Photobiocatalysis for Abiological Transformations

Catalytic Promiscuity of Fatty Acid Photodecarboxylase Enables Stereoselective Synthesis of Chiral α-Tetralones

In the field of biocatalysis, discovering novel reactivity from known enzymes has been a longstanding challenge. Fatty acid photo-decarboxylase from Chlorella variabilis (CvFAP) has drawn considerable attention as a promising photoenzyme with potential green chemistry applications; however, its non-natural reactivity has rarely been exploited to date. Herein we report a non-natural reductive dehalogenation (deacetoxylation) reactivity of CvFAP inspired by its natural oxidative decarboxylation process, enabling the stereoselective synthesis of a series of chiral α-substituted tetralones with high yields (up to 99 %) and e.r. values (up to 99 : 1). Mechanistic studies demonstrated that the native photoenzyme catalyzed the reductive dehalogenation via a novel mechanism involving oxidized state (FADox)/semiquinone state (FADsq) redox pair and an electron transfer (ET)/proton transfer (PT) process of radical termination, distinct from the previous reports. To our knowledge, this study represents a new example of CvFAP promiscuity, and thus expands the reactivity repertoire of CvFAP and highlights the versatility of CvFAP in asymmetric synthesis.

Biocatalytic Generation of Trifluoromethyl Radicals by Nonheme Iron Enzymes for Enantioselective Alkene Difunctionalization

The trifluoromethyl (-CF3) group represents a highly prevalent functionality in pharmaceuticals. Over the past few decades, significant advances have been made in the development of synthetic methods for trifluoromethylation. In contrast, there are currently no metalloenzymes known to catalyze the formation of C(sp3)-CF3 bonds. In this work, we demonstrate that a nonheme iron enzyme, hydroxymandelate synthase from Amycolatopsis orientalis (AoHMS), is capable of generating CF3 radicals from hypervalent iodine(III) reagents and directing them for enantioselective alkene trifluoromethyl azidation. A high-throughput screening (HTS) platform based on Staudinger ligation was established, enabling the rapid evaluation of AoHMS variants for this abiological transformation. The final optimized variant accepts a range of alkene substrates, producing the trifluoromethyl azidation products in up to 73% yield and 96:4 enantiomeric ratio (e.r.). The biocatalytic platform can be further extended to alkene pentafluoroethyl azidation and diazidation by altering the iodine(III) reagent. In addition, anion competition experiments provide insights into the radical rebound process for this abiological transformation. This study not only expands the catalytic repertoire of metalloenzymes for radical transformations but also creates a new enzymatic space for organofluorine synthesis.

Mechanism of ultrafast flavin photoreduction in the active site of flavoenzyme LSD1 histone demethylase

Photoreduction of oxidized flavins has a functional role in photocatalytic and photoreceptor flavoproteins. In flavoproteins without light-dependent physiological functions, ultrafast, reversible flavin photoreduction is supposedly photoprotective by nature, and holds potential for nonnatural photocatalytic applications. In this work, we combine protein mutagenesis, ultrafast spectroscopy, molecular dynamics simulations and quantum mechanics calculations to investigate the nonfunctional flavin photoreduction in a flavoenzyme, lysine-specific demethylase 1 (LSD1) which is pivotal in DNA transcription. LSD1 harbors an oxidized flavin adenine dinucleotide (FAD) cofactor and multiple electron-donating residues in the active site. Upon photoexcitation, the FAD cofactor is photoreduced in <200 fs by electron transfer (ET) from nearby residue(s), and the charge pairs recombine in ca. 2 ps. Site-directed mutagenesis pinpoints a specific tryptophan residue, W751, as the primary electron donor, whereas a tyrosine residue, Y761, despite being located closer to the flavin ring, does not effectively contribute to the process. Based on a hybrid quantum-classical computational approach, we characterize the W751-FAD and Y761-FAD charge-transfer states (CTW751 and CTY761, respectively), as well as the FAD locally excited state (LEFAD), and demonstrate that the coupling between LEFAD and CTW751 is larger than those involving CTY761 by an order of magnitude, rationalizing the experimental observations. More generally, this work highlights the role of the intrinsic protein environment and details of donor-acceptor molecular configurations on the dynamics of short-range ET involving a flavin cofactor and amino acid residue(s).

