The selective enzymatic reduction of nitroaliphatic and nitroaromatic compounds to aliphatic amines and amino-, azoxy- and azo-aromatics, respectively, remains a persisting challenge for biocatalysis. Here we demonstrate the light-powered, selective photoenzymatic synthesis of aliphatic amines and amino-, azoxy- and azo-aromatics from the corresponding nitro compounds. The nitroreductase from Bacillus amyloliquefaciens, in synergy with a photocatalytic system based on chlorophyll, promotes selective conversions of electronically-diverse nitroarenes into a series of aromatic amino, azoxy and azo products with excellent yield (up to 97%). The exploitation of an alternative nitroreductase from Enterobacter cloacae enables the tailoring of a photoenzymatic system for the challenging synthesis of aliphatic amines from nitroalkenes and nitroalkanes (up to 90% yield). This photoenzymatic reduction overcomes the competing bio-Nef reaction, typically hindering the complete enzymatic reduction of nitroaliphatics. The results highlight the usefulness of nitroreductases to create selective photoenzymatic systems for the synthesis of precious chemicals, and the effectiveness of chlorophyll as an innocuous photocatalyst, enabling the use of sunlight to drive the photobiocatalytic reactions. 2873 ncomms Nature Communications Nat Commun Nature Publishing Group PMC10482925 10482925 10482925 37673927 10.1038/s41467-023-41194-w Tailored photoenzymatic systems for selective reduction of aliphatic and aromatic nitro compounds fueled by light Luján Alejandro Prats 1 Bhat Mohammad Faizan 1 Tsaturyan Sona 1 van Merkerk Ronald 1 Fu Haigen 2 Poelarends Gerrit J 1 ✉ 1 Department of Chemical and Pharmaceutical Biology, Groningen Research Institute of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands 2 Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14850 USA ✉ Corresponding author. Flavoenzymes such as nitroreductases are able to reduce nitro compounds in aqueous conditions with a certain degree of chemo- and regio-selectivity. These enzymes catalyze the reduction of nitro (–NO 2 ) groups into amines (–NH 2 ) via nitroso (–NO) and hydroxylamine (–NHOH) intermediates by using prosthetic flavin and NAD(P)H cofactors 28 – 32 . However, the selective reduction of nitro compounds is a persisting challenge for biocatalysis, frequently resulting in a mixture of intermediates and incomplete reductions 30 , 33 , 34 . Photoenzymatic synthesis of aromatic amines using BaNTR1 Next, we sought to explore the scope and limitations of this photoenzymatic reaction. Pleasingly, a wide variety of nitroarenes are well accepted by BaNTR1 (Fig. 2 , Suppl. Fig. 2 , and Suppl. Fig. 3 ). Nitroarenes possessing electron-withdrawing groups such as halogens, nitrile and trifluoromethyl at different positions are efficiently converted to Nitroarene substrates containing electron-withdrawing and electron-donating groups at different positions are well accepted by BaNTR1, providing the desired aromatic azoxy products 37–51 with excellent conversions and selectivity (Fig. 3 and Suppl. Fig. 4 ). Remarkably, BaNTR1 is capable of chemoselectively reducing the nitro group over the carbonyl group ( 48–49 ). Moreover, the heterocyclic azoxy compound 52 can also be synthesized using this photoenzymatic system. However, starting substrates containing amine, hydroxy, and more complex structures resulted in a mixture of products without the formation of the corresponding azoxy product. By screening an in-house panel of flavoenzymes 45 , we found that the nitroreductase EcNR, which also exhibits activity towards nitroaromatics, achieved the formation of minor amounts of the respective aliphatic amine product from both nitro compounds (Suppl. Table 5 ). During the optimization of the reaction conditions, we observed that the addition of the mild reducing agent, ascorbic acid, improved the conversion into amine to 25–37% (Suppl. Table 6 ). Alternatively, when the reaction was performed under photobiocatalytic conditions (chlorophyll and red LEDs), the amine synthesis also showed a remarkable increase in conversion (64–73%). 5 , Suppl. Fig. 6 ). Interestingly, EcNR can process nitroalkenes and nitroalkanes harboring a phenyl ring (or cyclohexene group), which can either be in conjugation with the nitro functionality or separated from the nitro group by a methylene bridge. For the panel of nitroalkenes, the EcNR-based photoenzymatic system performed the reduction of electronically diverse substrates, reducing the nitro group as well as the carbon-carbon double bond, achieving the synthesis of the corresponding aliphatic amines ( 68–77 , up to >99%). On the other hand, the addition of a strong oxidant (H 2 O 2 ) promotes the photoenzymatic formation of nitroso and hydroxylamine intermediates, followed by their spontaneous condensation into azoxy products. Interestingly, the addition of a milder oxidant (molecular oxygen) to the photoenzymatic reaction leads toward the accumulation of azoxy products while allowing a further reduction into aromatic azo compounds. Finally, the selection of an alternative nitroreductase, EcNR, and the inclusion of a mild reducing agent, ascorbic acid, allows for the efficient photobiocatalytic synthesis of a series of aliphatic amines. We recently reported that nitroreductase BaNTR1 can perform the reduction of ketones following a presumed ketyl radical formation under photoenzymatic conditions 45 . In addition, the reduction of nitroarenes via single electron transfers has been reported previously using light-mediated photochemical radical pathways 25 . The photoenzymatic systems reported herein represent an alternative to chemical and photochemical methods, overcoming the inability to selectively achieve multiple products from a single starting material and effectively producing highly valuable aliphatic amines from the corresponding nitro compounds.