Catching Water in the Act: X-rays Reveal How Electrons, Protons and Water Move Together

Plants turn sunlight into stored energy. Your cells burn food for fuel. Fuel cells and catalysts split and make chemical bonds. Behind a surprising number of these processes sits the same quiet trick: an electron and a proton moving together, in a coordinated dance. Chemists call it proton-coupled electron transfer, or PCET, and until now nobody had managed to watch the whole choreography, including the water around it, in a single experiment.

Why moving two particles together is so efficient

When a molecule moves an electron and a proton in a coordinated way, it can skip energetically costly intermediate steps. That makes reactions faster and far more energy efficient, which is why nature leans on PCET for photosynthesis and metabolism. Researchers still debate the details in many PCET reactions: do the proton and electron move at the same moment, at which site of the molecule, and how does the surrounding water network help the proton hop along?

The difficulty is speed and scale. Electrons move on timescales that are very hard to track, protons are almost as fast, and the water molecules around them reorganize in ways that are notoriously difficult to observe directly. Earlier experiments captured pieces of the picture, but not a combined view of the molecule’s electronic changes and the rearranging solvent.

The experiment: a light flash, then two X-ray views

A team led by the Department of Energy’s Pacific Northwest National Laboratory (PNNL), working with SLAC National Accelerator Laboratory and several university labs, chose a well-studied model system: a ruthenium-based molecule dissolved in acidic water. When light hits it, it shifts an electron internally and then picks up a proton from the surrounding solution. Importantly, this particular molecule does not undergo extra electronic or structural rearrangements that would muddy the X-ray signals, so the team could isolate the electron, proton and solvent contributions.

At SLAC’s Linac Coherent Light Source, an X-ray free-electron laser, they combined two complementary probes:

·         Ultrafast X-ray absorption spectroscopy (the chemRIXS instrument), which is sensitive to specific atoms and shows how electrons redistribute between molecular sites.

·         Time-resolved X-ray scattering (the XCS instrument), which tracks how atoms rearrange, including the motions of the surrounding water molecules.

Quantum chemistry calculations and molecular dynamics simulations were then essential for turning those complex signals into a molecular-level story.

What they saw

In this system the two steps are neatly separated in time. The light-driven electron shift happens faster than the instrument’s resolution (on the order of 180 femtoseconds), while the proton arrives hundreds of picoseconds later, around 460 picoseconds in the published analysis. That gap of more than a thousandfold let the researchers examine each step on its own.

The measurements showed the excited-state electron density becoming strongly localized at the nitrogen atom that picks up the proton. At the same time, the first shell of water around the molecule switched its hydrogen-bonding pattern, from one type of bond arrangement to another. In the words of lead PNNL scientist Elisa Biasin, the team captured for the first time how electronic changes tied to proton transfer are coupled to reorganization of the solvent.

An honest limitation

The proton itself is invisible here. X-ray scattering mostly sees electron-rich atoms, so a lone proton does not show up directly. Instead, the team infers its behavior from the local electronic changes and the global rearrangement of the water network, checked against the calculations. This was also a model system where the basic mechanism was already understood, so the real test will be applying the method to messier reactions where the order of proton and electron movement is still debated.

Why it matters

This is less a finished invention than a new way of looking. The multimodal X-ray approach gives researchers a framework for studying PCET in more complex systems, which could inform the design of better catalysts, fuel cells, flow batteries and artificial photosynthesis. And the upgraded LCLS-II, with its better signal quality, should help push these methods toward real-world problems in catalysis and energy harvesting. Sometimes understanding how water gets out of the way (or gets involved) is the key to making chemistry efficient.

Source

·         Press release: PNNL, “Scientists Capture First Molecular-Level Images of Water Reorganization During a Reaction Crucial for Life” (August 28, 2026)

·         Original paper: Kahraman A., Sachs M., Ghosh S., Schoenlein B.I., Vauthey E., Andersen A., Govind N., Larsen C.B., Biasin E., et al. “Electronic and solvent reorganization in proton-coupled electron transfer captured by ultrafast X-rays.” Nature Communications 17, 9053 (2026)

·         Preprint: arXiv:2510.03693

Source: Catching Water in the Act: X-rays Reveal How Electrons, Protons and Water Move Together