Photoenzymatic enantioselective intermolecular radical hydroalkylation

An evolved artificial radical cyclase enables the construction of bicyclic terpenoid scaffolds via an H-atom transfer pathway

While natural terpenoid cyclases generate complex terpenoid structures via cationic mechanisms, alternative radical cyclization pathways are underexplored. The metal-catalysed H-atom transfer reaction (M-HAT) offers an attractive means for hydrofunctionalizing olefins, providing access to terpenoid-like structures. Artificial metalloenzymes offer a promising strategy for introducing M-HAT reactivity into a protein scaffold. Here we report our efforts towards engineering an artificial radical cyclase (ARCase), resulting from anchoring a biotinylated [Co(Schiff-base)] cofactor within an engineered chimeric streptavidin. After two rounds of directed evolution, a double mutant catalyses a radical cyclization to afford bicyclic products with a cis-5-6-fused ring structure and up to 97% enantiomeric excess. The involvement of a histidine ligation to the Co cofactor is confirmed by crystallography. A time course experiment reveals a cascade reaction catalysed by the ARCase, combining a radical cyclization with a conjugate reduction. The ARCase exhibits tolerance towards variations in the dienone substrate, highlighting its potential to access terpenoid scaffolds.

Chemogenetic Evolution of Diversified Photoenzymes for Enantioselective [2 + 2] Cycloadditions in Whole Cells

Artificial photoenzymes with novel catalytic modes not found in nature are in high demand; yet, they also present significant challenges in the field of biocatalysis. In this study, a chemogenetic modification strategy is developed to facilitate the rapid diversification of photoenzymes. This strategy integrates site-specific chemical conjugation of various artificial photosensitizers into natural protein cavities and the iterative mutagenesis in cell lysates. Through rounds of directed evolution, prominent visible-light-activatable photoenzyme variants were developed, featuring a thioxanthone chromophore. They successfully enabled the enantioselective [2 + 2] photocycloaddition of 2-carboxamide indoles, a class of UV-sensitive substrates that are traditionally challenging for known photoenzymes. Furthermore, the versatility of this photoenzyme is demonstrated in enantioselective whole-cell photobiocatalysis, enabling the efficient synthesis of enantioenriched cyclobutane-fused indoline tetracycles. These findings significantly expand the photophysical properties of artificial photoenzymes, a critical factor in enhancing their potential for harnessing excited-state reactivity in stereoselective transformations.

Unlocking the function promiscuity of old yellow enzyme to catalyze asymmetric Morita-Baylis-Hillman reaction

Exploring the promiscuity of native enzymes presents a promising strategy for expanding their synthetic applications, particularly for catalyzing challenging reactions in non-native contexts. In this study, we explore the promiscuous potential of old yellow enzymes (OYEs) to facilitate the Morita-Baylis-Hillman reaction (MBH reaction), leveraging substrate similarities between MBH reaction and reduction reaction. Using mass spectrometry and spectroscopic techniques, we confirm promiscuity of GkOYE in both MBH and reduction reactions. By blocking H- and H+ transfer pathways, we engineer GkOYE.8, which loses its reduction ability but enhances its MBH activity. The structural basis of MBH reaction catalyzed by GkOYE.8 is obtained through mutation studies and kinetic simulations. Furthermore, enantiocomplementary mutants GkOYE.11 and GkOYE.13 are obtained by directed evolution, exhibiting the ability to accept various aromatic aldehydes and alkenes as substrates. This study demonstrates the potential of leveraging substrate similarities to unlock enzyme functionalities, enabling the catalysis of new-to-nature reactions.

Threonine Aldolase-Catalyzed Enantioselective α-Alkylation of Amino Acids through Unconventional Photoinduced Radical Initiation

Visible light-driven pyridoxal radical biocatalysis has emerged as a promising strategy for the stereoselective synthesis of valuable noncanonical amino acids (ncAAs). Previously, the use of well-tailored photoredox catalysts represented the key to enable efficient pyridoxal phosphate (PLP) enzyme-catalyzed radical reactions. Here, we report a PLP-dependent threonine aldolase-catalyzed asymmetric α-C-H alkylation of abundant amino acids using Katritzky pyridinium salts as alkylating agents. The use of engineered threonine aldolases allowed for this redox-neutral radical alkylation to proceed efficiently, giving rise to challenging α-trisubstituted and -tetrasubstituted ncAA products in a protecting-group-free fashion with excellent enantiocontrol. Mechanistically, this enantioselective α-alkylation capitalizes on the unique reactivity of the persistent enzymatic quinonoid intermediate derived from the PLP cofactor and the amino acid substrate to allow for novel radical C-C coupling. Surprisingly, this photobiocatalytic process does not require the use of well-established photoredox catalysts and operates through an unconventional photoinduced radical generation involving a PLP-derived aldimine. The ability to develop photobiocatalytic reactions without relying on classic photocatalysts or photoenzymes opens up new avenues for advancing stereoselective intermolecular radical reactions that are not known in either organic chemistry or enzymology.

Repurposing Visible-Light-Excited Ene-Reductases for Diastereo- and Enantioselective Lactones Synthesis

Repurposing enzymes to catalyze non-natural asymmetric transformations that are difficult to achieve using traditional chemical methods is of significant importance. Although radical C-O bond formation has emerged as a powerful approach for constructing oxygen-containing compounds, controlling the stereochemistry poses a great challenge. Here we present the development of a dual bio-/photo-catalytic system comprising an ene-reductase and an organic dye for achieving stereoselective lactonizations. By integrating directed evolution and photoinduced single electron oxidation, we repurposed engineered ene-reductases to steer non-natural radical C-O formations (one C-O bond for hydrolactonizations and lactonization-alkylations while two C-O bonds for lactonization-oxygenations). This dual catalysis gave a new approach to a diverse array of enantioenhanced 5- and 6-membered lactones with vicinal stereocenters, part of which bears a quaternary stereocenter (up to 99 % enantiomeric excess, up to 12.9 : 1 diastereomeric ratio). Detailed mechanistic studies, including computational simulations, uncovered the synergistic effect of the enzyme and the externally added organophotoredox catalyst Rh6G.

