X-rays Reveal How Protons and Electrons Move Together in Nature’s Energy-Reaction

Researchers have recorded pictures of a chemical reaction that nature uses to move energy with great efficiency. The images connect shifts in electrons, protons and water molecules, and they might help create better catalysts, fuel cells and flow batteries.

Researchers from the Department of Energy’s Pacific Northwest National Laboratory led a team that included collaborators at SLAC National Accelerator Laboratory and several universities. Their work was published in Nature Communications.

Capturing the Reaction

The process depends on positively and negatively charged particles moving together in an organized way. Plants make use of comparable methods to take in energy from the sun and hold it as chemical energy. Animals employ related methods to turn food into energy.

Protons and electrons can travel as a pair without needing any extra stages, which saves energy and speeds up the process.

A light pulse was delivered to a molecule, setting off the reaction, while ultrafast X-rays were employed to observe its aftermath. The team witnessed a chain of events unfolding together: alterations in electronic structure, the arrival of a proton, and changes in the surrounding water environment.

“We have captured for the first time how electronic changes associated with proton transfer are coupled to reorganization of the surrounding solvent,” said Elisa Biasin, a PNNL experimental chemical physicist.

Why It Matters

Nature relies on proton-coupled electron transfer, or PCET, for its operations. The process drives photosynthesis, and it also powers biological energy conversion.

The fundamental process behind the system has been figured out by researchers, yet the exact sequence and timing of how electrons and protons move through it still aren’t fully known.

“Are they happening together or not? At which molecular site? And how is the water network facilitating the proton hop?” Biasin asked. “These are some of the possible open questions.”

Researchers now have a means to address these questions, and the findings reveal that localized shifts in electronic structure accompany a wider reorganization of the surrounding water network when the molecule picks up a proton.

Following Electrons, Protons and Water

The real difficulty is pace. Electrons travel at an extremely rapid rate, and protons keep up nearly as quickly. The water molecules around the reacting particles rearrange themselves continually, and this constant shifting is notoriously hard to watch directly.

Earlier studies were able to measure certain aspects of this procedure individually, though none managed to observe both the internal electronic alterations within the molecule and the rearrangement of matter surrounding it simultaneously.

Biasin and her team chose a ruthenium-based molecule that has been studied extensively because it makes the system simpler to understand. That particular compound takes in light and grabs a proton from its environment when placed in an acid.

“We identified the metal complex used in this study because it does not undergo additional electronic and structural rearrangements that complicate interpretation of X-ray signals, allowing us to isolate signals associated with the electron, proton and solvent motion,” said Christopher Larsen, a co-investigator and senior lecturer at the University of Auckland, New Zealand.

At the University of Geneva, the scientists applied time-resolved characterization techniques to trace the phases of the reaction.

What Comes Next

Scientists combined ultrafast X-ray spectroscopy, X-ray scattering and advanced simulations to document key moments in the reaction. That approach may ultimately aid researchers in building better catalysts, fuel cells and flow batteries.

Researchers now have a direct means to probe these linked transformations, which may eventually aid in designing more efficient catalysts, fuel cells, flow batteries and other energy conversion and storage technologies.

This study moves the field forward by exploring how molecules and their surroundings change together throughout key chemical reactions.

1. Ultrafast X-ray spectroscopy captures electronic structure changes.
2. X-ray scattering documents the arrival of a proton.
3. Advanced simulations model the surrounding water network’s reorganization.

Source material: “Scientists finally see how nature moves energy so efficiently,” ScienceDaily.

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