On the Physics of Size Selectivity

TL;DR

This paper reveals that biological ion channels use entropy-driven mechanisms for size selectivity, with simple bulk models effectively capturing the core physics despite complex geometries.

Contribution

It demonstrates that entropy-driven effects explain size selectivity in ion channels and shows that simple bulk models can accurately describe these phenomena.

Findings

Attractive channels favor small particles

Repulsive channels favor large particles

Bulk models match detailed density-functional theory results

Abstract

We demonstrate that two mechanisms used by biological ion channels to select particles by size are driven by entropy. With uncharged particles in an infinite cylinder, we show that a channel that attracts particles is small-particle selective and that a channel that repels water from the wall is large-particle selective. Comparing against extensive density-functional theory calculations of our model, we find that the main physics can be understood with surprisingly simple bulk models that neglect the confining geometry of the channel completely.