Abstract
The free energy, polarization and susceptibility of a linear chain of molecular dipoles modeling the water molecules in a transmembrane ion channel are computed as functions of the cation location in the channel, taking into account the influence of the channel forming peptide in a primitive fashion, by treating it as a distribution of fixed charges. The membrane is simulated by a low dielectric (ϵ
M = 2) slab surrounded by high dielectric (ϵ = ∞) continua. For parameters representative of gramicidin, we find that water molecules are still orientationally frozen by the field of an ion; the fixed peptide charges do not alter this major result of our previous calculation. These charges affect the free energy mainly through direct electrostatic interactaction with the ion.