Proton transport in water is doubly gated by sequential hydrogen-bond exchanges

The transport of excess protons in water is central to acid-base chemistry, biochemistry, and energy production. However, elucidating its mechanism has been challenging. Recent nonlinear vibrational spectroscopy experiments could not be explained by existing models. Here, we combine neural network-based molecular dynamics simulations considering nuclear quantum effects for all atoms and vibrational spectroscopy calculations to determine the proton transport mechanism. Our simulations reveal the equilibrium between two stable proton-localized structures with distinct Eigen-like and Zundel-like hydrogen-bond motifs. Proton transport follows a three-step mechanism gated by two successive hydrogen-bond exchanges: the first reduces the proton-acceptor water coordination, leading to proton transfer, and the second, the rate-limiting step, prevents rapid back-transfer by increasing the proton-donor coordination. This sequential mechanism is consistent with experimental characterizations of proton diffusion, explaining the low activation energy and the prolonged intermediate lifetimes in vibrational spectroscopy. These results are crucial for understanding proton dynamics in biochemical and technological systems.