Molecular scale spectroscopy and simulation of stressed lipid bilayers

Lipid-lipid interactions play a very important role in mediating membrane protein activity, through alterations in membrane physical properties or lateral phase separation. However, the underlying physical principles governing lipid organization remain unclear and there are very few experiemtnal tools to measure organization and structural properties on the molecular scale. In this work, the effect of molecular packing on some of the key membrane properties including bilayer thickness, acyl chain conformational flexibility, diffusive mobility, and electrostatic potential, and dynamics of DiI, was characterized using atomistic computational simulations. Lateral tension increases acyl chain interdigitation, and lateral/rotational mobility, and decreases hydrophobic thickness and electrostatic potential. DiI dynamics indicated that it is likely to be a good reporter for changes in bilayer fluidity. Combining the diffusion coefficient and fluorescence lifetime of DiI, the membrane microviscosity near the headgroup and tail regions was measured. Further, it is shown for the first time that fluorescence lifetime of DiI can report free-area in lipid bilayers to a precision of 1A2. This property was used to measure thermal undulations in giant vesicles and nanoliposomes from fluctuations in lifetime of DiI. The methods developed here enable characterization of membrane mechanics, which may aid in identifying the role of membrane in endothelial mechanotransduction. Finally, a decrease in hydrophobic thickness induced by non-lipid amphiphiles was found to promote raft formation, and vice versa, emphasizing the role of hydrophobic mismatch. This finding brings a new perspective to how non-lipid amphiphiles regulate raft formation in cell membranes.