Following the evolution of paramagnetic species during redox reactions in situ and under catalysis: film electrochemistry - EPR spectroscopy

The combination of film electrochemistry (FE) and electron paramagnetic resonance (EPR) spectroscopy promises simultaneous insight into the reactivity and the nature of key paramagnetic intermediates formed during redox reactions. Therefore, we developed a novel ‘in-situ’ FE-EPR setup and demonstrated the sensitive detection of paramagnetic centres during the course of an electrochemical reaction in real time, both under non-turnover conditions and during catalysis. Using hierarchical indium-tin-oxide based electrodes, we reduced the current response time of the electrode and increased the surface area for maximum EPR sensitivity, enabling both simultaneous cyclic voltammetry and chronoamperometry EPR measurements. A first application on STEMPO, a silatrane-anchored nitroxide radical, was chosen to: i) demonstrate the effectiveness of the newly developed in operando FE-EPR setup and ii) enable new insight into the mechanism of alcohol oxidation by nitroxide molecules using FE-EPR. Our results resolve a long-standing controversy over whether a radical is formed during catalysis. We further show the applicability of our setup to a molybdo-protein, the periplasmic methionine sulfoxide reductase (MsrP) of E. coli, allowing to follow in-situ the transition from Mo(IV) to Mo(V) and more importantly it allowed the first experimental detection of the transition from Mo(V) to Mo(VI).