Phase behavior of a binary lipid system containing long- and short-chain phosphatidylcholines

Abstract

Bilayered micelles, or so-called bicelles, are generally made of long- and short-chain lipids. They are extensively used as model membranes to study the structure of membrane-associated peptides or proteins and their interactions with membranes. However, the phase behavior of lipid mixtures composed of long- and short-chain lipids, especially at low temperatures, is still not very clear. In this work, the most commonly used long-chain lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and a short-chain lipid, 1,2-dioctanoyl-sn-glycero-3-phosphocholine (diC8PC), were selected as a bicellar model to study their phase behavior. Over the whole range of DPPC/diC8PC molar ratios (q) studied in this work, a lamellar crystalline phase (L_c′) enriched in DPPC was found to be the most stable phase at 5 °C, together with a diC8PC-enriched micelle phase. Interestingly, a metastable phase, named the U phase in this work, was observed in the mixtures with a DPPC/diC8PC molar ratio between 1 and 4. The metastable U phase was found to be lacking in long-range order in the direction of the bilayer surface normal, but bearing a different “crystalline phase-like” hydrocarbon chain packing mode, in comparison with the lamellar crystalline phase. The kinetic properties of the U phase were also studied in detail, and it was found that the phase acts as a precursor phase in the process of forming the most stable crystalline phase. This work deepens our understanding of lipid crystallization behavior, and is also a step forward towards a more detailed picture of the phase behavior of lipid mixtures composed of long- and short-chain lipids.