Structural and morphological transition of long-chain phospholipid vesicles induced by mixing with short-chain phospholipid.

Effects of a short-chain phospholipid, dihexanoylphosphatidylcholine (DHPC), on the structure and morphology of membrane assemblies of a long-chain phospholipid, dimyristoylphosphatidylcholine (DMPC), were examined by fluorescence spectroscopy, differential scanning calorimetry (DSC), and cryogenic transmission electron microscopy (cryo-TEM). It was found by fluorescence measurements that DHPC affects on the gel and liquid crystalline state of DMPC vesicle membranes in different ways. Further, the result of DSC suggested that, along the transition process from DMPC vesicle to DMPC-DHPC mixed micelle, there are at least three different concentration regions which are characterized by the individual variation pattern of the transition temperature and enthalpy change. The cryo-TEM micrographs demonstrated the formation of thread-like assemblies in the second region and the coexistence of the assemblies and spherical micelles in the third region. Thus, it was concluded that the structural transition from DMPC vesicle to DMPC-DHPC mixed micelle could occur in a stepwise manner through the formation of the thread-like assembly, which cannot be described by the three-stage model of vesicle to micelle transition.