Calcium ion binding between lipid bilayers: the four-component system of phosphatidylserine, phosphatidylcholine, calcium chloride, and water.

Ca2+ binding between lamellae of phosphatidylserine (PS) and phosphatidylcholine (PC) gives rise to a rigid phase of Ca(PS)2. When aqueous Ca2+, hydrated PS/PC, and Ca(PS)2 coexist at equilibrium, the aqueous Ca2+ concentration is invariant and is characteristic of the PS/PC ratio. This characteristic Ca2+ concentration is 0.040 microM for palmitoyloleoylphosphatidylserine without PC and increases as the inverse square of the PS mole fraction at high PS concentration (Raoult's law) and as the inverse square of the PS mole fraction multiplied by a constant at low PS concentration (Henry's law). For example, for palmitoyloleoylphosphatidylserine/palmitoyloleoylphosphatidylcholi ne = 0.6/0.4 or 0.2/0.8, this characteristic Ca2+ concentration is about 0.1 or about 6 microM, respectively. These observations at constant temperature are summarized in a quaternary phase diagram for the four-component system CaCl2/PS/PC/water.