Internal electrostatic potentials in bilayers: measuring and controlling dipole potentials in lipid vesicles

Abstract

The binding and translocation rates of hydrophobic cation and anion spin labels were measured in unilamellar vesicle systems formed from phosphatidylcholine. As a result of the membrane dipole potential, the binding and translocation rates for oppositely charged hydrophobic ions are dramatically different. These differences were analyzed using a simple electrostatic model and are consistent with the presence of a dipole potential of approximately 280 mV in phosphatidylcholine. Phloretin, a molecule that reduces the magnitude of the dipole potential, increases the translocation rate of hydrophobic cations, while decreasing the rate for anions. In addition, phloretin decreases the free energy of binding of the cation, while increasing the free energy of binding for the anion. The incorporation of 6-ketocholestanol also produces differential changes in the binding and translocation rates of hydrophobic ions, but in an opposite direction to those produced by phloretin. This is consistent with the view that 6-ketocholestanol increases the magnitude of the membrane dipole potential. A quantitative analysis of the binding and translocation rate changes produced by ketocholestanol and phloretin is well accounted for by a point dipole model that includes a dipole layer due to phloretin or 6-ketocholestanol in the membrane-solution interface. This approach allows dipole potentials to be estimated in membrane vesicle systems and permits predictable, quantitative changes in the magnitude of the internal electrostatic field in membranes. Using phloretin and 6-ketocholestanol, the dipole potential can be altered by over 200 mV in phosphatidylcholine vesicles.