O 2 -tolerant electro- and photocatalytic CO 2 -to-CO conversion by carbon monoxide dehydrogenases operating in deep eutectic solvents Léonard Olivotto 1,2,3,4 *, Claudio Righetti 1,3 , Julien Pérard 1 , Christine Cavazza 1 , Alan Le Goff 3 , Moritz F. Kühnel 2,4 1 Univ. Grenoble Alpes, CEA, CNRS, IRIG, CBM, F-38000 Grenoble, France, 2 Department of Chemistry, Swansea University, Swansea, Wales, 3 Département de Chimie Moléculaire UMR 5250, Université Grenoble Alpes, Grenoble, FrancE, 4 Institut für Chemie, Universität Hohenheim, Stuttgart, Germany Carbon monoxide dehydrogenases (CODHs) show an exceptional catalytic performance for CO 2 reduction. These enzymes can reversibly convert CO 2 into CO with minimal overpotential, making them attractive catalysts for future applications turning CO 2 into valuable chemicals. CODHs rely on polymetallic active sites based on two abundant metals, iron and nickel, buried inside a protein scaffold [1,2] . The thermophilic enzyme from Carboxydothermus hydrogenoformans Ch CODH-II is one of the most active at high temperature and with a fast turnover frequency. In order to utilise CODHs ex vivo , i.e., for electro- and photobiocatalytic processes, the design of tailored interfaces to efficiently provide electrons to CODH is essential. However, these enzymes are also known to be very quickly and irreversibly damaged by oxygen [3] . Consequently, this drawback limits their use to strictly anaerobic conditions. Here, we investigate Ch CODH-II immobilised on carbon nanotube-based electrodes and on functionalised graphitic carbon nitride. In particular, we study the effect of non-natural environments as a means of increasing the CODH oxygen tolerance. Recently, we have shown that tuning the solvent is a powerful strategy to prevent O 2 inhibition of enzymes. Deep Eutectic Solvents (DESs), an alternative to ionic liquids with low toxicity and low production cost [4] , have oxygen diffusion limiting properties [5] . We have thus used DESs for the photocatalytic hydrogen evolution by hydrogenases absorbed on photocatalytic nanomaterials showing the biocompatibility of such a medium and the enhancement of O 2 tolerance of the enzyme [6] . We have now translated this approach to CODHs and demonstrate the biocompatibility of DESs with Ch CODH-II. We will show electrocatalytic turnover of CODH immobilized on modified carbon nanotube-based electrodes in DESs as well as the O 2 -shielding effect on CODH. Furthermore, a photocatalytic system based on Ch CODH-II and carbon nitride (CN x ) is presented. CN x has attracted much interest due to its visible light absorption, low cost, biocompatibility and favourable band positions for CO 2 reduction [7] . We show that CN x can also be chemically modified to improve its interaction with CODH, thus enabling efficient enzymatic CO 2 -to-CO photoreduction. References 1. U. Contaldo, M. Curtil, J. Pérard, C. Cavazza, A. Le Goff, Angew.Chem. Int.Ed., 2022 , 61, 7 2. U. Contaldo, B. Guigliarelli, J. Perard, C. Rinaldi, A. Le Goff, C. Cavazza, ACS Catal. 2021 , 11 , 5808 3. M. Merrouch,J. Hadj-Saïd,L. Domnik,H. Dobbek,C. Léger,S. Dementin,V. Fourmond Chem. – Eur. J. 2015 , 21 , 18934–18938 4. E. L. Smith, A. P. Abbott, K. S. Ryder, Chem. Rev. 2014 , 114 (21), 11060–11082 5. M. G. Allan, M. J. McKee, F. Marken, M. F. Kuehnel, Energy Environ. Sci. 2021 , 14 (10), 5523–5529. 6. M. G. Allan, T. Pichon, J. A. McCune, C. Cavazza, A. Le Goff, M. F. Kühnel, Angew. Chem. Int. Ed. , 2023 , 62, e202219176 7. Y. Fang and X. Wang, Chem. Commun. , 2018 , 54 ,5674
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