Scientists finally see how nature moves energy so efficiently

Some of the most important reactions in nature depend on positively and negatively charged particles moving in a coordinated way. These reactions are essential to processes such as photosynthesis, catalysis and biological energy conversion, but they happen so quickly that they are extremely difficult to observe.

A team led by the Department of Energy's Pacific Northwest National Laboratory, working with researchers at SLAC National Accelerator Laboratory and several universities, has now captured snapshots of these events after they are triggered by light striking a molecule.

The findings, published in Nature Communications, could deepen scientists' understanding of these reactions and eventually contribute to improved flow batteries, fuel cells and catalysts.

At the center of the study is the coordinated motion of positively charged protons and negatively charged electrons. This type of energy transfer is among the most efficient known in nature. Plants use related processes to capture energy from sunlight and convert it into stored chemical energy.

When electrons and protons move in a coordinated way, molecules can avoid intermediate steps that would otherwise require more energy. That can make a reaction both faster and much more efficient. The researchers investigated how changes in a molecule's electronic structure, the arrival of a proton and shifts in the surrounding water environment are connected during this process.

Scientists have studied this interaction for decades, but no previous experiment had captured the process in a single study with both local and structural sensitivity. Advanced X-ray techniques at the Linac Coherent Light Source at SLAC, combined with state-of-the-art quantum chemistry calculations and molecular dynamics simulations, provided an unusually detailed view.

PNNL experimental chemical physicist Elisa Biasin, former PNNL scientist Abdullah Kahraman and PNNL theorists Niranjan (Niri) Govind and Amity Andersen worked with collaborators to study a light-driven proton-coupled electron transfer reaction, or PCET.

The team combined ultrafast X-ray spectroscopy, X-ray scattering and advanced simulations to capture important stages of the reaction. For the first time with structural sensitivity, the researchers were able to show how a molecule's electronic structure changes at specific locations as it gains a proton, while the surrounding water environment reorganizes at the same time.

"We have captured for the first time how electronic changes associated with proton transfer are coupled to reorganization of the surrounding solvent," said Biasin. "This gives us a new way to understand how molecules and their environments evolve together during fundamental chemical transformations."

PCET plays an important role throughout nature. Plants rely on it during photosynthesis to harvest light, while animals use related processes to efficiently convert food into energy. It is also involved in many other forms of biological and chemical energy conversion.

For the particular experimental system examined in this study, researchers already understand the basic mechanism. But many questions remain about other PCET reactions, especially the precise timing and order in which electrons and protons move.

"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. To answer them, you need ultra-fast time resolution, chemical and structural sensitivity, and alignment with theory. We have made a step forward to shed light on these questions."