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.