Scientists Capture First Molecular Images of Water Reorganization in Life's Key Reactions

In a laboratory at the Pacific Northwest National Laboratory in Richland, Washington, a team of researchers has done something that chemists have been trying to accomplish for decades: they have watched, in real time and at the molecular level, what happens when a molecule gains a proton and the water around it reorganizes in response. The images they captured reveal a fundamental process that powers photosynthesis in plants, fuels energy conversion in animals, and underpins some of the most efficient chemical reactions known to exist.

The reaction they studied is called proton-coupled electron transfer, or PCET. It works like this: electrons and protons move in a coordinated dance, transferring energy between molecular sites without taking expensive detours through intermediate steps. This coordination makes reactions dramatically faster and more efficient than they would be if electrons and protons moved independently. In nature, plants use this mechanism to harvest sunlight and convert it into stored chemical energy. Animals use it to metabolize food. The same principle could, if better understood, help engineers design superior fuel cells, flow batteries, and catalysts for industrial processes.

What made this observation possible was a combination of tools that had never been used together in quite this way. The team, led by experimental chemical physicist Elisa Biasin and including theorists Niranjan Govind and Amity Andersen, along with collaborators at SLAC National Accelerator Laboratory and academic institutions, deployed ultrafast X-ray spectroscopy to track how electrons move between molecular sites. They paired this with time-resolved X-ray scattering to watch how atoms—including the water molecules surrounding the reacting molecule—shifted position. They then used quantum chemistry calculations and molecular dynamics simulations to interpret what the X-rays were showing them. The result, published in Nature Communications, is the first unified picture of how electronic structure, proton addition, and solvent reorganization are linked during this reaction.

The challenge was formidable. Electrons move on timescales measured in femtoseconds—millionths of a billionth of a second. Protons move almost as quickly. Water molecules reorganize themselves in ways that are notoriously difficult to observe directly. Previous experiments could capture pieces of the story but never the whole thing at once. The team chose to study a ruthenium-based molecule that absorbs light and, in acidic conditions, captures a proton from its surroundings. This particular molecule was ideal because it does not undergo additional electronic rearrangements that would muddy the X-ray signals, allowing the researchers to isolate the specific signatures of electron, proton, and solvent motion.

Using the Linac Coherent Light Source at SLAC, the researchers watched as the molecule gained a proton and the electronic structure at specific molecular sites reshaped itself. Simultaneously, they observed the water network surrounding the molecule reorganize. The X-ray absorption spectroscopy revealed local electronic changes; the X-ray scattering revealed global reorganization of the water environment. Theory and simulation filled in the gaps, translating the complex X-ray signals into a coherent picture of what was actually happening at the atomic scale.

There is one limitation worth noting: the proton itself remained invisible to the X-rays. Protons are so small and electron-poor that X-ray scattering passes through them without registering. But by observing the electronic reorganization of the molecule and the structural reorganization of the surrounding water, and by comparing these observations to theoretical predictions, the researchers could infer the proton's movement with confidence. As Biasin explained, the agreement between experimental data and calculations allowed them to draw reliable conclusions about the proton's behavior even though they could not see it directly.

The significance of this work lies not just in what it reveals about this particular reaction, but in the framework it establishes for studying more complex PCET reactions in the future. Many of the most important chemical reactions in nature and industry involve electrons, protons, and their environment moving together on ultrafast timescales. The multimodal X-ray approach developed here provides a template for understanding these coupled processes. With the planned upgrade to the LCLS facility, which will offer better signal-to-noise ratios, researchers will be able to apply these methods to real-world catalytic and energy-harvesting problems. The first clear image of water reorganization during proton-coupled electron transfer is not the end of the story—it is the beginning of a new chapter in understanding how to design more efficient energy-conversion technologies.