MIXING BEHAVIOR OF SATURATED SHORT-CHAIN PHOSPHATIDYLCHOLINES AND FATTY-ACIDS - EUTECTIC POINTS, LIQUID AND SOLID-PHASE IMMISCIBILITY, NON-LAMELLAR PHASES

Phase diagrams of the hydrated pseudo-binary mixtures dilauroylphospha tidylcholine (DLPC)/lauric acid (LA) and dimyristoylphosphatidylcholin e (DMPC)/myristic acid (MA) have been constructed using high-sensitivi ty DSC and time-resolved X-ray diffraction. They are of a different ty pe compared to those of the longer chain phosphatidylcholine (PC)/fatt y acid (FA) mixtures, the latter being of maximum azeotropic point typ e. Eutectic points were distinguished in the phase diagrams, at 75 mol % MA and 49 degrees C for the DMPC/MA mixture, and at ca. 67 mol% LA a nd 29 degrees 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 regio ns (2-4 degrees C) of liquid-liquid phase immiscibility exist at compo sitions with slightly prevailing fatty acid molar content. For instanc e, at 67 mol% MA, a phase separation between L-alpha phase enriched in DMPC and H-II phase enriched in MA takes place in the temperature ran ge 51-55 degrees C. Solid phase immiscibility is detected between 60 a nd 90 mol%, fatty acid. The studied PC/FA mixtures form compound subge l polymorphic phases (one in the DMPC/MA mixture, with DMPC/MA 1:2 mol ar stoichiometry, and two in the DLPC/LA mixture, with about 40 and 60 mol% LA, respectively) upon low-temperature equilibration. In the liq uid crystalline phase region, non-lamellar phases dominate the phase d iagrams, 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 cu bic 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 cubi c phase of space group Fd3m form in the two PC/FA mixtures. At lower F A content (<75 mol%), at least three cubic phases (Ia3d, Pn3m, Im3m) f orm. The order of their appearance with increasing temperature varies with the PC/FA ratio. They exist either as single phases or concurrent ly to the H-II phase. (C) 1997 Elsevier Science Ireland Ltd.