Researchers capture molecular water reorganization

Pacific Northwest National Laboratory researchers said on August 28 they captured the first molecular-level images of water reorganizing during proton-coupled electron transfer, or PCET, a class of reactions that underpins photosynthesis, catalysis and biological energy conversion. The work was published in Nature Communications and combined ultrafast X-ray spectroscopy, time-resolved X-ray scattering and computer simulations to track both a reacting molecule and the water around it. Elisa Biasin, a PNNL chemical physicist, said the team could watch how charge moved to a specific atomic site and how nearby water molecules changed in response. The experiments were carried out with collaborators at SLAC National Accelerator Laboratory and other academic labs. ### What exactly did the researchers capture? The study captured a sequential PCET reaction in water with atomic-site specificity, according to the Nature Communications paper. The team followed how a light-excited ruthenium polypyridyl complex redistributed electron density and then gained a proton, while the surrounding solvent reorganized at the same time. Biasin told Discover that the measurements showed, on picosecond timescales, what happens when a specific atomic site in a molecule receives a proton. (pnnl.gov) PNNL said the combined approach revealed for the first time with structural sensitivity how proton gain reshapes electronic structure at specific sites and reorganizes the surrounding water environment. (nature.com) ### Why is water reorganization part of the story? PCET reactions depend on the coupled movement of electrons and protons, and the water surrounding the reacting molecule can help stabilize or redirect those changes. The Nature Communications abstract said the measurements resolved not only protonation but also a concomitant rearrangement of the first solvation shell. The paper reported a specific hydrogen-bonding change: the local environment switched from N···HO to NH···O hydrogen bonds after protonation. (discovermagazine.com) That detail matters because it shows solvent motion was not background noise; it was part of the reaction pathway the researchers were trying to observe. ### How fast did this happen? The Nature Communications abstract said protonation in the model system occurred after about 460 picoseconds. Discover described the observations as snapshots taken on picosecond, or trillionth-of-a-second, timescales. (pubmed.ncbi.nlm.nih.gov) SLAC’s Linac Coherent Light Source supplied the advanced X-ray methods used in the work, while theory and molecular dynamics simulations helped interpret the measurements. PNNL said that combination gave the team both local and structural sensitivity in a single study, something it said had not previously been achieved for this interplay of charge, proton motion and solvent response. (pubmed.ncbi.nlm.nih.gov) ### Why did the team use a ruthenium molecule instead of a living system? The researchers used a ruthenium-based model complex that absorbs light and captures protons from its surroundings under acidic conditions, Discover reported. Model systems like that let scientists isolate a fundamental step of PCET without the added complexity of a whole biological assembly. (pnnl.gov) Nature Communications described the experiment as a way to capture electronic and solvent reorganization in a model PCET reaction at atomic scale. PNNL said the goal was to understand a basic mechanism that also appears in much larger settings, including photosynthesis and other energy-conversion processes. ### Where could this show up outside the lab? PNNL said the findings could help researchers better understand and ultimately design better flow batteries, fuel cells and catalysts. (discovermagazine.com) The lab said the new multimodal X-ray approach could also be used to study chemical transformations important for biological and energy-conversion processes. The paper itself does not promise a commercial device. (nature.com) What it provides is a measurement framework: ultrafast, site-specific views of how electronic redistribution, protonation and solvent motion unfold together. Future studies using the same approach would likely involve other PCET systems and additional reactions relevant to catalysis and energy conversion, based on the scope described by PNNL and the journal abstract. (pnnl.gov)