Boundary potential of lipid bilayers: methods, interpretations and biological applications

The electric field distribution at boundaries of cell membranes consists of diffuse part of the electrical double layer and the potential drop over the polar area generally attributed to dipole effects. This report focuses on the molecular nature of dipole components of boundary potential and its relation to bilayer structure as it follows from different experimental approaches and molecular dynamic (MD) simulations. Alterations of the total boundary potential (BP) of planar bilayer lipid membranes (BLM) can be detected by the method of Intramembraneous Field Compensation, developed in our laboratory. When combined with traditional electrokinetic measurements in liposome suspension and Volta potential control at lipid monolayers it reveals alterations of the dipole potential induced by multivalent cations (Be 2+ , Gd 3+ ) about 100-150 mV. It is related to the lipid phase transition detected by isotherm titration calorimetry (ITC) measurements. IFC method combined with perfusion of the cell show reversible electrostatic effects due to lysine adsorption and irreversible binding of polylysines accompanied by fast positive changes of BP as electrokinetic measurements, and slow negative ones attributed to BP dipole component. According to model proposed it related with changes of lipid hydration state varied by incorporation of ions or organic molecules into the lipid monolayer. Molecular dynamic simulations support this idea and relate dipole effects with H-bonded water molecules and lipid lateral coordination. Both effects are assumed as the principal reason of Gd 3+ blocking effect on E.coli mechanosensitive channels.