Electrostatic gating of a nanometer water channel

TLDR

Molecular dynamics simulations were used to study how a mobile external charge influences water permeation in a single‑walled carbon nanotube, estimating a critical distance from the balance of interactions between the nearest water molecule, its neighbors, and the charge. The nanopore exhibits sharp on‑off gating by a single +1e charge, being highly responsive when the charge is within 0.85 Å of the wall and largely insensitive beyond that, with flow and net flux decaying exponentially as the charge approaches and becoming negligible once it crosses the wall—an effect that may inform the behavior of biological membrane water channels.

Abstract

Water permeation across a single-walled carbon nanotube (SWNT) under the influence of a mobile external charge has been studied with molecular dynamics simulations. This designed nanopore shows an excellent on-off gating behavior by a single external charge (of value +1.0e): it is both sensitive to the available charge signal when it is close (less than a critical distance of 0.85 A or about half the size of a water molecule) and effectively resistant to charge noise, i.e., the effect on the flow and net flux across the channel is found to be negligible when the charge is >0.85 A away from the wall of the nanopore. This critical distance can be estimated from the interaction balance for the water molecule in the SWNT closest to the imposed charge with its neighboring water molecules and with the charge. The flow and net flux decay exponentially with respect to the difference between these two interaction energies when the charge gets closer to the wall of the SWNT and reaches a very small value once the charge crosses the wall, suggesting a dominating effect on the permeation properties from local water molecules near the external charge. These findings might have biological implications because membrane water channels share a similar single-file water chain inside these nanoscale channels.