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Proton-coupled electron transfer (PCET) reactions are studied in a wide range of scientific disciplines, such as organic chemistry, electrochemistry, theoretical chemistry, thermodynamic chemistry, chemical kinetics and enzymology. As a consequence, this interdisciplinary research area entails a multitude of different, partly ambiguous definitions. In this chapter, we provide a short summary of the theoretical analysis of PCET reactions and introduce a selection of experimental thermodynamic and kinetic techniques that are used to investigate PCET reactions in biological redox systems. We highlight the experimental challenge of distinguishing concerted PCET mechanisms with a single transition state from stepwise, i.e. kinetically linked, reactions. The stepwise mechanisms can be sub-divided into gated and coupled PCET reactions. One example for a kinetically coupled PCET would be a fast, yet thermodynamically unfavourable PT preceding the ET step. In contrast, a PT that rate-limits an ET step is defined as a gated PCET mechanism. Experimentally, both stepwise and concerted PCET reactions become apparent as “ET reactions that exhibit a solvent kinetic isotope effect” emphasising the adequacy of this comprehensive definition. Consequently, hydride-transfer reactions are also contained within this classification. To illustrate the experimental approaches and some associated challenges, we discuss two enzymatic systems that have recently been analysed in our lab: the inter-copper ET in a copper-containing blue nitrite reductase and the H-transfer steps in the old yellow enzyme family.

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