Phase Behaviors And Phase Transition Mechanisms Of Some Representative Binary Lipid Systems

Phospholipids are one of the most important components of cell membranes.Phase states of phospholipids are of vital importance to the biophysical properties of membranes,hence the detailed studies on phase behaviors and phase transition mechanisms of phospholipids can deepen our understandings on cell membrane properties.For binary lipid systems,except simple thermal phase transitions,lateral phase separation and transbilayer asymmetry phenomen on make the condition closer to a real cell membrane,therefore binary lipid mixtures can be models for the study of cell membranes.The studies on phase behaviors and phase transition mechanisms can be helpful on the way of further understanding of the structural properties of cell membranes and their biological function basis.By using differential scanning calorimetry,spectroscopy?including infrared spectroscopy,fluorescence spectroscopy and circular dichroism?,synchrotron X-ray scattering?both wide-angle and small-angle scattering?and cryogenic transmission electron microscopy,we have studied the phase behaviors and phase transition mechanisms of three typical binary lipid mixtures,which are multilamellar vesicles composed of DPPC and diC8PC,unilamellar vesicles composed of DPPC and DOPS,and non-lamellar phase composed of DOPE and DOPS.We focused on the lateral phase separation and crystallization process in the DPPC-diC8PC binary system,the formation kinetics and mechanism of transbilayer asymmetry in the DPPC-DOPS binary system and the protein-induced phase separation and the lamellar to the non-lamellar phase transition in the DOPE-DOPS binary system.These studies represent phase behaviors of three typical self-assemblies of phospholipids,and provide new insights for further studies.In the work related to DPPC–diC8PC binary system,we have studied mixtures with a series of diC8PC percentages of total lipids.We identified a new phase,named U phase,when the percentage of diC8PC is above 25%.The structural and kinetic properties were studied in detail.In the meantime,we have also focused on the crystallization process of the binary system DPPC–diC8PC and found that when the percentage of diC8PC is above 25%,the crystallization pathway can be altered and U phase behaves as a precursor phase during the crystallization process.In the transbilayer asymmetry study of the DPPC-DOPS binary system,we found that the addition of Ca²⁺induced transbilayer asymmetry of DOPS lipids,which is caused by the change of the flip-flop rate constants in the presence of Ca²⁺as revealed by kinetic studies.The conclusion can be extended to other PS-containing systems and other divalent cations such as Mg²⁺and Zn²⁺.In the study of DOPE-DOPS binary system and its interaction with lysozyme,we found that in the absence of lysozyme,the phase state of DOPE-DOPS binary mixture was inversed hexagonal(H_II)phase,while in the presence of lysozyme,phase separation was observed.The H_II phase transformed into a DOPS-rich lamellar phase and a DOPE-rich H_II phase under the influence of native lysozyme at room temperature.Upon heating,the DOPS-rich lamellar phase transformed into a new H_II phase and this phase transition strongly affected the structures of lysozyme during its thermal denaturation process.