Water’s ability to move positive charge, through the transport of protons, is a fundamental part of both nature and technology. It plays a role in processes as varied as cellular function, energy storage, catalysis, and fuel cells. Yet despite its importance, proton motion within small, well-defined water networks has remained difficult to measure experimentally. For more than two centuries, scientists have studied how protons “hop” across molecules, but the process has been challenging to capture at the molecular scale.
A research group led by Professor Mark Johnson at Yale University has now taken a major step forward. Using a highly customized mass spectrometer combined with laser spectroscopy, the Johnson Lab has provided the first experimental benchmarks for proton transfer within water clusters. The study, published in Science, shows how protons move through a finite water network and how long the transfer process takes.
Rana, A., Harville, P. A., Khuu, T., & Johnson, M. A. (2025). Microcanonical kinetics of water-mediated proton transfer in microhydrated 4-aminobenzoic acid. Science, 389(6765), 1143–1146. https://doi.org/10.1126/science.ady1723
Professor Mark Johnson at Yale University stated,
“We show what happens in a tiny molecular system where there is no place for the protons to hide. We’re able to provide parameters that will give theorists a well-defined target for their chemical simulations, which are ubiquitous but have been unchallenged by experimental benchmarks.”
The molecule at the center of the experiment was microhydrated 4-aminobenzoic acid (4ABA), which can hold an extra proton in two different locations: at its amino group or at its carboxyl group. These two “protomer” sites absorb light at distinct wavelengths, making it possible to track where the proton is located at any given moment. By attaching controlled numbers of water molecules; three or more in the earliest experiments, and up to six in the most recent; the researchers were able to observe how hydration makes proton transfer possible.
To monitor this delicate process, the team used a cryogenic ion trap coupled to a 30-foot-long mass spectrometer, adapted over many years of development. The instrument chilled molecular clusters to near absolute zero, allowed them to be selectively excited with laser light, and then destructively analyzed their products at a rate of ten times per second. This design provided a window into the motion of protons across a small number of water molecules, something that had not been previously achieved.
One of the key findings was that proton transfer does not occur in the absence of a minimal water network. With fewer than three water molecules, the proton remains fixed at its initial site, even when energy is added to the system. Once three or more molecules are present, however, proton transfer becomes possible, suggesting that water clusters must reach a certain size before the familiar “proton relay” mechanism emerges. In larger clusters, such as those with six water molecules, the team was able to measure how long the proton takes to move between sites. Although they could not observe the intermediate steps directly, they could define the start and end points of the process and quantify the transfer time.
The significance of this work goes beyond the experimental achievement itself. For years, computational models have tried to describe how protons move between water molecules, but those models have largely relied on assumptions rather than direct measurements. The Yale team’s results now give theorists a solid set of benchmarks, allowing simulations to be tested against real data and refined to better match what actually happens at the molecular level.
The findings also point to practical opportunities. A clearer understanding of proton motion can inform the design of new proton-conducting materials, such as membranes for fuel cells or catalytic systems that rely on rapid charge transfer. In biology, where enzymes and cells depend on proton relays within water networks, the data help fill in some long-standing gaps in our knowledge. While the current experiments were carried out in a controlled, cryogenic setting, they provide a foundation for future studies that will examine proton transport under more complex and realistic conditions.
Even though the researchers were not able to capture the proton’s movement at every step along its path, they did establish where it begins, where it ends, and how long the transfer takes. For a process that has puzzled scientists for centuries, that alone is a major step forward. The work gives both chemists and engineers a clearer framework to build on as they continue to explore how water, one of the most common molecules on Earth, manages the fundamental task of moving charge.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