Genetic Incorporation of a Thioxanthone-Containing Amino Acid for the Design of Artificial Photoenzymes

Genetically encodable photosensitizers allow the design of artificial photoenzymes to expand the scope of abiological reactions. Herein, we report the genetic incorporation of a thioxanthone-containing amino acid into a protein scaffold via an engineered pyrrolysyl-tRNA/pyrrolysyl-tRNA synthetase pair. The designer enzyme was engineered to catalyze a dearomative [2+2] cycloaddition reaction in high yields (up to>99 % yield) with excellent enantioselectivity (up to 98 : 2 e.r.). This work provides a robust and facile method for photoenzyme design and lays the foundation for the development of further photoenzymatic reactions.

Spatiotemporal Encapsulation of Tandem Enzymes in Hierarchical Metal-Organic Frameworks for Cofactor-Dependent Photoenzymatic CO2 Conversion

The photo-enzyme coupling system (PECS) holds immense potential in "green" biomanufacturing, encompassing the realms of pharmaceuticals, fuels, and carbon sequestration. Nevertheless, the intricate nature of enzymes' structures significantly impedes the seamless integration of multiple enzymes in a precise, tandem fashion, with exact control over their distribution, posing a formidable challenge. Herein, it has devised a mesoporous csq-type metal organic framework (Zr-MOF) from meso-tetrakis-(4-((phenyl)ethynyl)benzoate)porphyrin (Por-PTP) and Zr6(μ3-O)4(μ3-OH)4(OH)4(H2O)4) nodes (Zr6 clusters), featuring intricate hierarchical hexagonal (5.8 nm) and triangular (2.9 nm) channels, enabling the simultaneous encapsulation of Formate dehydrogenase from Candida boidinii (CbFDH) and ferredoxin-NADP+ reductase (FNR) via a spatiotemporally controlled strategy for cofactor-dependent photoenzymatic carbon dioxide (CO2) conversion. Upon illumination, photoexcited electrons originating from the Zr-MOF frameworks migrate to the adjacent FNR for cofactor NADH regeneration, which is then harnessed by proximal CbFDH for CO2 fixation. Concurrently, the resulting holes are neutralized by AA for system recovery. The results demonstrated the confinement of tandem enzymes within MOF channels significantly enhanced the performance of multi-enzyme cascade pathways as well as augmenting the local NAD+/NADH, which leading to a further improvement in the efficiency of tandem biocatalytic formic acid generation (55 mm) from CO2. Crucially, the photo-enzyme-coupled factories exhibited remarkable stability alongside exceptional recyclability, attributed to the preservation of MOF skeletons.

Single-Electron Oxidation Triggered by Visible-Light-Excited Enzymes for Asymmetric Biocatalysis

By integrating enzymatic catalysis with photocatalysis, photoenzymatic catalysis emerges as a powerful strategy to enhance enzyme catalytic capabilities and provide superior stereocontrol in reactions involving reactive intermediates. Repurposing naturally occurring enzymes using visible light is among the most active directions of photoenzymatic catalysis. This Minireview focuses on a cutting-edge strategy in this direction, namely single-electron-oxidation-triggered non-natural biotransformations catalyzed by photoexcited enzymes. These straightforward transformations feature a unique radical mechanism initiated by single-electron oxidation, achieving redox-neutral non-natural C-C, C-O, and C-S bond formation, and expanding the chemical toolbox of enzymes. By highlighting recent advances in this field and emphasizing their catalytic mechanisms and synthetic potential, innovative approaches for photobiomanufacturing are anticipated.

