Abstract
We reinvestigate the electrostatics of a narrow, water-filled pore, taking into consideration recent demonstrations that ion interaction with such pores dramatically alters the dielectric properties of water. In the absence of an ion the water dipoles are easily reoriented (high <); with an ion present their orientations are frozen (low e). We account for this nonlinear dielectric behavior of pore water within the traditional continuum two-dielectric model of a transmembrane ion channel, introducing the charge-dependent effective dielectric constant e,(<?) for the water in the channel. We describe simplified approximations to e,(q), comparing our results for the ion energy barrier with the previous microscopic analysis of a gramicidin-like model channel. These parameters depend significantly on the ion’s position within the channel. Our analysis strongly suggests that the dielectric constant of the channel is significantly altered during the process of ion entry and its effective value is much lower than the bulk value (~80), traditionally used in the continuum models of the channel. This finding reopens the question of the origin of low-energy barriers for ion transport through the channel.