Calcium and Zinc Differentially Affect the Structure of Lipid Membranes

Interactions of calcium (Ca²⁺) and zinc (Zn²⁺) cations with biomimetic membranes made of dipalmitoylphosphatidylcholine (DPPC) were studied by small angle neutron diffraction (SAND). Experiments show that the structure of these lipid bilayers is differentially affected by the two divalent cations. Initially, both Ca²⁺ and Zn²⁺ cause DPPC bilayers to thicken, while further increases in Ca²⁺ concentration result in the bilayer thinning, eventually reverting to having the same thickness as pure DPPC. The binding of Zn²⁺, on the other hand, causes the bilayers to swell to a maximum thickness, and the addition of more Zn²⁺ does not result in a further thickening of the membrane. Agreement between our results obtained using oriented planar membranes and those from vesicular samples implies that the effect of cations on bilayer thickness is the result of electrostatic interactions, rather than geometrical constraints due to bilayer curvature. This notion is further reinforced by MD simulations. Finally, the radial distribution functions reveal a strong interaction between Ca²⁺ and the phosphate oxygens, while Zn²⁺ shows a much weaker binding specificity.