Interaction of alkaline earth cations with the negatively charged phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol:: A differential scanning and isothermal titration calorimetric study

The interaction of the alkaline earth cations Mg2+, Ca2+, and Sr2+ with 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), dispersed in 0.1 M NaCl at pH 7, was investigated by differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC). Both calorimetric methods revealed that the interaction of Mg2+,and Sr2+ With DMPG is of similar nature but different from the behavior of Ca2+. In the first heating scans of DMPG:Mg2+ and DMPG:Sr2+ complexes, transition peaks at 31-35 and 60 degrees C are observed. The higher transition peak disappears in consecutive heating and cooling cycles and only the transition at 31-35 degrees C remains. The transition at lower temperature occurs from a metastable gel phase into a liquid-crystalline phase, whereas the high-temperature transition occurs from the stable quasicrystalline lamellar phase. The thermotropic behavior of DMPC:Ca2+ complexes is different. The high-temperature transition from the stable state is centered at 80 degrees C. Cooling induces a metastable gel phase at low temperatures. However, this converts upon heating into the stable gel phase at a temperature of 40-50 degrees C, which then melts at 80 degrees C. From the ITC experiments, the binding enthalpies and the change in heat capacity (Delta c(p),) for the binding reaction was obtained. Delta c(p), has contributions arising from hydrophobic (Delta c(p)(hydrophobic)) and/or hydrophilic (Delta c(p)(hydrophilic)) hydration or dehydration of the bilayer. The ITC results showed that a similar hydrophobic dehydration of the metastable gel phase was induced in the presence of Mg2+ and Sr2+, whereas Ca2+ binding leads to a much stronger dehydration of hydrophobic regions. In the stable or metastable Liquid-crystalline phase, this dehydration effect is strongly reduced for all complexes with divalent cations. On the basis of the DSC and ITC results, enthalpy-temperature diagrams were constructed that illustrate the enthalpies of the different metastable states and the enthalpic changes observed in the DSC and ITC experiments.