Curved Lipid Bilayers: Structure, Dynamics, Phase Properties and Surface Electrostatics

Abstract

A growing body of experimental data suggests that at least some of membrane-anchored and membrane-associated proteins are capable of sensing the membrane curvature. Further, highly curved lipid bilayers and small vesicles are involved in such important cellular processes as membrane fusion, endo- and exocytosis, and tubules' formation. Finally, Golgi apparatus represents an example of highly curved lipid structure. While significance of membrane curvature in cellular regulatory processes is emerging, limited data exist on biophysical properties of highly curved lipid bilayers. Here we summarize results of differential scanning calorimetry and spin labeling EPR studies of unilamellar vesicles (SUV) with average diameter ranging from 200 to 30 nm. Analysis of DSC data at multiple scan rates revealed broadening and shifts of the main phase transition of DMPC from ca. 22.9 to 23.6 oC. This observation is consistent with bilayer compression and an increase in local order parameter reveled by EPR and oxygen accessibility measurements. To assess the surface electrostatics of lipid vesicles we employed EPR of a recently introduced phospholipid (IMTSL-PTE) bearing a pH-sensitive nitroxide covalently attached to the lipid head group (Biophys. J. 2013, 104: 106). The magnitude of the negative surface electrostatic potential, Ψ, for POPG increased from −137 to −167 mV upon decrease in the vesicle diameter from 107 to 31 nm even though zeta-potentials were identical. This effect could be again rationalized by increase in lipid packing upon increase in curvature for the bilayer in fluid phase. However, the effect vanished for the gel phase. We conclude that biologically relevant fluid bilayer phase allows for a larger variability in the lipid packing density in the lipid polar head group region than a more ordered gel phase. Supported by U.S. DOE Contract DE-FG02-02ER15354.