Phase equilibria in DOPC/DPPC: Conversion from gel to subgel in two component mixtures

Biological membranes contain a mixture of phospholipids with varying degrees of hydrocarbon chain unsaturation. Mixtures of long chain saturated and unsaturated lipids with cholesterol have attracted a lot of attention because of the formation of two coexisting fluid bilayer phases in such systems over a broad range of temperature and composition. Interpretation of the phase behavior of such ternary mixtures must be based on a thorough understanding of the phase behavior of the binary mixtures formed with the same components. This article describes the phase behavior of mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) with 1,2-di-d31-palmitoyl-sn-glycero-3-phosphocholine (DPPC) between −20 and 50 °C. Particular attention has been paid to the phase coexistence below about 16 °C where the subgel phase appears. The changes in the shape of the spectrum (and its spectral moments) during the slow transformation process leads to the conclusion that below 16 °C the gel phase is metastable and the gel component of the two-phase mixture slowly transforms to the subgel phase with a slightly different composition. This results in a line of three-phase coexistence near 16 °C. Analysis of the transformation of the metastable gel domains into the subgel phase using the nucleation and growth model shows that the subgel domain growth is a two dimensional process.

Phase equilibria in DOPC/DPPC: Conversion from gel to subgel in two component mixtures Journal Articles