Abstract
Enzymes are recognized as exceptional catalysts for achieving high stereoselectivities1,2,3, but their ability to control the reactivity and stereoinduction of free radicals lags behind that of chemical catalysts4. Thiamine diphosphate (ThDP)-dependent enzymes5 are well-characterized systems that inspired the development of N-heterocyclic carbenes (NHCs)6,7,8 but have not yet been proved viable in asymmetric radical transformations. There is a lack of a biocompatible and general radical-generation mechanism, as nature prefers to avoid radicals that may be harmful to biological systems9. Here we repurpose a ThDP-dependent lyase as a stereoselective radical acyl transferase (RAT) through protein engineering and combination with organophotoredox catalysis10. Enzyme-bound ThDP-derived ketyl radicals are selectively generated through single-electron oxidation by a photoexcited organic dye and then cross-coupled with prochiral alkyl radicals with high enantioselectivity. Diverse chiral ketones are prepared from aldehydes and redox-active esters (35 examples, up to 97% enantiomeric excess (e.e.)) by this method. Mechanistic studies reveal that this previously elusive dual-enzyme catalysis/photocatalysis directs radicals with the unique ThDP cofactor and evolvable active site. This work not only expands the repertoire of biocatalysis but also provides a unique strategy for controlling radicals with enzymes, complementing existing chemical tools.
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Data availability
All data are available in the main text or the supplementary materials and from the Cambridge Crystallographic Data Centre (CCDC; https://www.ccdc.cam.ac.uk/structures/); crystallographic data are available free of charge under CCDC reference numbers 2256735 (3e) and 2256734 (3g).
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Acknowledgements
We thank N. Jiao, E. Meggers and H. Zhao for insightful discussions. S. Zhang, C. Zhang, J. Feng and B. Wang are appreciated for their help with mechanistic studies. We appreciate the financial support from the National Key Research and Development Program of China (2022YFA0913000 to X.H., 2019YFA0405600 to L.Y.), the National Natural Science Foundation of China (21825703 and 21927814 to C.T., 22277053 to X.H.), the Natural Science Foundation of Jiangsu Province (BK20220760 to X.H., BK20230018 to Y.L.), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB37000000 to C.T.), the Youth Innovation Promotion Association of Chinese Academy of Sciences (2022455 to L.Y.) and Fundamental Research Funds for the Central Universities (020514380253 to Y.L.). Part of this work was performed at the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, Chinese Academy of Sciences. All theoretical calculations were performed at the High Performance Computing Center (HPCC) of Nanjing University.
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Y.X. developed the photocatalytic/biocatalytic system and performed most of the experiments. X.P., J.Z., Z.X. and Y.B. assisted in synthetic experiments and mechanistic investigations. H.C. and Y.L. performed theoretical calculations. L.Y. and A.L. carried out EPR measurements and analysis under the supervision of C.T. Y.Z. performed X-ray crystal-structure analysis. X.H. and Y.X. wrote the manuscript, with input from all authors. X.H. coordinated and conceived the project.
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X.H., Y.X., X.P., J.Z., Z.X. and Y.B. are the inventors of a patent application submitted by Nanjing University that covers the photoenzymatic approach to chiral ketones under application number 202310939697.8. The other authors declare no competing interests.
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Xu, Y., Chen, H., Yu, L. et al. A light-driven enzymatic enantioselective radical acylation. Nature 625, 74–78 (2024). https://doi.org/10.1038/s41586-023-06822-x
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DOI: https://doi.org/10.1038/s41586-023-06822-x
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