Unmasking the reverse catalytic activity of 'ene'-reductases for asymmetric carbonyl desaturation

Carbonyl desaturation is a fundamental reaction widely practised in organic synthesis. While numerous methods have been developed to expand the scope of this important transformation, most of them necessitate multi-step protocols or suffer from the use of high loadings of metal or strong oxidizing conditions. Moreover, approaches that can achieve precise stereochemical control of the desaturation process are extremely rare. Here we report a biocatalytic platform for desymmetrizing desaturation of cyclohexanones to generate diverse cyclohexenones bearing a remote quaternary stereogenic centre, by reengineering 'ene'-reductases to efficiently mediate dehydrogenation, the reverse process of their native activity. This 'ene'-reductase-based desaturation system operates under mild conditions with air as the terminal oxidant, tolerates oxidation-sensitive or metal-incompatible functional groups and, more importantly, exhibits unparalleled stereoselectivity compared with those achieved with small-molecule catalysts. Mechanistic investigations suggest that the reaction proceeded through α-deprotonation followed by a rate-determining β-hydride transfer.

Photobiocatalytic Platform for the Efficient Enantio-Divergent Synthesis of β-Fluoromethylated Ketones

β-Fluoromethyl (CH2F, CHF2, and CF3)-substituted chiral ketones are essential moieties and are vital building blocks in pharmaceutical and agrochemistry. However, general and convenient methods for enantio-diverse access to diverse β-fluoromethylated ketones are lacking, hindering the further development of these functional moieties. In this study, we developed an ene-reductase-based photobiocatalytic platform for efficient synthesis of enantio-divergent β-fluoromethylated chiral ketones. Our method highlights substrate-type diversity, excellent enantioselectivity, enzymatic enantio-divergent synthesis, as well as a dicyanopyrazine (DPZ)-type photosensitizer for biocompatible olefin E/Z isomerization in enzymatic stereoconvergent olefin asymmetric reduction, thereby providing a general photobiocatalytic solution to diverse β-fluoromethylated chiral ketones.

Noncanonical Amino Acids: Bringing New-to-Nature Functionalities to Biocatalysis

Biocatalysis has become an important component of modern organic chemistry, presenting an efficient and environmentally friendly approach to synthetic transformations. Advances in molecular biology, computational modeling, and protein engineering have unlocked the full potential of enzymes in various industrial applications. However, the inherent limitations of the natural building blocks have sparked a revolutionary shift. In vivo genetic incorporation of noncanonical amino acids exceeds the conventional 20 amino acids, opening new avenues for innovation. This review provides a comprehensive overview of applications of noncanonical amino acids in biocatalysis. We aim to examine the field from multiple perspectives, ranging from their impact on enzymatic reactions to the creation of novel active sites, and subsequent catalysis of new-to-nature reactions. Finally, we discuss the challenges, limitations, and promising opportunities within this dynamic research domain.

Photo-induced imine reduction by a photoredox biocatalyst consisting of a pentapeptide and a Ru bipyridine terpyridine complex

Imine reduction is a useful reaction in the preparation of amine derivatives. Various catalysts have been reported to promote this reaction and photoredox catalysts are promising candidates for sustainable amine synthesis. Improvement of this reaction using biomolecule-based reaction scaffolds is expected to increase the utility of the reaction. In this context, we have recently investigated photoredox Ru complexes with pentapeptide scaffolds via coordination bonds as catalysts for photoreduction of dihydroisoquinoline derivatives. First, Ru bipyridine terpyridine complexes coordinated with five different pentapeptides (XVHVV: X = V, F, W, Y, C) were prepared and characterized by mass spectrometry. Catalytic activities of the Ru complexes with XVHVV were evaluated for photoreduction of dihydroisoquinoline derivatives in the presence of ascorbate and thiol compounds as sacrificial reagents and hydrogen sources. Interestingly, the turnover number of the Ru complex with VVHVV is 531, which is two-fold higher than that of a simple Ru complex with an imidazole ligand. The detailed emission lifetime measurements indicate that the enhanced catalytic activity provided by the peptide scaffold is caused by an efficient reaction with the thiol derivative to accelerate reductive quenching of Ru complex. The quenching behavior suggests formation of an active species such as a Ru(I) complex. These findings reveal that the simple pentapeptide serves as an effective scaffold to enhance the photocatalytic activity of a photoactive Ru complex.

