THERMODYNAMICS OF ION BINDING TO PHOSPHATIDIC-ACID BILAYERS - TITRATION CALORIMETRY OF THE HEAT OF DISSOCIATION OF DMPA

The heat of dissociation of the second proton of 1,2-dimyristoylphosphatidic acid (DMPA) was studied as a function of temperature using titration calorimetry. Tbe dissociation of the second proton of DMPA was induced by addition of NaOH. From the calorimetric titration experiment, the intrinsic pK0 for the dissociation reaction could be determined by applying the Gouy-Chapman theory. pK0 decreases with temperature from ca. 6.2 at 11-degrees-C to 5.4 at 54-degrees-C. From tbe total heat of reaction, the dissociation enthalpy, DELTA-H(diss), was determined by subtracting the heat of neutralization of water and the heat of dilution of NaOH. In the temperature range between 2 and 23-degrees-C, DELTA-H(diss), is endothermic with an average value of ca. 2.5 kcal.mol-1 and shows no clear-cut temperature dependence. In tbe temperature range between 23 and 52-degrees-C, DELTA-H(diss) calculated after subtraction of the heat of neutralization and dilution is not the true dissociation enthalpy but includes contributions from the phase transition enthalpy, DELTA-H(trans), as the pH jump induces a transition from the gel to the liquid-crystalline phase. Tbe DELTA-C(p) for tbe reaction enthalpy observed in this temperature range is positive. Above 53-degrees-C, the pH jump induces again only the dissociation of the second proton, and the bilayers stay in the liquid-crystalline phase. In this temperature range, DELTA-H(diss) seems to decrease with temperature. The thermodynamic data from titration calorimetry and differential scanning calorimetry as a function of pH can be combined to construct a complete enthalpy-temperature diagram of DMPA in its two ionization states.