Enhanced Photobiocatalytic Cascades at Pickering Droplet Interfaces

Developing new methods to engineer photobiocatalytic reactions is of utmost significance for artificial photosynthesis, but it remains a grand challenge due to the intrinsic incompatibility of biocatalysts with photocatalysts. In this work, photocatalysts and enzymes were spatially colocalized at Pickering droplet interfaces, where the reaction microenvironment and the spatial distance between two distinct catalysts were exquisitely regulated to achieve unprecedented photobiocatalytic cascade reactions. As proof of the concept, ultrathin graphitic carbon nitride nanosheets loaded with Au nanoparticles were precisely positioned in the outer interfacial layer of Pickering oil droplets to produce H2O2 under light irradiation, while enzymes were exactly placed in the inner interfacial layer to catalyze the subsequent biocatalytic oxidation reactions using in situ formed H2O2 as an oxidant. In the alkene epoxidation and thioether oxidation, our interfacial photobiocatalytic cascades showed a 2.0-5.8-fold higher overall reaction efficiency than the photobiocatalytic cascades in the bulk water phase. It was demonstrated that spatial localization of the photocatalyst and the enzyme at Pickering oil droplet interfaces not only provided their respective preferable reaction environments and intimate proximity for rapid H2O2 transport but also protected the enzyme from oxidative inactivation caused by the photogenerated species. These remarkable interfacial effects contributed to the significantly enhanced photobiocatalytic cascading efficiency. Our work presents an innovative photobiocatalytic reaction system with manifold benefits, providing a cutting-edge platform for solar-driven chemical transformations via photobiocatalysis.

A New Age of Biocatalysis Enabled by Generic Activation Modes

Biocatalysis is currently undergoing a profound transformation. The field moves from relying on nature's chemical logic to a discipline that exploits generic activation modes, allowing for novel biocatalytic reactions and, in many instances, entirely new chemistry. Generic activation modes enable a wide range of reaction types and played a pivotal role in advancing the fields of organo- and photocatalysis. This perspective aims to summarize the principal activation modes harnessed in enzymes to develop new biocatalysts. Although extensively researched in the past, the highlighted activation modes, when applied within enzyme active sites, facilitate chemical transformations that have largely eluded efficient and selective catalysis. This advance is attributed to multiple tunable interactions in the substrate binding pocket that precisely control competing reaction pathways and transition states. We will highlight cases of new synthetic methodologies achieved by engineered enzymes and will provide insights into potential future developments in this rapidly evolving field.

Cobalt-Catalyzed Enantioselective Reductive α-Chloro-Carbonyl Addition of Ketimine to Construct the β-Tertiary Amino Acid Analogues

β-Tertiary amino acid derivatives constitute one of the most frequently occurring units in natural products and bioactive molecules. However, the efficient asymmetric synthesis of this motif still remains a significant challenge. Herein, we disclose a cobalt-catalyzed enantioselective reductive addition reaction of ketimine using α-chloro carbonyl compound as a radical precursor, providing expedient access to a diverse array of enantioenriched β-quaternary amino acid analogues. This protocol exhibits outstanding enantioselectivity and broad substrate scope with excellent functional group tolerance. Preliminary mechanism studies rule out the possibility of Reformatsky-type addition and confirm the involvement of radical species in stereoselective addition process. The synthetic utility has been demonstrated through the rapid assembly of iterative amino acid units and oligopeptide, showcasing its versatile platform for late-stage modification of drug candidates.

A Type of Chiral C2-Symmetric Arylthiol Catalyst for Highly Enantioselective Anti-Markovnikov Hydroamination

The development of chiral hydrogen donor catalysts is fundamental in the expansion and innovation of asymmetric organocatalyzed reactions via an enantioselective hydrogen atom transfer (HAT) process. Herein, an unprecedented type of chiral C2-symmetric arylthiol catalysts derived from readily available enantiomeric lactate ester was developed. With these catalysts, an asymmetric anti-Markovnikov alkene hydroamination-cyclization reaction was established, affording a variety of pharmaceutically interesting 3-substituted piperidines with moderate to high enantioselectivity. Results of the designed control experiments and theoretical computation rationalized the origin of stereocontrol and disclosed the spatial effect of the moiety of chiral thiols on the enantioselectivity. We believed the facile synthesis, flexible tunability, and effective enantioselectivity-controlling capability of these catalysts would shed light on the development of versatile chiral HAT catalysts and related asymmetric reactions.

Modular Access to Chiral Amines via Imine Reductase-Based Photoenzymatic Catalysis

The development of catalysts serves as the cornerstone of innovation in synthesis, as exemplified by the recent discovery of photoenzymes. However, the repertoire of naturally occurring enzymes repurposed by direct light excitation to catalyze new-to-nature photobiotransformations is currently limited to flavoproteins and keto-reductases. Herein, we shed light on imine reductases (IREDs) that catalyze the remote C(sp3)-C(sp3) bond formation, providing a previously elusive radical hydroalkylation of enamides for accessing chiral amines (45 examples with up to 99% enantiomeric excess). Beyond their natural function in catalyzing two-electron reductive amination reactions, upon direct visible-light excitation or in synergy with a synthetic photoredox catalyst, IREDs are repurposed to tune the non-natural photoinduced single-electron radical processes. By conducting wet mechanistic experiments and computational simulations, we unravel how engineered IREDs direct radical intermediates toward the productive and enantioselective pathway. This work represents a promising paradigm for harnessing nature's catalysts for new-to-nature asymmetric transformations that remain challenging through traditional chemocatalytic methods.

