Structural changes in dipalmitoylphosphatidylcholine bilayer promoted by Ca2+ ions: a small-angle neutron scattering study

Small-angle neutron scattering (SANS) curves of unilamellar dipalmitoylphosphatidylcholine (DPPC) vesicles in 1–60 mM CaCl2 were analyzed using a strip-function model of the phospholipid bilayer. The fraction of Ca2+ ions bound in the DPPC polar head group region was determined using Langmuir adsorption isotherm. In the gel phase, at 20 °C, the lipid bilayer thickness, dL, goes through a maximum as a function of CaCl2 concentration (dL = 54.4 Å at ∼2.5 mM of CaCl2). Simultaneously, both the area per DPPC molecule AL, and the number of water molecules nW located in the polar head group region decrease (ΔAL = AL(DPPC) − AL(DPPC+Ca) = 2.3 Å2 and Δn = nW(DPPC) − nW(DPPC+Ca) = 0.8 mol/mol at ∼2.5 mM of CaCl2). In the fluid phase, at 60 °C, the structural parameters dL, AL, and nW show evident changes with increasing Ca2+ up to a concentration cCa2+≤10 mMcCa2+≤10 mM. DPPC bilayers affected by the calcium binding are compared to unilamellar vesicles prepared by extrusion. The structural parameters of DPPC vesicles prepared in 60 mM CaCl2 (at 20 and 60 °C) are nearly the same as those for unilamellar vesicles without Ca2+.