Surface Electrostatics Associated with Lipid Bilayer Curvature

Spontaneous lipid bilayer bending and related curvature are being recognized as an essential mechanism associated with many cellular functions. Experimental observations indicate that tubular vesicles of just 50 to 100 nm in diameter are commonly formed within the Golgi body and also between the endoplasmic reticulum and the Golgi. While changes in various conditions may promote the tubulation of the Golgi membrane, detailed understanding of basic biophysical processes beyond the bilayer bending are missing in the literature and so are convenient models of highly curved lipid tubules. Here we report a systematic study of a surface potential for both small unilamellar monodisperse lipid vesicles (SUMV) with diameters ranging from 30 to 100 nm and also lipid tubules that are stabilized by confining these structures within rigid homogeneous nanopores of similar diameters. For example, for SUMVs composed of negatively charged lipids the magnitude of the surface potential increased with bilayer bending from ca. −106 mV for 100 nm SUMV to −166 mV for 30 nm SUMV. These measurements were carried out by spin probe EPR method using recently synthesized lipids having pH-reporting nitroxides covalently tethered to the lipid polar head. Parallel differential scanning calorimetry experiments indicated the presence of at least two components within the lipid phase. These phase components were characterized by measurably different phase transition temperatures and correlation times of the lipid thermal relaxation. Overall, the data indicate that the bilayer bending affects the local electrostatic potential and lipid fluctuation properties in a rather large degree and is likely associated with a mechanism for cellular machinery function. Supported by U.S. DOE Contract DE-FG02-02ER15354.