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

Abstract

Dipalmitoylphosphatidylcholine (DPPC) was hydrated in 0.2–60 mM solution of CaCl₂ in heavy water and thoroughly 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)Q²] vs. Q² of SANS intensity I(Q) in the range of scattering vectors Q corresponding to the interval 0.001 Å⁻²≤Q²≤0.006 Å⁻², the vesicle bilayer radius of gyration R_g and the bilayer thickness parameter d_g 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 d_g indicate nonlinear changes in the lipid bilayer thickness, with a maximum at ~5 mM CaCl₂. In the liquid-crystalline phase (at 60°C), the parameter of the lipid bilayer thickness d_g=43.2±0.3 Å is constant within the concentration range 1≤c_Ca≤40 mM. Vesicles prepared at 60 mM CaCl₂ show within experimental error, the same values of d_g as pure DPPC unilamellar vesicles prepared by extrusion using polycarbonate filter with pores of diameter 500 Å.