Model calculations of polarization effects in elastic membrane channels

Rate theoretical analysis of ion-selectivity in membrane channels with elastically bound ligands

This article demonstrates why a gramicidin-like pore features ion-specific conductivity with the sequence Cs+ greater than Rb+ greater than K+ greater than Na+ greater than Li+. The starting point is a generalized transition state method for escape across multi-dimensional barriers. This model-independent procedure provides an adequate description of the transport process for ions along the interior of a channel-like molecule with flexible ligands. Moreover, the proper treatment of three-dimensional motion of ions within the channel allows for a cross-coupling with the ligands' motion. It is shown that the particle migrating along a sequence of binding sites actually corresponds to a polaron, where the individual dwelling times strongly depend on the ion's radius.

A molecular model for ion selectivity in membrane channels

In this article, the three-dimensional motion of an ion within a molecular channel is discussed for the first time; escape rates from binding sites are calculated using the transition state method. For a given ligand configuration and a particular pore radius the rates depend upon ion size and mass. It is found that the activation energies depend strongly on the ion size, i.e., they increase with decreasing ion radius. In contrast to the rates obtained from the mass dependence alone, the rates depending on both mass and size of the alkali ions yield the completely inverted sequence, namely the Eisenman sequence I.