Effect of calcium ion on the mechanical properties of lipid bilayer membrane

Elastic properties of dipalmitoyl phosphatidylcholine bilayers were studied by an ultrasonic technique as a function of the concentration of calcium ion. The ultrasonic velocity showed a broad minimum at about 10 mM CaCl2, indicating that the bulk modulus of membrane is smallest at this calcium ion concentration. Since the amount of calcium ions adsorbed to the membrane monotonously increases, calcium ions have two kinds of effects on the phosphatidylcholine membrane, namely softening effect at lower concentration and hardening at higher concentration. It is suggested that the softening is due to some structural change of hydrated water and the hardening is due to chelation of lipid molecules by adsorbed calcium ions. The estimation of bulk modulus and Young's modulus revealed that Young's modulus and rigidity are much smaller than the bulk modulus even in the gel phase.