Electron spin resonance analysis of photoenzymatic catalysis

This Perspective highlights recent research progress and prospects in elucidating the catalytic mechanism of photoenzymes using ESR (electron spin resonance) spectroscopy, which is emerging as a unique and crucial method for identifying radical intermediates, illustrating electron transfer events and the underlying mechanisms of photoenzymatic catalysis.

A metalloenzyme platform for catalytic asymmetric radical dearomatization

Catalytic asymmetric dearomatization represents a powerful means to convert flat aromatic compounds into stereochemically well-defined three-dimensional molecular scaffolds. Using new-to-nature metalloredox biocatalysis, we describe an enzymatic strategy for catalytic asymmetric dearomatization via a challenging radical mechanism that has eluded small-molecule catalysts. Enabled by directed evolution, new-to-nature radical dearomatases P450rad1-P450rad5 facilitated asymmetric dearomatization of a broad spectrum of aromatic substrates, including indoles, pyrroles and phenols, allowing both enantioconvergent and enantiodivergent radical dearomatization reactions to be accomplished with excellent enzymatic control. Computational studies revealed the importance of additional hydrogen bonding interactions between the engineered metalloenzyme and the reactive intermediate in enhancing enzymatic activity and enantiocontrol. Furthermore, designer non-ionic surfactants were found to significantly accelerate this biotransformation, providing an alternative means to promote otherwise sluggish new-to-nature biotransformations. Together, this evolvable metalloenzyme platform opens up new avenues to advance challenging catalytic asymmetric dearomatization processes involving free radical intermediates.

Applications of Ene-Reductases in the Synthesis of Flavors and Fragrances

Flavors and fragrances (F&F) are interesting organic compounds in chemistry. These compounds are widely used in the food, cosmetic, and medical industries. Enzymatic synthesis exhibits several advantages over natural extraction and chemical preparation, including a high yield, stable quality, mildness, and environmental friendliness. To date, many oxidoreductases and hydrolases have been used to biosynthesize F&F. Ene-reductases (ERs) are a class of biocatalysts that can catalyze the asymmetric reduction of α,β-unsaturated compounds and offer superior specificity and selectivity; therefore, ERs have been increasingly considered an ideal alternative to their chemical counterparts. This review summarizes the research progress on the use of ERs in F&F synthesis over the past 20 years, including the achievements of various scholars, the differences and similarities among the findings, and the discussions of future research trends related to ERs. We hope this review can inspire researchers to promote the development of biotechnology in the F&F industry.

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.

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.

Asymmetric photoenzymatic incorporation of fluorinated motifs into olefins

Enzymes capable of assimilating fluorinated feedstocks are scarce. This situation poses a challenge for the biosynthesis of fluorinated compounds used in pharmaceuticals, agrochemicals, and materials. We developed a photoenzymatic hydrofluoroalkylation that integrates fluorinated motifs into olefins. The photoinduced promiscuity of flavin-dependent ene-reductases enables the generation of carbon-centered radicals from iodinated fluoroalkanes, which are directed by the photoenzyme to engage enantioselectively with olefins. This approach facilitates stereocontrol through interaction between a singular fluorinated unit and the enzyme, securing high enantioselectivity at β, γ, or δ positions of fluorinated groups through enzymatic hydrogen atom transfer-a process that is notably challenging with conventional chemocatalysis. This work advances enzymatic strategies for integrating fluorinated chemical feedstocks and opens avenues for asymmetric synthesis of fluorinated compounds.