From Ground-State to Excited-State Activation Modes: Flavin-Dependent "Ene"-Reductases Catalyzed Non-natural Radical Reactions

Enzymes are desired catalysts for chemical synthesis, because they can be engineered to provide unparalleled levels of efficiency and selectivity. Yet, despite the astonishing array of reactions catalyzed by natural enzymes, many reactivity patterns found in small molecule catalysts have no counterpart in the living world. With a detailed understanding of the mechanisms utilized by small molecule catalysts, we can identify existing enzymes with the potential to catalyze reactions that are currently unknown in nature. Over the past eight years, our group has demonstrated that flavin-dependent "ene"-reductases (EREDs) can catalyze various radical-mediated reactions with unparalleled levels of selectivity, solving long-standing challenges in asymmetric synthesis.This Account presents our development of EREDs as general catalysts for asymmetric radical reactions. While we have developed multiple mechanisms for generating radicals within protein active sites, this account will focus on examples where flavin mononucleotide hydroquinone (FMNhq) serves as an electron transfer radical initiator. While our initial mechanistic hypotheses were rooted in electron-transfer-based radical initiation mechanisms commonly used by synthetic organic chemists, we ultimately uncovered emergent mechanisms of radical initiation that are unique to the protein active site. We will begin by covering intramolecular reactions and discussing how the protein activates the substrate for reduction by altering the redox-potential of alkyl halides and templating the charge transfer complex between the substrate and flavin-cofactor. Protein engineering has been used to modify the fundamental photophysics of these reactions, highlighting the opportunity to tune these systems further by using directed evolution. This section highlights the range of coupling partners and radical termination mechanisms available to intramolecular reactions.The next section will focus on intermolecular reactions and the role of enzyme-templated ternary charge transfer complexes among the cofactor, alkyl halide, and coupling partner in gating electron transfer to ensure that it only occurs when both substrates are bound within the protein active site. We will highlight the synthetic applications available to this activation mode, including olefin hydroalkylation, carbohydroxylation, arene functionalization, and nitronate alkylation. This section also discusses how the protein can favor mechanistic steps that are elusive in solution for the asymmetric reductive coupling of alkyl halides and nitroalkanes. We are aware of several recent EREDs-catalyzed photoenzymatic transformations from other groups. We will discuss results from these papers in the context of understanding the nuances of radical initiation with various substrates.These biocatalytic asymmetric radical reactions often complement the state-of-the-art small-molecule-catalyzed reactions, making EREDs a valuable addition to a chemist's synthetic toolbox. Moreover, the underlying principles studied with these systems are potentially operative with other cofactor-dependent proteins, opening the door to different types of enzyme-catalyzed radical reactions. We anticipate that this Account will serve as a guide and inspire broad interest in repurposing existing enzymes to access new transformations.

Contact-electro-catalysis (CEC)

Contact-electro-catalysis (CEC) is an emerging field that utilizes electron transfer occurring at the liquid-solid and even liquid-liquid interfaces because of the contact-electrification effect to stimulate redox reactions. The energy source of CEC is external mechanical stimuli, and solids to be used are generally organic as well as in-organic materials even though they are chemically inert. CEC has rapidly garnered extensive attention and demonstrated its potential for both mechanistic research and practical applications of mechanocatalysis. This review aims to elucidate the fundamental principle, prominent features, and applications of CEC by compiling and analyzing the recent developments. In detail, the theoretical foundation for CEC, the methods for improving CEC, and the unique advantages of CEC have been discussed. Furthermore, we outline a roadmap for future research and development of CEC. We hope that this review will stimulate extensive studies in the chemistry community for investigating the CEC, a catalytic process in nature.

Stereoselective amino acid synthesis by photobiocatalytic oxidative coupling

Photobiocatalysis-where light is used to expand the reactivity of an enzyme-has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts1. So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes2-9. Here we report photobiocatalytic asymmetric sp3-sp3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5'-phosphate-dependent enzymes, threonine aldolases10-12, for the α-C-H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids 13-15 possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox-pyridoxal biocatalysis provides a platform for sp3-sp3 oxidative coupling16, permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology.

Photoenzymatic Asymmetric Hydroamination for Chiral Alkyl Amine Synthesis

Chiral alkyl amines are common structural motifs in pharmaceuticals, natural products, synthetic intermediates, and bioactive molecules. An attractive method to prepare these molecules is the asymmetric radical hydroamination; however, this approach has not been explored with dialkyl amine-derived nitrogen-centered radicals since designing a catalytic system to generate the aminium radical cation, to suppress deleterious side reactions such as α-deprotonation and H atom abstraction, and to facilitate enantioselective hydrogen atom transfer is a formidable task. Herein, we describe the application of photoenzymatic catalysis to generate and harness the aminium radical cation for asymmetric intermolecular hydroamination. In this reaction, the flavin-dependent ene-reductase photocatalytically generates the aminium radical cation from the corresponding hydroxylamine and catalyzes the asymmetric intermolecular hydroamination to furnish the enantioenriched tertiary amine, whereby enantioinduction occurs through enzyme-mediated hydrogen atom transfer. This work highlights the use of photoenzymatic catalysis to generate and control highly reactive radical intermediates for asymmetric synthesis, addressing a long-standing challenge in chemical synthesis.

Stereodivergent photobiocatalytic radical cyclization through the repurposing and directed evolution of fatty acid photodecarboxylases

Despite their intriguing photophysical and photochemical activities, naturally occurring photoenzymes have not yet been repurposed for new-to-nature activities. Here we engineered fatty acid photodecarboxylases to catalyse unnatural photoredox radical C-C bond formation by leveraging the strongly oxidizing excited-state flavoquinone cofactor. Through genome mining, rational engineering and directed evolution, we developed a panel of radical photocyclases to facilitate decarboxylative radical cyclization with excellent chemo-, enantio- and diastereoselectivities. Our high-throughput experimental workflow allowed for the directed evolution of fatty acid photodecarboxylases. An orthogonal set of radical photocyclases was engineered to access all four possible stereoisomers of the stereochemical dyad, affording fully diastereo- and enantiodivergent biotransformations in asymmetric radical biocatalysis. Molecular dynamics simulations show that our evolved radical photocyclases allow near-attack conformations to be easily accessed, enabling chemoselective radical cyclization. The development of stereoselective radical photocyclases provides unnatural C-C-bond-forming activities in natural photoenzyme families, which can be used to tame the stereochemistry of free-radical-mediated reactions.

