Two Co-existing and Opposing Mechanisms of Proton Transfer in One-Dimensional Open-end Water Chains

The proton transport in one-dimensional (1D) confined water chains has been extensively studied as a model for ion channels in cell membrane and fuel cell, etc. However, the mechanistic understanding of proton transfer (PT) process in 1D water chains remain incomplete. In this study, we demonstrate that the two limiting structures of the hydrated excess proton, H₅O₂⁺ (Zundel) and H₃O⁺ (linear H₇O₃⁺), undergo a change in dominance as the water chain grows, causing two co-existing and opposing PT mechanisms. Specifically, H₅O₂⁺ is stable in the middle of the chain, whereas H₃O⁺ serves as a transition state (TS). Except for this region, H₃O⁺ is stabilized while H₅O₂⁺ serving as a TS. The interaction analysis shows that the electrostatic interaction plays a crucial role in the difference of PT mechanisms. Our work fills a knowledge gap between the various PT mechanisms reported in bulk water and long 1D water chains, contributing to a deeper understanding of biological ion channels at the atomic level.