Phase separation behavior of binary mixture of photopolymerizable diacetylene and unsaturated phospholipids in liposomes

Abstract

Photopolymerizable lipids, such as diacetylene lipids have attracted much attention for their ability to stabilize lipid bilayer structure. In this study, we investigated the phase separation behavior of a lipid bilayer containing 1,2‑bis(10,12‑tricosadiynoyl)‑sn‑glycero‑3‑phosphocholine (DiynePC). The phase separation behavior of liposomes composed of DiynePC and the non-polymerizable lipid, 1,2‑dioleoyl‑sn‑glycero‑3‑phosphocholine (DOPC) was evaluated using a fluorescence resonance energy transfer (FRET) assay during both the cooling and heating processes. The polymerization behavior and orientation of DiynePC was evaluated by measuring the absorbance contributed by polymerized diacetylene groups. The main phase transition temperature (Tm) and orientation temperature (To) of DiynePC were determined from differential scanning calorimetry (DSC) and absorbance measurements. As a comparison, liposomes composed of 1,2‑dipalmitoyl‑sn‑glycero‑3‑phosphocholine (DPPC), which has a similar Tm as DiynePC, but a dissimilar To, were evaluated as well as, DiynePC-containing liposomes. The DPPC/DOPC liposomes showed phase separation at the Tm of DPPC during both the cooling and heating processes. On the other hand, the phase separation of the DiynePC/DOPC liposomes occurred near To of DiynePC but slightly increased during the cooling process. Interestingly, once the liposomes were cooled to a much lower temperature than the To, the phase separation was promoted by the subsequent heating process, and reached a maximum state of the phase separation near the To. These results indicated that the phase separation of DiynePC is affected by both the orientation state and thermal molecular motion.