Covalent Organic Frameworks Based Photoenzymatic Nano-reactor for Asymmetric Dynamic Kinetic Resolution of Secondary Amines

Bio-catalysis represents a highly efficient and stereoselective method for the synthesis of valuable chiral compounds, however, the poor stability and limited reaction types of free enzymes restrict their wide application in industrial production. In this work, to overcome these problems, a multifunctional photoenzymatic nanoreactor CALB@COF-Ir was developed through the encapsulation of Candida antarctica lipase B (CALB) in a photosensitive covalent organic framework COF-Ir. This bio-nanocluster serves as efficient catalysts in asymmetric dynamic kinetic resolution (DKR) of secondary amines to give a series of chiral amines in high yields (up to 99 %) and enantioselectivities (up to 99 % ee). The well-designed COF-Ir not only acts as safety cover to prevent CALB from deactivation but promotes racemization of secondary amines via photo-induced hydrogen atom transfer (HAT) process. Photoelectric characterization and TDDFT calculation revealed that (ppy)2Ir units in COF-Ir play crucial role in this photocatalytic system which enhance its photo-redox properties through facilitating the separation between photoelectrons (e-) and holes (h+). Furthermore, the heterogeneous photoenzymatic nanoreactor could be recycled for five rounds with slight decline of catalytic reactivity.

Flavin-N5OOH Functions as both a Powerful Nucleophile and a Base in the Superfamily of Flavoenzymes

Flavoenzymes can mediate a large variety of oxidation reactions through the activation of oxygen. However, the O2 activation chemistry of flavin enzymes is not yet fully exploited. Normally, the O2 activation occurs at the C4a site of the flavin cofactor, yielding the flavin C4a-(hydro)hydroperoxyl species in monooxygenases or oxidases. Using extensive MD simulations, QM/MM calculations and QM calculations, our studies reveal the formation of the common nucleophilic species, Flavin-N5OOH, in two distinct flavoenzymes (RutA and EncM). Our studies show that Flavin-N5OOH acts as a powerful nucleophile that promotes C-N cleavage of uracil in RutA, and a powerful base in the deprotonation of substrates in EncM. We reason that Flavin-N5OOH can be a common reactive species in the superfamily of flavoenzymes, which accomplish generally selective general base catalysis and C-X (X=N, S, Cl, O) cleavage reactions that are otherwise challenging with solvated hydroxide ion base. These results expand our understanding of the chemistry and catalysis of flavoenzymes.

Multifunctional Biocatalysts for Organic Synthesis

Biocatalysis is becoming an indispensable tool in organic synthesis due to high enzymatic catalytic efficiency as well as exquisite chemo- and stereoselectivity. Some biocatalysts display great promiscuity including a broad substrate scope as well as the ability to catalyze more than one type of transformation. These promiscuous activities have been applied individually to efficiently access numerous valuable target molecules. However, systems in which enzymes possessing multiple different catalytic activities are applied in the synthesis are less well developed. Such multifunctional biocatalysts (MFBs) would simplify chemical synthesis by reducing the number of operational steps and enzyme count, as well as simplifying the sequence space that needs to be engineered to develop an efficient biocatalyst. In this Perspective, we highlight recently reported MFBs focusing on their synthetic utility and mechanism. We also offer insight into their origin as well as comment on potential strategies for their discovery and engineering.

Asymmetric Synthesis of α-Chloroamides via Photoenzymatic Hydroalkylation of Olefins

Photoenzymatic intermolecular hydroalkylations of olefins are highly enantioselective for chiral centers formed during radical termination but poorly selective for centers set in the C-C bond-forming event. Here, we report the evolution of a flavin-dependent "ene"-reductase to catalyze the coupling of α,α-dichloroamides with alkenes to afford α-chloroamides in good yield with excellent chemo- and stereoselectivity. These products can serve as linchpins in the synthesis of pharmaceutically valuable motifs. Mechanistic studies indicate that radical formation occurs by exciting a charge-transfer complex templated by the protein. Precise control over the orientation of molecules within the charge-transfer complex potentially accounts for the observed stereoselectivity. The work expands the types of motifs that can be prepared using photoenzymatic catalysis.

Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation

The photochemical properties of Electron Donor-Acceptor (EDA) complexes present exciting opportunities for synthetic chemistry. However, these strategies often require an inert atmosphere to maintain high efficiency. Herein, we develop an EDA complex photocatalytic system through rational design, which overcomes the oxygen-sensitive limitation of traditional EDA photocatalytic systems and enables aerobic oxygenation reactions through dioxygen activation. The mild oxidation system transfers electrons from the donor to the effective catalytic acceptor upon visible light irradiation, which are subsequently captured by molecular oxygen to form the superoxide radical ion, as demonstrated by the specific fluorescent probe, dihydroethidine (DHE). Furthermore, this visible-light mediated oxidative EDA protocol is successfully applied in the aerobic oxygenation of boronic acids. We believe that this photochemical dioxygen activation strategy enabled by EDA complex not only provides a practical approach to aerobic oxygenation but also promotes the design and application of EDA photocatalysis under ambient conditions.

Regiodivergent Radical Termination for Intermolecular Biocatalytic C-C Bond Formation

Radical hydrofunctionalizations of electronically unbiased dienes are challenging to render regioselective, because the products are nearly identical in energy. Here, we report two engineered FMN-dependent "ene"-reductases (EREDs) that catalyze regiodivergent hydroalkylations of cyclic and linear dienes. While previous studies focused exclusively on the stereoselectivity of alkene hydroalkylation, this work highlights that EREDs can control the regioselectivity of hydrogen atom transfer, providing a method for selectively preparing constitutional isomers that would be challenging to prepare using traditional synthetic methods. Engineering the ERED from Gluconabacter sp. (GluER) furnished a variant that favors the γ,δ-unsaturated ketone, while an engineered variant from a commercial ERED panel favors the δ,ε-unsaturated ketone. The effect of beneficial mutations has been investigated using substrate docking studies and the mechanism probed by isotope labeling experiments. A variety of α-bromo ketones can be coupled with cyclic and linear dienes. These interesting building blocks can also be further modified to generate difficult-to-access heterocyclic compounds.