First and Second Reductions in an Aprotic Solvent: Comparing Computational and Experimental One-Electron Reduction Potentials for 345 Quinones

Using reference reduction potentials of quinones recently measured relative to the saturated calomel electrode (SCE) in N,N-dimethylformamide (DMF), we benchmark absolute one-electron reduction potentials computed for 345 Q/Q•- and 265 Q•-/Q2- half-reactions using adiabatic electron affinities computed with density functional theory and solvation energies computed with four continuum solvation models: IEF-PCM, C-PCM, COSMO, and SM12. Regression analyses indicate a strong linear correlation between experimental and absolute computed Q/Q•- reduction potentials with Pearson's correlation coefficient (r) between 0.95 and 0.96 and the mean absolute error (MAE) relative to the linear fit between 83.29 and 89.51 mV for different solvation methods when the slope of the regression is constrained to 1. The same analysis for Q•-/Q2- gave a linear regression with r between 0.74 and 0.90 and MAE between 95.87 and 144.53 mV, respectively. The y-intercept values obtained from the linear regressions are in good agreement with the range of absolute reduction potentials reported in the literature for the SCE but reveal several sources of systematic error. The y-intercepts from Q•-/Q2- calculations are lower than those from Q/Q•- by around 320-410 mV for IEF-PCM, C-PCM, and SM12 compared to 210 mV for COSMO. Systematic errors also arise between molecules having different ring sizes (benzoquinones, naphthoquinones, and anthraquinones) and different substituents (titratable vs nontitratable). SCF convergence issues were found to be a source of random error that was slightly reduced by directly optimizing the solute structure in the continuum solvent reaction field. While SM12 MAEs were lower than those of the other solvation models for Q/Q•-, SM12 had larger MAEs for Q•-/Q2- pointing to a larger error when describing multiply charged anions in DMF. Altogether, the results highlight the advantages of, and further need for, testing computational methods using a large experimental data set that is not skewed (e.g., having more titratable than nontitratable substituents on different parent groups or vice versa) to help further distinguish between sources of random and systematic errors in the calculations.

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.

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.

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.

Engineering a Dual-Functionalized PolyHIPE Resin for Photobiocatalytic Flow Chemistry

The use of a dual resin for photobiocatalysis, encompassing both a photocatalyst and an immobilized enzyme, brings several challenges, including effective immobilization, maintaining photocatalyst and enzyme activity and ensuring sufficient light penetration. However, the benefits, such as integrated processes, reusability, easier product separation, and potential for scalability, can outweigh these challenges, making dual resin systems promising for efficient and sustainable photobiocatalytic applications. In this study, we employed a photosensitizer-containing porous emulsion-templated polymer as a functional support that is used to covalently anchor a chloroperoxidase from Curvularia inaequalis (CiVCPO). We demonstrate the versatility of this heterogeneous photobiocatalytic material, which enables the bromination of four aromatic substrates, including rutin-a natural occurring flavonol-under blue light (456 nm) irradiation and continuous flow conditions.

Catalytic Mechanism of Fatty Acid Photodecarboxylase: On the Detection and Stability of the Initial Carbonyloxy Radical Intermediate

In fatty acid photodecarboxylase (FAP), light-induced formation of the primary radical product RCOO⋅ from fatty acid RCOO- occurs in 300 ps, upon which CO2 is released quasi-immediately. Based on the hypothesis that aliphatic RCOO⋅ (spectroscopically uncharacterized because unstable) absorbs in the red similarly to aromatic carbonyloxy radicals such as 2,6-dichlorobenzoyloxy radical (DCB⋅), much longer-lived linear RCOO⋅ has been suggested recently. We performed quantum chemical reaction pathway and spectral calculations. These calculations are in line with the experimental DCB⋅ decarboxylation dynamics and spectral properties and show that in contrast to DCB⋅, aliphatic RCOO⋅ radicals a) decarboxylate with a very low energetic barrier and on the timescale of a few ps and b) exhibit little red absorption. A time-resolved infrared spectroscopy experiment confirms very rapid, ≪300 ps RCOO⋅ decarboxylation in FAP. We argue that this property is required for the observed high quantum yield of hydrocarbons formation by FAP.

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.

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.

