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 degrees C, the lipid bilayer thickness, d(L), goes through a maximum as a function of CaCl2 concentration (d(L)=54.4 angstrom at similar to 2.5 mM of CaCl2). Simultaneously, both the area per DPPC molecule A(L), and the number of water molecules n(W) located in the polar head group region decrease (Delta A(L) = A(L(DPPC)) -A(L(DPPC+Ca)) =2.3 angstrom(2) and Delta n=n(W(DPPC)) - n(W(DPPC+Ca)) =0.8 mol/mol at similar to 2.5mM of CaCl2). In the fluid phase, at 60 degrees C. the structural parameters dL, AL, and nw show evident changes with increasing Ca2+ up to a concentration c(Ca2+) <= 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 degrees C) are nearly the same as those for unilamellar vesicles without Ca2+. (C) 2008 Elsevier Ireland Ltd. All rights reserved.