Reagent Engineering for Group Transfer Biocatalysis

Biocatalysis has become a major driver in the innovation of preparative chemistry. Enzyme discovery, engineering and computational design have matured to reliable strategies in the development of biocatalytic processes. By comparison, substrate engineering has received much less attention. In this Minireview, we highlight the idea that the design of synthetic reagents may be an equally fruitful and complementary approach to develop novel enzyme-catalysed group transfer chemistry. This Minireview discusses key examples from the literature that illustrate how synthetic substrates can be devised to improve the efficiency, scalability and sustainability, as well as the scope of such reactions. We also provide an opinion as to how this concept might be further developed in the future, aspiring to replicate the evolutionary success story of natural group transfer reagents, such as adenosine triphosphate (ATP) and S-adenosyl methionine (SAM).

Remote stereocontrol with azaarenes via enzymatic hydrogen atom transfer

Strategies for achieving asymmetric catalysis with azaarenes have traditionally fallen short of accomplishing remote stereocontrol, which would greatly enhance accessibility to distinct azaarenes with remote chiral centres. The primary obstacle to achieving superior enantioselectivity for remote stereocontrol has been the inherent rigidity of the azaarene ring structure. Here we introduce an ene-reductase system capable of modulating the enantioselectivity of remote carbon-centred radicals on azaarenes through a mechanism of chiral hydrogen atom transfer. This photoenzymatic process effectively directs prochiral radical centres located more than six chemical bonds, or over 6 Å, from the nitrogen atom in azaarenes, thereby enabling the production of a broad array of azaarenes possessing a remote γ-stereocentre. Results from our integrated computational and experimental investigations underscore that the hydrogen bonding and steric effects of key amino acid residues are important for achieving such high stereoselectivities.

Single-Electron Oxidation-Initiated Enantioselective Hydrosulfonylation of Olefins Enabled by Photoenzymatic Catalysis

Controlling the enantioselectivity of hydrogen atom transfer (HAT) reactions has been a long-standing synthetic challenge. While recent advances on photoenzymatic catalysis have demonstrated the great potential of non-natural photoenzymes, all of the transformations are initiated by single-electron reduction of the substrate, with only one notable exception. Herein, we report an oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins using a novel mutant of gluconobacter ene-reductase (GluER-W100F-W342F). Compared to known photoenzymatic systems, our approach does not rely on the formation of an electron donor-acceptor complex between the substrates and enzyme cofactor and simplifies the reaction system by obviating the addition of a cofactor regeneration mixture. More importantly, the GluER variant exhibits high reactivity and enantioselectivity and a broad substrate scope. Mechanistic studies support the proposed oxidation-initiated mechanism and reveal that a tyrosine-mediated HAT process is involved.

Nature-Inspired Photocatalytic Azo Bond Cleavage with Red Light

The emerging field of photoredox catalysis in mammalian cells enables spatiotemporal regulation of a wealth of biological processes. However, the selective cleavage of stable covalent bonds driven by low-energy visible light remains a great challenge. Herein, we report that red light excitation of a commercially available dye, abbreviated NMB+, leads to catalytic cleavage of stable azo bonds in both aqueous solutions and hypoxic cells and hence a means to photodeliver drugs or functional molecules. Detailed mechanistic studies reveal that azo bond cleavage is triggered by a previously unknown consecutive two-photon process. The first photon generates a triplet excited state, 3NMB+*, that is reductively quenched by an electron donor to generate a protonated NMBH•+. The NMBH•+ undergoes a disproportionation reaction that yields the initial NMB+ and two-electron-reduced NMBH (i.e., leuco-NMB, abbreviated as LNMB). Interestingly, LNMB forms a charge transfer complex with all four azo substrates that possess an intense absorption band in the red region. A second red photon induces electron transfer from LNMB to the azo substrate, resulting in azo bond cleavage. The charge transfer complex mediated two-photon catalytic mechanism reported herein is reminiscent of the flavin-dependent natural photoenzyme that catalyzes bond cleavage reactions with high-energy photons. The red-light-driven photocatalytic strategy offers a new approach to bioorthogonal azo bond cleavage for photodelivery of drugs or functional molecules.

Novel applications of photobiocatalysts in chemical transformations

Photocatalysis has proven to be an effective approach for the production of reactive intermediates under moderate reaction conditions. The possibility for the green synthesis of high-value compounds using the synergy of photocatalysis and biocatalysis, benefiting from the selectivity of enzymes and the reactivity of photocatalysts, has drawn growing interest. Mechanistic investigations, substrate analyses, and photobiocatalytic chemical transformations will all be incorporated in this review. We seek to shed light on upcoming synthetic opportunities in the field by precisely describing mechanistically unique techniques in photobiocatalytic chemistry.

One-pot chemoenzymatic syntheses of non-canonical amino acids

Despite their prevalent use in drug discovery and protein biochemistry, non-canonical amino acids are still challenging to synthesize through purely chemical means. In recent years, biocatalysis has emerged as a transformative paradigm for small-molecule synthesis. One strategy to further empower biocatalysis is to use it in combination with modern chemical reactions and take advantage of the strengths of each method to enable access to challenging structural motifs that were previously unattainable using each method alone. In this Mini-Review, we highlight several recent case studies that feature the synergistic use of chemical and enzymatic transformations in one pot to synthesize novel non-canonical amino acids.

ONE-SENTENCE SUMMARY

This Mini-Review highlights several recent case studies that feature the synergistic use of chemical and enzymatic transformations in one pot to synthesize novel non-canonical amino acids.

A light-driven enzymatic enantioselective radical acylation

Enzymes are recognized as exceptional catalysts for achieving high stereoselectivities1-3, 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)6-8 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.

Photoenzymatic Enantioselective Synthesis of Oxygen-Containing Benzo-Fused Heterocycles

New-to-nature biocatalysis in organic synthesis has recently emerged as a green and powerful strategy for the preparation of valuable chiral products, among which chiral oxygen-containing benzo-fused heterocycles are important structural motifs in pharmaceutical industry. However, the asymmetric synthesis of these compounds through radical-mediated methods is challenging. Herein, a novel asymmetric radical-mediated photoenzymatic synthesis strategy is developed to realize the efficient enantioselective synthesis of oxygen-containing benzo-fused heterocycles through structure-guided engineering of a flavin-dependent 'ene'-reductase GluER. It shows that variant GluER-W100H could efficiently produce various benzoxepinones, chromanone and indanone with different benzo-fused rings in high yields with great stereoselectivities under visible light. Moreover, these results are well supported by mechanistic experiments, revealing that this photoenzymatic process involves electron donor-acceptor complex formation, single electron transfer and hydrogen atom transfer. Therefore, we provide an alternative green approach for efficient chemoenzymatic synthesis of important chiral skeletons of bioactive pharmaceuticals.

