Water alignment, dipolar interactions, and multiple proton occupancy during water-wire proton transport

TL;DR

This study develops a kinetic model for water-wire proton transport, incorporating water dipole alignment, interactions, and multiple proton occupancy, revealing how these factors influence current-voltage behaviors and proton conduction mechanisms.

Contribution

The paper introduces a novel multistate kinetic model that includes water dipole alignment, interactions, and multiple proton occupancy, providing new insights into proton transport dynamics.

Findings

Alignment fields decrease proton current.

Water channel interactions can cause superlinear and sublinear I-V relationships.

A lubrication mechanism suppresses dipolar interactions during multiple proton occupancy.

Abstract

A discrete multistate kinetic model for water-wire proton transport is constructed and analyzed using Monte-Carlo simulations. The model allows for each water molecule to be in one of three states: oxygen lone pairs pointing leftward, pointing rightward, or protonated (H$_{3}$O$^{+}$). Specific rules for transitions among these states are defined as protons hop across successive water oxygens. We then extend the model to include water-channel interactions that preferentially align the water dipoles, nearest-neighbor dipolar coupling interactions, and coulombic repulsion. Extensive Monte-Carlo simulations were performed and the observed qualitative physical behaviors discussed. We find the parameters that allow the model to exhibit superlinear and sublinear current-voltage relationships and show why alignment fields, whether generated by interactions with the pore interior or by membrane potentials {\it always} decrease the proton current. The simulations also reveal a ``lubrication'' mechanism that suppresses water dipole interactions when the channel is multiply occupied by protons. This effect can account for an observed sublinear-to-superlinear transition in the current-voltage relationship.