THE MEMBRANE DIPOLE POTENTIAL IN A TOTAL MEMBRANE-POTENTIAL MODEL - APPLICATIONS TO HYDROPHOBIC ION INTERACTIONS WITH MEMBRANES

The total potential energy profile for hydrophobic ion interactions with lipid bilayers can be written as the sum of four terms: the electrical Born, image and dipole contributions, and a neutral energy term. We introduce a specific model for the membrane dipole potential, treating it as a two-dimensional array of point dipoles located near each membrane-water interface. Together with specific theoretical models for the other energy terms, a total potential profile is developed that successfully describes the complete set of thermodynamic parameters for binding and translocation for the two hydrophobic ion structural analogues, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-). A reasonable fit to the data is possible if the dipole potential energy has a magnitude of 5.5 .+-. 0.5 kcal/mol (240 .+-. 20 mV), positive inside, and if the neutral energy contribution for TPP+ and TPB- is -7.0 .+-. 1.0 kcal/mol. These results may also have important implications for small ion interactions with membranes and the energetics of charged groups in membrane proteins.