Catching the proton-electron handoff that powers photosynthesis

A proton-electron handoff sits at the heart of photosynthesis, respiration, and most modern catalysts. It is also so fast and so tightly coupled that no single experiment had ever caught it cleanly. A team led by the Department of Energy's Pacific Northwest National Laboratory (PNNL), working with SLAC National Accelerator Laboratory and several universities, has now done it in a ruthenium polypyridyl model complex, publishing the result in Nature Communications.

The work targets proton-coupled electron transfer (PCET), a reaction in which an electron and a proton move in the same step, with each one shaping the other's path. Photosynthesis uses PCET to split water and build sugars; fuel cells and flow batteries use it to move charge; catalysts use it to make or break bonds. The handoff is also one of the most efficient reactions in nature, which is exactly what has made it so hard to study. The electron moves on femtosecond timescales (one quadrillionth of a second per step), and the proton follows almost as quickly, with the surrounding water molecules rearranging around both. For decades, chemists had to choose between watching the local electronic picture and watching the broader structural one.

The team combined three ultrafast techniques. Femtosecond optical spectroscopy tracked the initial light-driven electron hop, a metal-to-ligand charge transfer triggered by a laser pulse. Ultrafast nitrogen K-edge X-ray absorption spectroscopy, performed at SLAC's Linac Coherent Light Source (LCLS), an X-ray free-electron laser that delivers X-ray pulses short enough to resolve chemistry as it happens, followed the electron as it redistributed around a specific nitrogen atom on the ligand. Time-resolved X-ray solution scattering, also at LCLS, captured the structural rearrangement of the molecule and its surrounding water shell at the same instant. Together, the authors describe the combination as the first experiment to capture PCET steps with both local electronic sensitivity and broader structural sensitivity in one place.

The team studied a ruthenium polypyridyl complex dissolved in water, a stand-in for the kind of metal complex chemists actually use in catalysis. After the light pulse, the electron moved first, leaving the ligand nitrogen electronically primed. About 460 picoseconds later, a timescale specific to this model compound, the proton arrived, switching the local hydrogen bond from N···HO to NH···O. The surrounding water shell rearranged in lockstep. Quantum-chemistry calculations and molecular-dynamics simulations, run alongside the experiments, helped the team assign each part of the signal to a specific atomic motion.

The advance is a measurement capability, not a deployed solution. The ruthenium complex is a model system, not photosynthesis itself. The 460-picosecond protonation time is a property of this specific molecule in this specific solvent, not a universal rate for biological energy transfer, and similar reactions in living systems will have their own, different rates. Downstream applications the paper's authors flag, including improved catalysts, fuel cells, and flow batteries, are aspirational, framed in the paper as "could contribute" and "eventually." None of those technologies is improved by this result alone.

Previous PCET experiments had to split the reaction into pieces: one instrument for the electron, another for the proton, a third for the surrounding solvent. The LCLS-based combination lets researchers see all three as one event, with the same time axis and the same molecule, a measurement that PNNL's institutional release and the press summary frame as a step toward energy technologies. The paper itself is more careful, calling it a measurement advance first and a contribution to future applications second. The next step, as the authors see it, is to point the same triple-lens approach at the catalysts, electrode surfaces, and battery chemistries that the field actually wants to design.