Evidence of a Distinctive Enantioselective Binding Mode for the Photoinduced Radical Cyclization of α-Chloroamides in Ene-Reductases

We demonstrate here through molecular simulations and mutational studies the origin of the enantioselectivity in the photoinduced radical cyclization of α-chloroacetamides catalyzed by ene-reductases, in particular the Gluconobacter oxidans ene-reductase and the Old Yellow Enzyme 1, which show opposite enantioselectivity. Our results reveal that neither the π-facial selectivity model nor a protein-induced selective stabilization of the transition states is able to explain the enantioselectivity of the radical cyclization in the studied flavoenzymes. We propose a new enantioinduction scenario according to which enantioselectivity is indeed controlled by transition-state stability; however, the relative stability of the prochiral transition states is not determined by direct interaction with the protein but is rather dependent on an inherent degree of freedom within the substrate itself. This intrinsic degree of freedom, distinct from the traditional π-facial exposure mode, can be controlled by the substrate conformational selection upon binding to the enzyme.

Photoredox/Enzymatic Catalysis Enabling Redox-Neutral Decarboxylative Asymmetric C-C Coupling for Asymmetric Synthesis of Chiral 1,2-Amino Alcohols

Photocatalysis offers tremendous opportunities for enzymes to access new functions. Herein, we described a redox-neutral photocatalysis/enzymatic catalysis system for the asymmetric synthesis of chiral 1,2-amino alcohols via decarboxylative radical C-C coupling of N-arylglycines and aldehydes by combining an organic photocatalyst, eosin Y, and carbonyl reductase RasADH. Notably, this protocol avoids using any sacrificial reductants. A possible reaction mechanism proposed is that the transformation proceeds through sequential photoinduced decarboxylative radical addition to an aldehyde and a photoenzymatic deracemization pathway. This redox-neutral photoredox/enzymatic strategy is promising not only for effective synthesis of a series of chiral amino alcohols in a green and sustainable manner but also for the design of other novel C-C radical coupling transformations for the synthesis of bioactive molecules.

In Situ Encapsulation of Cytochrome c within Covalent Organic Frames Using Deep Eutectic Solvents under Ambient Conditions

In situ integration of enzymes with covalent organic frameworks (COFs) to form hybrid biocatalysts is both significant and challenging. In this study, we present an innovative strategy employing deep eutectic solvents (DESs) to synergistically synthesize COFs and shield cytochrome c (Cyt c). By utilizing DESs as reaction solvents in combination with water, we successfully achieved rapid and in situ encapsulation of Cyt c within COFs (specifically COF-TAPT-TFB) under ambient conditions. The resulting Cyt c@COF-TAPT-TFB composite demonstrates a remarkable preservation of enzymatic activity. This encapsulation strategy also imparts exceptional resistance to organic solvents and exhibits impressive recycling stability. Additionally, the enhanced catalytic efficiency of Cyt c@COF-TAPT-TFB in a photoenzymatic cascade reaction is also showcased.

P450-catalyzed atom transfer radical cyclization

Cytochromes P450 have been extensively studied for both fundamental enzymology and biotechnological applications. Over the past decade, by taking inspiration from synthetic organic chemistry, new classes of P450-catalyzed reactions that were not previously encountered in the biological world have been developed to address challenging problems in organic chemistry and asymmetric catalysis. In particular, by repurposing and evolving P450 enzymes, stereoselective biocatalytic atom transfer radical cyclization (ATRC) was developed as a new means to impose stereocontrol over transient free radical intermediates. In this chapter, we describe the detailed experimental protocol for the directed evolution of P450 atom transfer radical cyclases. We also delineate protocols for analytical and preparative scale biocatalytic atom transfer radical cyclization processes. These methods will find application in the development of new P450-catalyzed radical reactions, as well as other synthetically useful processes.

EDA Photochemistry: Mechanistic Investigations and Future Opportunities

Recently, organic synthesis has seen a renaissance in radical chemistry due to the accessibility of mild methods for radical generation using visible light. While renewed interest in synthetic radical chemistry has been driven by the advent of photoredox catalysis, a resurgence of electron donor-acceptor (EDA) photochemistry has also led to many new radical transformations. Similar to photoredox catalysis, EDA photochemistry involves light-promoted single-electron transfer pathways. However, the mechanism of electron transfer in EDA systems is unique wherein the lifetimes of radical intermediates are often shorter due to competitive back-electron transfer. Distinguishing between EDA and photoredox mechanisms can be challenging since they can form identical products. In this perspective, we seek to provide insight on the mechanistic studies which can distinguish between EDA and photoredox manifolds. Additionally, we highlight some key challenges in EDA photochemistry and suggest future goals which could advance the synthetic potential of this field of research.

Recent Advances in the Enantioselective Radical Reactions

The review covers research published since 2017 and is focused on enantioselective synthesis using radical reactions. It describes recent approaches to the asymmetric synthesis of chiral molecules based on the application of the metal catalysis, dual metal and organocatalysis and finally, pure organocatalysis including enzyme catalysis. This review focuses on the synthetic aspects of the methodology and tries to show which compounds can be obtained in enantiomerically enriched forms.

Using Data Science for Mechanistic Insights and Selectivity Predictions in a Non-Natural Biocatalytic Reaction

The study of non-natural biocatalytic transformations relies heavily on empirical methods, such as directed evolution, for identifying improved variants. Although exceptionally effective, this approach provides limited insight into the molecular mechanisms behind the transformations and necessitates multiple protein engineering campaigns for new reactants. To address this limitation, we disclose a strategy to explore the biocatalytic reaction space and garner insight into the molecular mechanisms driving enzymatic transformations. Specifically, we explored the selectivity of an "ene"-reductase, GluER-T36A, to create a data-driven toolset that explores reaction space and rationalizes the observed and predicted selectivities of substrate/mutant combinations. The resultant statistical models related structural features of the enzyme and substrate to selectivity and were used to effectively predict selectivity in reactions with out-of-sample substrates and mutants. Our approach provided a deeper understanding of enantioinduction by GluER-T36A and holds the potential to enhance the virtual screening of enzyme mutants.

