Dipalmitoylphosphatidylcholine (DPPC) was hydrated in 0.2–60 mM solution of CaCl2in 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.Q2of SANS intensityI(Q)in the range of scattering vectorsQcorresponding to the interval 0.001 Å−2≤Q2≤0.006 Å−2, the vesicle bilayer radius of gyrationRgand the bilayer thickness parameterdgwere 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 ofdgindicate 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 thicknessdg=43.2±0.3 Å is constant within the concentration range 1≤cCa≤40 mM. Vesicles prepared at 60 mM CaCl2show within experimental error, the same values ofdgas pure DPPC unilamellar vesicles prepared by extrusion using polycarbonate filter with pores of diameter 500 Å.
Response to Energetics of Fluctuations in Lipid Bilayer Thickness by I. R. Miller
