Proton-Coupled Electron Transfer Mechanisms

Proton-coupled electron transfer (PCET) encompasses reactions in which the movement of an electron is directly linked to the transfer of a proton. This coupling can proceed by sequential steps—electron transfer followed by proton transfer or vice versa—or by a concerted process in which both species move simultaneously. PCET underpins key processes in photosynthesis, respiration and artificial energy conversion, owing to its ability to minimise energy barriers and avoid charged intermediates. Control over mechanism arises through modulation of thermodynamic driving forces for proton and electron transfers, the strength of hydrogen-bonding networks, solvent dynamics and external perturbations such as pressure. Advances in spectroscopic and kinetic techniques have enabled real-time characterisation of PCET pathways and allowed detailed mapping of how reaction coordinates shift between stepwise and concerted regimes. Understanding these pathways informs the design of catalysts for solar-fuel production, molecular electronics and biomimetic systems.

Research from Nature Portfolio

Recent studies have employed high-pressure pump–probe techniques on metal-complex chromophores to probe the mechanistic crossover between stepwise and concerted PCET pathways in excited states. Under low quencher concentrations, a proton-first mechanism was observed, whereas elevated pressures slowed proton motion relative to electron transfer, revealing the distinct pressure-sensitivity of sequential steps. By contrast, at high quencher levels a concerted pathway prevailed, displaying no pressure dependence due to simultaneous charge and proton relocation. These results establish pressure as a powerful variable for distinguishing PCET pathways. In parallel, investigations of mixed-valence complexes bridged by hydrogen bonds have demonstrated that thermal electron transfer can proceed through both proton-coupled and proton-uncoupled channels. Mechanistic switching between the two pathways was achieved by tuning electronic coupling and hydrogen-bond strength, highlighting how interface design governs charge transport. This work extends the concept of PCET beyond proton-bound routes, with implications for charge propagation in proteins and artificial membranes.

Research from all publishers

Studies of de novo proteins containing non-canonical phenol derivatives have revealed how local environment and hydrogen-bonding networks shape PCET rates and mechanisms. By embedding mercaptophenol moieties within a well-folded protein scaffold and varying solvent exposure or internal hydrogen bonds, researchers showed that changes in proton-transfer driving force can switch the mechanism from stepwise to concerted under different pH and oxidant conditions. This work underscores the importance of tuning both electrostatic and structural factors to control PCET in enzymes. A comprehensive review of proton transfer in photochemical and biochemical systems has emphasised the link between ultrafast proton motion and slower protein conformational changes, proposing a unified framework that connects light-driven proton transfers with transmembrane gradients in bioenergetics. Finally, investigations into oxidation of tryptophan derivatives by photosensitisers in aqueous solution have clarified a longstanding debate: with water as the primary proton acceptor, certain systems undergo concerted electron-proton transfer rather than a sequential mechanism. These findings refine our understanding of radical-chain processes in natural systems and inform the design of water-compatible redox catalysts.

Proton-Coupled Electron Transfer Mechanisms publication trend

The graph below shows the total number of articles in proton-coupled electron transfer mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Proton-coupled electron transfer (PCET): A reaction in which electron transfer and proton transfer are linked, either in a single concerted step or via rapid sequential steps.

Concerted mechanism (CPET): A pathway in which proton and electron move simultaneously, avoiding high-energy charged intermediates.

Stepwise mechanism: A PCET process occurring in two distinct stages—either proton transfer first then electron transfer (PT-ET) or electron transfer first then proton transfer (ET-PT).

Driving force (ΔG°): The free-energy change that governs the thermodynamic favourability of proton or electron transfer steps.

References

1. High-pressure pump–probe experiments reveal the mechanism of excited-state proton-coupled electron transfer and a shift from stepwise to concerted pathways. Nature Chemistry (2025).
2. Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways. Nature Communications (2019).
3. Switching the proton-coupled electron transfer mechanism for non-canonical tyrosine residues in a de novo protein. Chemical Science (2024).
4. Proton transfer reactions: From photochemistry to biochemistry and bioenergetics. BBA Advances (2023).
5. Concerted and Stepwise Proton-Coupled Electron Transfer for Tryptophan-Derivative Oxidation with Water as the Primary Proton Acceptor: Clarifying a Controversy. Journal of the American Chemical Society (2022).