Asymmetric Carbohydroxylation of Alkenes Using Photoenzymatic Catalysis

Alkene difunctionalizations enable the synthesis of structurally elaborated products from simple and ubiquitous starting materials in a single chemical step. Carbohydroxylations of olefins represent a family of reactivity that furnish structurally complex alcohols. While examples of this type of three-component coupling have been reported, catalytic asymmetric examples remain elusive. Here, we report an enzyme-catalyzed asymmetric carbohydroxylation of alkenes catalyzed by flavin-dependent "ene"-reductases to produce enantioenriched tertiary alcohols. Seven rounds of protein engineering reshape the enzyme's active site to increase activity and enantioselectivity. Mechanistic studies suggest that C-O bond formation occurs via a 5-endo-trig cyclization with the pendant ketone to afford an α-oxy radical which is oxidized and hydrolyzed to form the product. This work demonstrates photoenzymatic reactions involving "ene"-reductases can terminate radicals via mechanisms other than hydrogen atom transfer, expanding their utility in chemical synthesis.

Photoenzymatic Enantioselective Intermolecular Radical Hydroamination

Since the discovery of Hofmann-Löffler-Freytag reaction more than 130 years ago, nitrogen-centered radicals have been widely studied in both structures and reactivities1-2. Nevertheless, catalytic enantioselective intermolecular radical hydroamination remains a challenge due to the existence of side reactions, short lifetime of nitrogen-centered radicals, and lack of understanding of the fundamental catalytic steps. In chemistry, nitrogen-centered radicals are produced with radical initiators, photocatalysts, or electrocatalysts. On the other hand, the generation and reaction of nitrogen-centered radicals are unknown in nature. Here we report a pure biocatalytic system by successfully repurposing an ene-reductase through directed evolution for the photoenzymatic production of nitrogen-centered radicals and enantioselective intermolecular radical hydroaminations. These reactions progress efficiently at room temperature under visible light without any external photocatalysts and exhibit excellent enantioselectivities. Detailed mechanistic study reveals that the enantioselectivity originates from the radical-addition step while the reactivity originates from the ultrafast photoinduced electron transfer (ET) from reduced flavin mononucleotide (FMNH-) to nitrogen-containing substrates.

Stereoselective amino acid synthesis by synergistic photoredox-pyridoxal radical biocatalysis

Developing synthetically useful enzymatic reactions that are not known in biochemistry and organic chemistry is an important challenge in biocatalysis. Through the synergistic merger of photoredox catalysis and pyridoxal 5'-phosphate (PLP) biocatalysis, we developed a pyridoxal radical biocatalysis approach to prepare valuable noncanonical amino acids, including those bearing a stereochemical dyad or triad, without the need for protecting groups. Using engineered PLP enzymes, either enantiomeric product could be produced in a biocatalyst-controlled fashion. Synergistic photoredox-pyridoxal radical biocatalysis represents a powerful platform with which to discover previously unknown catalytic reactions and to tame radical intermediates for asymmetric catalysis.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

Enzyme-controlled stereoselective radical cyclization to arenes enabled by metalloredox biocatalysis

The effective induction of high levels of stereocontrol for free radical-mediated transformations represents a notorious challenge in asymmetric catalysis. Herein, we describe a novel metalloredox biocatalysis strategy to repurpose natural cytochromes P450 to catalyse asymmetric radical cyclisation to arenes through an unnatural electron transfer mechanism. Empowered by directed evolution, engineered P450s allowed diverse radical cyclisation selectivities to be accomplished in a catalyst-controlled fashion: P450arc1 and P450arc2 facilitated enantioconvergent transformations of racemic substrates, giving rise to either enantiomer of the product with excellent total turnover numbers (up to 12,000). In addition to these enantioconvergent variants, another engineered radical cyclase, P450arc3, permitted efficient kinetic resolution of racemic chloride substrates (S factor = 18). Furthermore, computational studies revealed a proton-coupled electron transfer (PCET) mechanism for the radical-polar crossover step, suggesting the potential role of the haem carboxylate as a base catalyst. Collectively, the excellent tunability of this metalloenzyme family provides an exciting platform for harnessing free radical intermediates for asymmetric catalysis.

Photoexcited Ru single-atomic sites for efficient biomimetic redox catalysis

The unsatisfactory catalytic activity of nanozymes owing to their inefficient electron transfer (ET) is the major challenge in biomimetic catalysis-related biomedical applications. Inspired by the photoelectron transfers in natural photoenzymes, we herein report a photonanozyme of single-atom Ru anchored on metal-organic frameworks (UiO-67-Ru) for achieving photoenhanced peroxidase (POD)-like activity. We demonstrate that the atomically dispersed Ru sites can realize high photoelectric conversion efficiency, superior POD-like activity (7.0-fold photoactivity enhancement relative to that of UiO-67), and good catalytic specificity. Both in situ experiments and theoretical calculations reveal that photoelectrons follow the cofactor-mediated ET process of enzymes to promote the production of active intermediates and the release of products, demonstrating more favorable thermodynamics and kinetics in H2O2 reduction. Taking advantage of the unique interaction of the Zr-O-P bond, we establish a UiO-67-Ru-based immunoassay platform for the photoenhanced detection of organophosphorus pesticides.

Ene Reductase Enabled Intramolecular β-C-H Functionalization of Substituted Cyclohexanones for Efficient Synthesis of Bridged Bicyclic Nitrogen Scaffolds

Herein we report that ene reductases (EREDs) can facilitate an unprecedented intramolecular β-C-H functionalization reaction for the synthesis of bridged bicyclic nitrogen heterocycles containing the 6-azabicyclo[3.2.1]octane scaffold. To streamline the synthesis of these privileged motifs, we developed a gram-scale one-pot chemoenzymatic cascade by combining iridium photocatalysis with EREDs, using readily available N-phenylglycines and cyclohexenones that can be obtained from biomass. Further derivatization using enzymatic or chemical methods can convert 6-azabicyclo[3.2.1]octan-3-one into 6-azabicyclo[3.2.1]octan-3α-ols, which can be potentially utilized for the synthesis of azaprophen and its analogues for drug discovery. Mechanistic studies revealed the reaction requires oxygen, presumably to produce oxidized flavin, which can selectively dehydrogenate the 3-substituted cyclohexanone derivatives to form the α,β-unsaturated ketone, which subsequently undergoes spontaneous intramolecular aza-Michael addition under basic conditions.

