Abstract
Many ion selective channels contain a narrow region in which water molecules cannot pass one another. We model this “single file” domain by a linear chain of molecular dipoles embedded in continuum, low dielectric (ϵm = 2 ) slab surrounded by high dielectric (ϵ = ∞) continua. We present exact results for the free energy, polarization and susceptibility of the molecular dipoles as functions of an applied electric field and of the location of a cation in the chain. For parameters typical of the gramicidin channel we find that for an ion-free channel, the dipolar susceptibility is high at low electric fields. When an ion replaces one of the water dipoles in the chain the susceptibility is reduced 100-fold.