Mixing behavior of saturated short-chain phosphatidylcholines and fatty acids: Eutectic points, liquid and solid phase immiscibility, non-lamellar phases

Abstract

Phase diagrams of the hydrated pseudo-binary mixtures dilauroylphosphatidylcholine (DLPC)/lauric acid (LA) and dimyristoylphosphatidylcholine (DMPC)/myristic acid (MA) have been constructed using high-sensitivity DSC and time-resolved X-ray diffraction. They are of a different type compared to those of the longer chain phosphatidylcholine (PC)/fatty acid (FA) mixtures, the latter being of maximum azeotropic point type. Eutectic points were distinguished in the phase diagrams, at 75 mol% MA and 49°C for the DMPC/MA mixture, and at ca. 67 mol% LA and 29°C for the DLPC/LA mixture. Regions of liquid–liquid and solid–solid phase separation have been located according to the shape of the phase diagrams and observed by X-ray diffraction. Limited regions (2–4°C) of liquid–liquid phase immiscibility exist at compositions with slightly prevailing fatty acid molar content. For instance, at 67 mol% MA, a phase separation between L α phase enriched in DMPC and H II phase enriched in MA takes place in the temperature range 51–55°C. Solid phase immiscibility is detected between 60 and 90 mol% fatty acid. The studied PC/FA mixtures form compound subgel polymorphic phases (one in the DMPC/MA mixture, with DMPC/MA 1:2 molar stoichiometry, and two in the DLPC/LA mixture, with about 40 and 60 mol% LA, respectively) upon low-temperature equilibration. In the liquid crystalline phase region, non-lamellar phases dominate the phase diagrams, especially in their fatty acid-rich part. With increasing FA content, the nonlamellar phases arrange in the sequence: bicontinuous cubic phases (Ia3d, Pn3m, Im3m)→hexagonal phase (H II)→micellar cubic phase (Fd3m)→isotropic phase (I). With the eutectic composition (75 mol% MA), only H II phase is detected above the melting transition in the DMPC/MA mixture. At higher FA content (85 mol%), micellar cubic phase of space group Fd3m form in the two PC/FA mixtures. At lower FA content (<75 mol%), at least three cubic phases (Ia3d, Pn3m, Im3m) form. The order of their appearance with increasing temperature varies with the PC/FA ratio. They exist either as single phases or concurrently to the H II phase.