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 (~ 100 µM). 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 ΔH 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.