On the nature of calcium ion binding between phosphatidylserine lamellae

Ca2+ binding between phosphatidylserine (PS) lamellae gives rise to a phase with the composition Ca(PS)2. When aqueous Ca2+, hydrated PS, and Ca(PS)2 coexist at equilibrium, the aqueous Ca2+ concentration is invariant. At Ca2+ concentrations below this critical value, no binding of Ca2+ to PS is detected. Above this value, Ca2+ binds to PS to form Ca(PS)2. The invariant Ca2+ concentration is 0.14 microM for palmitoyloleoylphosphatidylserine (POPS) and 3.0 microM for dioleoylphosphatidylserine (DOPS). For the mixed acyl chain PS derived from bovine brain (BBPS) this Ca2+ concentration ranges from 0.25 to 0.7 microM. The observed phase behavior is described by the phase rule for the three-component system of water, Ca2+, and PS, with temperature and pressure constant. In order for Ca2+ to bind between PS lamellae to form the Ca(PS)2 phase, the aqueous Ca2+ concentration must be supersaturated. The equilibrium Ca2+ concentration is determined by dissolving Ca(PS)2 by use of Ca2+ chelators.