Application of titration calorimetry to study binding of ions, detergents, and polypeptides to lipid bilayers

Abstract

The binding of ions, the incorporation of detergents and polypeptides into lipid bilayers, and the CMC and heat of micellization of detergents were studied by titration calorimetry. The heat of dissociation of dimyridstoylphosphatidic acid (DMPA⁻+OH⁻⇌DMPA²⁻+H₂O) was investigated as a function of temperature covering the phase transition of singly and doubly charged DMPA. The intrinsic pK₀ for the dissociation was determined from the tiration curves applying the Gouy-Chapman theory. pK₀ decreases with temperature from ca.6.2 at 11°C to 5.4 at 54°C. The temperature dependence of the dissociation enthalpy ΔH_Diss was combined with DSC data on the transition enthalpies ΔH_Trans for DMPA in its two ionization states to construct a complete enthalpy vs. temperature diagram. Titration calorimetry was also used to determine the CMC and the heat of micellization of the detergents SDS, octylglucoside, Na-cholate, and Na-deoxycholate. From the temerature dependence of the CMC and the heat of micellization the thermodynamic functions ΔG and ΔS were determined as a function of temperature. The interaction of Na-deoxycholate with lipid bilayers was studied at a temperature where the heat of mizellization was zero. Complex titration peaks were observed, indicating incorporation reactions on different time scales. The heats of reaction depended on the nature of the phospholipid. Titration calorimetry was also used to study the binding of melitton to lipid vesicles. The sign of the heat of incorporation depended on whether the lipid was in the gel or the liquid-crystalline state and on the chemical nature of the phospholipid.