Modulation of the polymorphism of the palmitic acid/cholesterol system by the pH

Differential scanning calorimetry, infrared spectroscopy, solid-state ¹³C and ²H NMR spectroscopy, and X-ray diffraction were used to characterize hydrated mixtures of palmitic acid (PA) and cholesterol (chol), at low (5.4) and high (8.5-9.0) pH. The pH affects the ionization state of the fatty acid, and it was found to modulate its interaction with cholesterol, and the architecture of the resulting complexes. At low pH, this system undergoes a transition from a phase-separated crystalline state to a lamellar liquid-ordered (lo) phase, in agreement with previous results (Langmuir 2001, 17, 5587-5594). The apparent pK_a of PA in these conditions is found to be about 8.2. At high pH, when the fatty acid is deprotonated and consequently negatively charged, both components participate in the formation of an lo lamellar structure that was found to be stable over a large temperature range (20-70°C). For 50/50 PA/chol mixtures, only this phase, with a d spacing of approximately 49 Å, is detected. These fluid bilayers can be characterized as having solubilized cholesterol and highly ordered PA hydrocarbon chains. It is thus proposed that the electrostatic repulsion between carboxylate headgroups allows cholesterol molecules to integrate into the palmitate matrix. The sterol promotes the bilayer formation and leads to an ordering of the PA acyl chain, a well-known effect of cholesterol with phospholipids. Therefore, the PA/chol system allows for the production of very simple lo lamellar structures that are pH-sensitive.