Voltage-Tuning of the Structure of a Lipid Monolayer

Abstract

The membrane potential can alter the local membrane structure as a result of the response of a charged or zwitterionic lipid to the local electric field. We have investigated the variation of lipid packing in response to interfacial electric potential differences across the monolayer that vary from −0.12 V to +0.28 V. Controlled variation of the potential difference was imposed on an SOPC (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) monolayer at 1,2-dichloroethane/water interfaces by the use of a 4-electrode liquid/liquid electrochemical cell. X-ray reflectivity and interfacial tension measurements reveal that the lipid layer is essentially unchanged until the potential reaches 0.18 V, at which point the lipid layer thickens decreases and the area per lipid increases. These data are qualitatively consistent with the expected variation of the lipid layer due to the torque on the headgroup dipole moment. Molecular dynamics simulations confirm the angular variation of the headgroup with electric potential difference. Electron density profiles determined by the simulations show the same trends as the x-ray reflectivity data.