Protoncoupled

Protoncoupled describes processes in which proton transfer is intrinsically linked to another chemical event, most commonly electron transfer, but also bond-making or breaking or conformational change. The coupling can be concerted, with the proton and the partner transfer occurring in a single kinetic step, or sequential, with proton transfer preceding or following the other event.

One major subclass is proton-coupled electron transfer (PCET), where a proton and an electron are transferred either simultaneously or in close succession. PCET is a central mechanism in biological energy conversion and catalysis, enabling redox reactions at high energy barriers to proceed under physiological conditions.

Mechanistic diversity: In concerted PCET, the proton moves through a hydrogen-bond network or relay to a base while an electron moves through its redox center, minimizing high-energy charged intermediates. In stepwise PCET, proton and electron transfer steps are separated by time.

Other forms include proton-coupled transport, in which proton translocation across membranes is coupled to substrate transport, using the proton motive force to drive uptake or extrusion; this is critical in mitochondria, chloroplasts, and bacteria.

Applications and examples: In biology, PCET underlies photosynthetic water splitting, respiration, and enzyme catalysis (e.g., some hydrogenases and oxidases). In chemistry and materials science, PCET informs catalysts for water oxidation, CO2 reduction, and organic redox reactions, and guides the design of electrochemical energy storage and conversion systems.

Measurement and theory: researchers use spectroscopy, electrochemistry, kinetic isotope effects, and quantum mechanical models to study PCET processes.