Stearic acid pH-dependent reactivity in dipalmitoyl phosphatidylcholine model membranes in Lβ′ gel phase. An electron spin resonance and differential scanning calorimetry experiment

Abstract

The study of stearic acid spin probe responsiveness to pH variations in a dipalmitoyl phosphatidylcholine (DPPC) matrix in the Lβ′ gel phase (at 25 °C, below the pre-transition termperature) has been carried out by e.s.r. spin labelling. An original approach was followed, taking into account both the changes in lipid–water electric surface potential, induced by proton concentrations and intrinsic polar head charges, and the change of the lipid–water partition coefficient of the spin probes, induced by protonation or ionization of the carboxylic group. Whereas differential scanning calorimetry illustrates pH-independent thermodynamic properties of the DPPC bilayers in the pH range 4.5–9, the e.s.r. lineshape experimental parameter modifications vs. bulk pH allow the determination of the apparent pK_a(at the lipid–water interface) of the probes. They also provide information about the molecular-dynamic environment and thus the location of the nitroxide group within the lipid bilayer in gel phase, in comparison with the Lα fluid phase previously described. From this information, a theoretical model of interaction can be constructed, the analytical description of which allows descrimination between the surface potential component and the ‘hydrophobic’ potential component of the probe; both of these contribute to the observed probe pK_a.