Calorimetric Investigations of Phosphatidic Acid Bilayers

Abstract

AbstractDifferential scanning calorimetry is a sensitive method for studying the phase transition of phospholipid bilayers. Up to now only few calorimetric investigations of the phase transition of charged phospholipids have been carried out [1–3]. We studied the phase behaviour of dihexadecyl‐phosphatidic acid (DHPA) as a function of the pH of the dispersion [4, 5]. Varying the pH induces different degrees of ionization of the phosphatidic acid head group. At high pH the head group has two negative charges leading to a decrease of the transition temperature to 49°C compared to 70°C at pH 9.0 and a decrease in transition enthalpy to 21.8 kJ/mol compared to 31.4 kJ/mol at pH 9.0. Lowering the pH below 9.0 leads to a gradual protonation of singly charged DHPA. With increasing protonation Tm rises to 74°C at pH 3.5 and then drops to 62°C at pH 1.5.The results obtained by calorimetry indicate, that probably two different phases can exist between pH 9.0 and 1.0. At half protonation at pH 3.5 a particular stable phase with high transition temperature and low transition enthalpy (11.3 kJ/mol) can be observed. Complete protonation leads to a phase, which has the same transition temperature as the melting point of dry crystalline protonated DHPA.The observed variation of the transition enthalpy with pH cannot be explained by assuming, that only the electrostatic potential of the bilayer surface is changed [6]. An influence of the degree of ionization of the head group on the hydrocarbon chain interactions has to be taken into account.From these experiments conclusions can be drawn upon the intermolecular interactions between the head groups of phosphatidic acids.