Chapter 5 Energetics of Membrane Permeability

Abstract The Gibbs free energy of dipole or ion permeation of lipid bilayers is calculated as the sum of all electrostatic, solvophobic and specific interactions. Partitioning models are consistent with dipole permeation and some features of ionic permeation, particularly, if the solvophobic energy is taken into account. Ionic and dipole permeability are extremely sensitive to the ionic/dipolar radius. Despite this sensitivity, calculations of the permeability can be carried out for typical monovalent cations, and provide reasonable estimates, but only for hydrated species. An alternative mechanism proposed for ionic permeation involves the occurrence of transient pore‐like defects in lipid bilayers which permit ions to bypass the Born energy barrier. The two alternative hypotheses, partitioning vs. transient pores, can be tested by measuring the ionic and dipolar permeation through bilayers of varying thickness. Experimental observations for ions permeability are consistent with the transient pore mechanism for shorter chain lipids, but tend towards the theoretical line for partitioning models for longer chain lipids. Results for small neutral solutes are best explained by the solubility‐diffusion mechanism. The proposed method of calculation of the Gibbs free energy of ion or dipole membrane transfer can be effectively used not only in describing the biophysical properties of bilayers, but also in extraction processes, pharmaceutical applications and liquid membrane separations. In this method it has been found that that the free energy of the solvophobic effect is opposite in sign to the electrostatic effect. As a result, the sum of the electrostatic and solvophobic components of the Gibbs free energy decreases with ionic size. The specific energy of ion/dipolar layer interaction depends on the dipolar membrane surface potential. These calculations yielded the permeability of different ions and molecules through bilayer membranes in good agreement with experimental data.