Effect ofn-alkanes and peptides on the phase equilibria in phosphatidylcholine—water systems

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The phase equilibria in phosphatidylcholine (PC)-n-alkane-2H2O systems have been studied to elucidate the driving forces for the transition between a lamellar liquid-crystalline (L α) phase and a reversed hexagonal (H II) phase. A tentative phase diagram for the system dioleoyl-PC (DOPC)-n-dodecane-2H2O was determined. DOPC forms an L α phase up to at least 90°C in excess water. However, an H II phase was formed at room temperature at both low and high water concentrations in DOPC-n-dodecane-2H2O mixtures. The phase equilibria were also studied in PC-n-dodecane-2H2O systems containing PC with different degrees of acyl chain unsaturation. The water and dodecane concentrations required to induce the formation of an H II (or isotropic) phase increase in the order dilinoleoyl-PC ∼ DOPC < 1-palmitoyl-2-oleoyl-PC < dipalmitoyl-PC. The effect of n-alkanes with different chain lengths (C8–C20) on the phase equilibria in DOPC-n-alkane-2H2O mixtures was studied. Although the number of alkane carbon atoms added per DOPC molecule was kept constant, the ability of the alkanes to promote the formation of an H II phase was strongly chain length dependent; the ability decreased when going from octane to eicosane. Finally, some PC-peptide-2H2O systems were investigated. Gramicidin (hydrophobic) had a similar influence on the phase equilibria as the alkanes. Melittin (amphiphilic) induced the formation of an isotropic phase, while insulin and duramycin (water soluble) had no, or a very limited, ability to induce a non-lamellar phase, respectively. Our results are discussed in the light of simple physical models dealing with the self-assembly of amphiphiles.