Electrostatic modeling of ion pores. Multipolar sources

We present calculations of the polarization energy required when a multipolar source enters a transmembrane ion channel. The polarization energy for polar, but uncharged, sources is nearly independent of channel length if the length/diameter ratio is greater than 1.25; however, it is strongly dependent on channel radius. All significant changes in the polarization energy occur within a distance of +/- one channel radius of the entrance to the channel. Our calculational method also provides a means of estimating the change in polarization energy that occurs when an ion is not located axially; this contribution can be significant for smaller ions, for which the binding sites may be far from the axis.