Stereodivergent Protein Engineering of Fatty Acid Photodecarboxylase for Light-Driven Kinetic Resolution of Sec-Alcohol Oxalates

Stereodivergent engineering of one enzyme to create stereocomplementary variants for synthesizing optically pure molecules with tailor-made (R) or (S) configurations on an optional basis is highly desirable and challenging. This study aimed to engineer fatty acid photodecarboxylase from Chlorella variabilis (CvFAP) using the focused rational iterative site-specific mutagenesis (FRISM) strategy to obtain two highly stereocomplementary variants with excellent selectivity (both giving products with up to 99 % e.e.). These variants were used for the CvFAP-catalyzed light-driven kinetic resolution of oxalates or oxamic acids prepared from the corresponding sec-alcohols or amines, providing a new biotransformation process for preparing chiral sec-alcohols and amines. Molecular dynamics simulation, kinetic data and transient spectra revealed the source of selectivity. This study represents the first example of the kinetic resolution of sec-alcohols or amines catalyzed by a pair of stereocomplementary CvFAPs.

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.

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.

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.

Anchoring a Structurally Editable Proximal Cofactor-like Module to Construct an Artificial Dual-center Peroxygenase

A recent novel strategy for constructing artificial metalloenzymes (ArMs) that target new-to-nature functions uses dual-functional small molecules (DFSMs) with catalytic and anchoring groups for converting P450BM3 monooxygenase into a peroxygenase. However, this process requires excess DFSMs (1000 equivalent of P450) owing to their low binding affinity for P450, thus severely limiting its practical application. Herein, structural optimization of the DFSM-anchoring group considerably enhanced their binding affinity by three orders of magnitude (Kd ≈10-8  M), thus approximating native cofactors, such as FMN or FAD in flavoenzymes. An artificial cofactor-driven peroxygenase was thus constructed. The co-crystal structure of P450BM3 bound to a DFSM clearly revealed a precatalytic state in which the DFSM participates in H2 O2 activation, thus facilitating peroxygenase activity. Moreover, the increased binding affinity substantially decreases the DFSM load to as low as 2 equivalents of P450, while maintaining increased activity. Furthermore, replacement of catalytic groups showed disparate selectivity and activity for various substrates. This study provides an unprecedented approach for assembling ArMs by binding editable organic cofactors as a co-catalytic center, thereby increasing the catalytic promiscuity of P450 enzymes.

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.

Enzymatic Photodecarboxylation on Secondary and Tertiary Carboxylic Acids

Photoenzymatic decarboxylation of bulky secondary and tertiary carboxylic acids catalyzed by engineered Chlorella variabilis fatty acid photodecarboxylase (CvFAP) is reported. Rational design and directed evolution of wild-type CvFAP are used to improve the reactivity and expand potential applications. Moreover, engineered CvFAP can catalyze light-driven kinetic resolution of α-substituted carboxylic acid. Our work sheds light on the production of chiral building blocks and bioactive molecules from bulky carboxylic acids via the photoenzymatic way.

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.

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.

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.

Immobilization of fatty acid photodecarboxylase in magnetic nickel ferrite nanoparticle

Fatty acid photodecarboxylase in Chlorella variabilis NC64A (CvFAP) performed excellent ability to exclusively decarboxylate renewable fatty acids for C1-shortened hydrocarbons fuel production under visible light. However, the large-scale application by such an approach is limited by the free state of CvFAP catalyst, which is unstable for efficient biofuel production. In this study, CvFAP was immobilized in magnetic nickel ferrite (NiFe₂O₄) nanoparticles for facile recovery by a simple procedure. The shift of Ni 2p in electron binding energy was detected to clarify the interaction between Ni²⁺ and histidine of CvFAP. The coordination of NiFe₂O₄ and CvFAP contributed to an efficient affinity binding with an immobilization capacity of 98 mg/g carrier. Hydrocarbon fuel concentration of 3.7 mM was obtained by NiFe₂O₄@CvFAP-induced photoenzymatic decarboxylation. The high stability of CvFAP in terms of residual enzyme activity of 79.7% and 68% at pH 9 and organic solvent ratio of 60%, respectively, were observed.

