Origin of calcium-induced minimum in bulk compressional modulus of lipid membranes. Configurational entropy of adsorbed Ca2+

Addition of Ca2+ to a dipalmitoylphosphatidylcholine lamellar system decreases the bulk compressional modulus (increases compressibility) of the membrane (S. Aruga, R. Kataoka and S. Mitaku, Biophys. Chem. 21 (1985) 265). The bulk modulus was reported to show a minimum value at 10 mM Ca2+ within the temperature range 20-45 degrees C. In the present report, the occurrence of this minimum in the bulk modulus is explained quantitatively as a result of fluctuation in the number of Ca2+ adsorbed onto the lipid bilayer surface. From this theory, the change in apparent molal volume of Ca2+ upon surface adsorption is estimated to be 5.7 cm3 mol-1, which appears to be a reasonable value. The number of adsorbed Ca2+ at the concentration where the bulk modulus assumes the minimum value is half of the number of allowable adsorption sites on lipid membranes. The configurational entropy of the adsorbed Ca2+ attains a maximum at the minimum point.