponding to the interval 0.001 A −2 Q 2 0.006 A −2 , the vesicle bilayer radius of gyration Rg and the bilayer thickness parameter dg were obtained. The structure of the bilayer displays different behavior for the gel phase and the liquid-crystalline phase: In the gel phase (at 20 ◦ C), the values of dg indicate nonlinear changes in the lipid bilayer thickness, with a maximum at ∼ 5m M CaCl 2. In the liquid-crystalline phase (at 60 ◦ C), the parameter of the lipid bilayer thick- ness dg = 43.2 ± 0.3 A is constant within the concentration range 1 cCa 40 mM. Vesicles prepared at 60 mM CaCl2 show within experimental error, the same values of dg as pure DPPC unilamellar vesicles prepared by extrusion using polycarbonate