Résumé
The study of stearic acid spin probe responsiveness to pH variations in a dipalmitoyl phosphatidylcholine (DPPC) matrix in the L
β
′ 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 p
K
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 L
α
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 p
K
a
.