Scientists Capture First Molecular Images of Water Reorganizing During a Reaction Crucial for Life

A research team led by the US Department of Energy's Pacific Northwest National Laboratory (PNNL), working with SLAC National Accelerator Laboratory and several academic labs, has captured the first snapshots showing how electrons and protons move together inside a molecule during a light-triggered chemical reaction, tracking both the molecule's electronic changes and the reorganization of the surrounding water. The findings were published in the journal Nature Communications.

The reaction, called proton-coupled electron transfer or PCET, is the coordinated movement of negatively charged electrons and positively charged protons. It is among the most efficient energy-transfer processes known in nature, and plants use it to capture sunlight and convert it into stored energy. Moving electrons and protons together lets molecules skip energetically costly intermediate steps, making the reaction faster and far more energy efficient. Understanding it could help researchers design better flow batteries, fuel cells and catalysts.

Using ultrafast X-ray methods at SLAC's Linac Coherent Light Source, together with quantum chemistry calculations and molecular dynamics simulations, the team observed for the first time, with structural sensitivity, how gaining a proton reshapes a molecule's electronic structure at specific sites while reorganizing the surrounding water molecules. "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.

The experiment used a well-studied ruthenium-based molecule that absorbs light and, in acidic conditions, captures a proton from its surroundings. "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.