Electrostatic interactions of alkaline earth cations with 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA) model membranes at neutral and acidic pH

The binding of alkaline earth cations Mg2+, Ca2+, and Sr2+ (M2+) to unilamellar 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA) vesicles was analysed by pH potentiometry, differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC) and FT-IR spectroscopy. The binding of alkaline earth cations induces deprotonation of the DMPA headgroup even at very low concentration of divalent cations (similar to 100 mu M). The amount of deprotonated DMPA was measured by pH potentiometry as a function of divalent cation concentration. The thermotropic phase behaviour of DMPA:M2+ complexes was studied by DSC and FT-IR as a function of pH of the dispersion (pH 7 and pH 3-5). The formation of metastable phases was observed, especially for Ca2+ and Sr2+ at pH 3-5. In unbuffered solutions, the divalent cations bind to single and/or double negatively charged DMPA, leading to the formation of different complexes and changes in the mixing behaviour of the two complexes. At pH 7, all three equimolar lipid/cation mixtures form a very stable, highly ordered 1:1 DMPA:M2+ complex. At lower divalence, the presence of a mixture of 2:1 and 1:1 complexes was observed. FT-IR spectroscopy experiments indicated an ordering of the acyl chains of DMPA after ion binding even in the liquid-crystalline phase and the induction of the dissociation of the second proton from the headgroup induced by Ca2+ or Sr2+ binding at pH 7. With ITC, the binding enthalpy Delta H of Mg2+, Ca2+, and Sr2+ to DMPA model membranes in the gel and in the liquid-crystalline phase was measured. Evidence for dehydration of hydrophobic surfaces due to cation binding was derived from changes in heat capacity.