Photobiocatalytic Strategies for Organic Synthesis

Biocatalysis has revolutionized chemical synthesis, providing sustainable methods for preparing various organic molecules. In enzyme-mediated organic synthesis, most reactions involve molecules operating from their ground states. Over the past 25 years, there has been an increased interest in enzymatic processes that utilize electronically excited states accessed through photoexcitation. These photobiocatalytic processes involve a diverse array of reaction mechanisms that are complementary to one another. This comprehensive review will describe the state-of-the-art strategies in photobiocatalysis for organic synthesis until December 2022. Apart from reviewing the relevant literature, a central goal of this review is to delineate the mechanistic differences between the general strategies employed in the field. We will organize this review based on the relationship between the photochemical step and the enzymatic transformations. The review will include mechanistic studies, substrate scopes, and protein optimization strategies. By clearly defining mechanistically-distinct strategies in photobiocatalytic chemistry, we hope to illuminate future synthetic opportunities in the area.

Enzyme Grafting with a Cofactor-Decorated Metal-Organic Capsule for Solar-to-Chemical Conversion

Semi-artificial approaches to solar-to-chemical conversion can achieve chemical transformations that are beyond the capability of natural enzymes, but face marked challenges to facilitate in vivo cascades, due to their inevitable need for cofactor shuttling and regeneration. Here, we report on an enzyme grafting strategy to build a metal-organic capsule-docking artificial enzyme (metal-organic-enzyme, MOE) that comprised the self-assembly of a cofactor-decorated capsule and the supramolecular enzyme-recognition features between the enzyme scaffold and the capsule to bypass cofactor shuttling and regeneration. The incorporated NADH mimics within the metal-organic capsule interacted with the imine intermediate that formed from the condensation of the amines and the dehydrogenation of alcohol substrates in the microenvironment to form complexes within the capsule and subsequently served as an in situ-generated photoresponsive cofactor. Upon illumination, the photoresponsive cofactor facilitates efficient proton/electron transport between the inner space (supramolecular hydrogenation) and outer space (enzymatic dehydrogenation) of the capsule to dehydrogenize the alcohols and hydrogenize the imine intermediates, respectively, circumventing the conventionally complex multistep cofactor shuttling and regeneration. The semi-artificial enzyme endows the conversion of diverse types of alcohol to amine products in both aqueous/organic solutions and Escherichia coli with high efficiency, offering a wide range of opportunities for sustainable and environmentally friendly biomanufacturing of commodity and fine chemicals.

Expanding the Promiscuity of a Copper-Dependent Oxidase for Enantioselective Cross-Coupling of Indoles

Herein, we disclose the highly enantioselective oxidative cross-coupling of 3-hydroxyindole esters with various nucleophilic partners as catalyzed by copper efflux oxidase. The biocatalytic transformation delivers functionalized 2,2-disubstituted indolin-3-ones with excellent optical purity (90-99 % ee), which exhibited anticancer activity against MCF-7 cell lines, as shown by preliminary biological evaluation. Mechanistic studies and molecular docking results suggest the formation of a phenoxyl radical and enantiocontrol facilitated by a suited enzyme chiral pocket. This study is significant with regard to expanding the catalytic repertoire of natural multicopper oxidases as well as enlarging the synthetic toolbox for sustainable asymmetric oxidative coupling.

Bioinspired Coassembly of Copper Ions and Nicotinamide Adenine Dinucleotides for Single-Site Nanozyme with Dual Catalytic Functions

Nanozymes can imitate the catalytic properties of natural enzymes while overcoming the limitations of natural enzymes such as high cost, poor robustness, and difficulty in recycling. However, rational design and facile preparation of nanozymes are still in demand. Inspired by the chemical structure of laccase, we report an amorphous metal-organic coordination nanocomposite named CuNAD, which is composed of copper ions and nicotinamide adenine dinucleotide (NAD⁺) via a simple coordinating coassembly process. As a single-site nanozyme, CuNAD exhibits excellent robustness under extreme conditions, significantly stronger catalytic activity for phenolic compounds, and 4.02-fold higher sensitivity for epinephrine detection than laccase. Furthermore, by breaking through the functional constraints of laccase, CuNAD is also able to activate H₂O₂ at neutral pH, benefiting a one-step chromogenic detection platform for cholesterol. This facile approach demonstrates the potential to develop single-site nanozymes by biomimicking natural enzymes and may boost more insights into the structure-function relationship of nanozymes.

Excited-State Properties of Fully Reduced Flavins in Ferredoxin-NADP+ Oxidoreductase

The fully reduced flavin cofactor (FAD_red) in ferredoxin-NADP⁺ oxidoreductase (FNR) is a functional intermediate that displays different catalytic and steady-state spectral properties for enzymes from Bacillus subtilis (BsFNR), Chlorobaculum tepidum (CtFNR), and Rhodopseudomonas palustris (RpFNR). Using ultrafast spectroscopy, we reveal that at physiological pH, photoexcited FAD_red in BsFNR and RpFNR exhibits unprecedentedly fast decays (dominantly in 6 and 8 ps, respectively), whereas in CtFNR the decay is much slower (∼400 ps), as in other flavoproteins. Correlating these observations with the protonation states of FAD_red and the dynamic properties of the protein environment, we conclude that the excited state of neutral FAD_red can be intrinsically short-lived even in proteins, contrasting with the well-documented behavior of the anionic form that systematically displays markedly increased excited-state lifetime upon binding to proteins. This work provides new insight into the photochemistry of fully reduced flavins, which are emerging as functional initial states in bioengineered photocatalysts.