Lipid spatial alignment and motional dynamics: The two important factors that determine membrane properties

Membranes are formed through lipid self-assembly. The properties of the self-assembled membrane are determined critically by lipid spatial alignment and lipid chain motional dynamics. The goal of this dissertation research was to obtain both lipid alignment information and motional dynamics information in order to have a fuller understanding of lipid membrane.;The study on lipid spatial alignment has been conducted in a lipid multilayer system. A new methodology utilizing phase contrast and confocal reflection microscopy has been developed to characterize the partially ordered layer system based on light-lipid interactions. Four types of lipid domain structures, oily streaks, polygonal arrays, bubble domains and stripe domains were visualized and analyzed in detail by phase contrast and confocal reflection microscopy. The images obtained through phase contrast and confocal reflection microscopy have better correlation with lipid alignment models than those through the conventional polarizing microscopy. The complementary nature of polygonal arrays has been revealed by confocal reflection microscopy. Both confocal reflection microscopy and phase contrast microscopy have been used to characterize the stripe to bubble domain transition processes.;Under thermal energy, the formation of a bubble domain is through a continuous contracting process of a stripe domain. Under hydrodynamic flow, a bending stripe corresponding to the activation state of the domain transition process has been clearly visualized. These experimental observations directly showed that the stripe-to-bubble domain transition process is a typical first order phase transition process. The domain transition has been further analyzed in terms of modulated phase based on the framework of competing interactions.;A novel membrane probe with pyrene moieties covalently attached to the end of both lipid chains and a positive charge at the lipid head group has been utilized to study membrane dynamics. This probe in isotropic solvents forms a strong intramolecular excimer through the trans-gauche isomerization process. The deviation of power dependence of excimer formation rate on solvent viscosity from unity in diffusive limit was analyzed in great detail in terms of reaction dynamics theory in solution phase and found to be due to the reactive mode coupling to non-reactive modes. The mode coupling and long barrier crossing and re-crossing time in the diffusive limit enable the reaction to choose a reaction pathway with least friction, which, in turn, enhances the excimer formation.;This novel membrane probe was also incorporated into DPPC large unilamellar vesicles. Based on fluorescence measurement, the enthalpy of main phase transition at low pH is significantly reduced compared to the enthalpy at neutral pH condition, which is in excellent agreement with the measurement based on differential scanning calorimeter. Analyzing the magnitude of E/M ratio and the apparent activation energy of the excimer formation under two pHs indicates that the membrane net surface charge strongly affects lipid chain mobility. The enthalpy reduction of main phase transition is the direct consequence of lipid chain mobility change.