MOLECULAR-ORIGIN OF THE INTERNAL DIPOLE POTENTIAL IN LIPID BILAYERS -ROLE OF THE ELECTROSTATIC POTENTIAL OF WATER

The mean electrostatic potential was calculated asa function of positi on across bilayers of dimyristoylphosphatidylcholine (DMPC) and dilaur oylphosphatidylethanolamine (DLPE). The lipid bilayers were held fixed in their energy minimized gel state conformations, as before (Zheng a nd Vanderkooi; Biophys. J. 1992: 63, 935-941), but the surrounding wat er was simulated using the GROMOS molecular dynamics program. The elec trostatic potential at the bilayer midplane is negative relative to th e zero of potential in vacuo, but the potential in bulk water is more negative than that of the bilayer, with the result that the bilayer mi dplane potential is positive with respect to the surrounding water. Th e calculated potential difference for DMPC is 138 mV. This value is of the same sign and of similar magnitude to the potential which must be assumed in order to explain the experimentally observed differential permeability of bilayers to positive and negative hydrophobic ions. Co pyright (C) 1996 Elsevier Science Ireland Ltd