Microscopic calculation of ion-transport rates in membrane channels

A method, based on rate theory, is described by which transport rates in ion channels can be calculated using only microscopic parameters, such as atomic coordinates, force constants and intermolecular energy parameters. The channel is treated as a system of elastically bound ligands interacting with the ion by coulombic and Lennard-Jones forces. Jump frequencies of the ion are obtained from the potential mean force which represents a thermal average over the different configurations of the ligand system. The method is illustrated by application to a special channel model, helical arrangement of dipolar ligands, which can be tilted toward the channel axis against harmonic restoring force. The jump frequency is found to be a non-monotonous function of ion radius. Furthermore, the ion specificity of the channel strongly depends on whether the ligand system is 'hard' or 'soft', i.e., on the extent to which the interaction with the ion can lead to a reorientation of the ligand groups.