Binding of calcium to phosphatidylcholine–phosphatidylserine membranes

We study the adsorption of calcium to charged lipid membranes composed of phosphatidylcholine (PC) – phosphatidylserine (PS) mixtures. An elaborate set of techniques is used to characterize the binding process for varying surface charge of the membrane and carefully chosen experimental conditions. Comparing results obtained from different approaches we aim at unifying and explaining scatter in literature data and giving a coherent picture of the phenomena. The presence of phopshotidylserine in the membrane strongly increases the calcium concentration at the vicinity of the membrane effectively enhancing the binding to the neutral lipid. Calcium binding lowers the surface potential as observed from ζ-potential measurements. The isotherms of the adsorption process as measured with isothermal titration calorimetry (ITC) show that the binding process is endothermic, counterintuitively to what is expected for Coulomb interactions. The process is entropy driven and related to liberation of water molecules from the hydration shells of the ion and the lipid headgroups. In addition, the data shows the presence of a second process, apart from binding, which occurs at relatively low calcium concentrations. Differential scanning calorimetry (DSC) measurements in the corresponding concentration interval discards the hypothesis that this process is related to phase separation of lipids in presence of calcium but rather to condensation effect of the calcium ions. Turbidity measurements on vesicle solutions support this observation. Finally, the data suggest that the presence of phosphatidylserine in the bilayers can finely tune the interaction of calcium with lipid membranes and give rise to a spectrum of phenomena strongly dependent on the ion concentration.