Photocatalytic Transformation of Biomass and Biomass Derived Compounds-Application to Organic Synthesis

Biomass and biomass-derived compounds have become an important alternative feedstock for chemical industry. They may replace fossil feedstocks such as mineral oil and related platform chemicals. These compounds may also be transformed conveniently into new innovative products for the medicinal or the agrochemical domain. The production of cosmetics or surfactants as well as materials for different applications are examples for other domains where new platform chemicals obtained from biomass can be used. Photochemical and especially photocatalytic reactions have recently been recognized as being important tools of organic chemistry as they make compounds or compound families available that cannot be or are difficultly synthesized with conventional methods of organic synthesis. The present review gives a short overview with selected examples on photocatalytic reactions of biopolymers, carbohydrates, fatty acids and some biomass-derived platform chemicals such as furans or levoglucosenone. In this article, the focus is on application to organic synthesis.

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.

The Evolving Nature of Biocatalysis in Pharmaceutical Research and Development

Biocatalysis is a highly valued enabling technology for pharmaceutical research and development as it can unlock synthetic routes to complex chiral motifs with unparalleled selectivity and efficiency. This perspective aims to review recent advances in the pharmaceutical implementation of biocatalysis across early and late-stage development with a focus on the implementation of processes for preparative-scale syntheses.

Advancement in algal bioremediation for organic, inorganic, and emerging pollutants

Rapidly changing bioremediation prospects are key drive to develop sustainable options that can offer extra benefits rather than only environmental remediation. Algal remediating is gaining utmost attention due to its mesmerising sustainable features, removing odour and toxicity, co-remediating numerous common and emerging inorganic and organic pollutants from gaseous and aqueous environments, and yielding biomass for a range of valuable products refining. Moreover, it also improves carbon footprint via carbon-capturing offers a better option than any other non-algal process for several high CO₂-emitting industries. Bio-uptake, bioadsorption, photodegradation, and biodegradation are the main mechanisms to remediate a range of common and emerging pollutants by various algae species. Bioadsorption was a dominant remediation mechanism among others implicating surface properties of pollutants and algal cell walls. Photodegradable pollutants were photodegraded by microalgae by adsorbing photons on the surface and intracellularly via stepwise photodissociation and breakdown. Biodegradation involves the transportation of selective pollutants intracellularly, and enzymes help to convert them into simpler non-toxic forms. Robust models are from the green microalgae group and are dominated by Chlorella species. This article compiles the advancements in microalgae-assisted pollutants remediation and value-addition under sustainable biorefinery prospects. Moreover, filling the knowledge gaps, and recommendations for developing an effective platform for emerging pollutants remediation and realization of commercial-scale algal bioremediation.

Photoexcited Enzymes for Asymmetric Csp3 -Csp3 Cross-Electrophile Couplings

Enzymes have several advantages over conventional catalysts for organic synthesis. Over the last two decades, much effort has been made to further extend the scope of biocatalytic reactions available to synthetic chemists, particularly by expanding the repertoire of enzymes for abiological transformations. In this regard, exciting new developments in the area of photobiocatalysis enable now the introduction of non-natural reactivity in enzymes to solve long-standing synthetic challenges. A recently described example from the Hyster group demonstrates in an unprecedented way how the combination of photochemistry with enzyme catalysis empowers the catalytic asymmetric construction of Csp³ -Csp³ bonds with high chemo- and enantioselectivity.

Enantioselective Radical Addition to Ketones through Lewis Acid-Enabled Photoredox Catalysis

Photocatalysis opens up a new window for carbonyl chemistry. Despite a multitude of photochemical reactions of carbonyl compounds, visible light-induced catalytic asymmetric transformations remain elusive and pose a formidable challenge. Accordingly, the development of simple, efficient, and economic catalytic systems is the ideal pursuit for chemists. Herein, we report an enantioselective radical photoaddition to ketones through a Lewis acid-enabled photoredox catalysis wherein the in situ formed chiral N,N'-dioxide/Sc(III)-ketone complex serves as a temporary photocatalyst to trigger single-electron transfer oxidation of silanes for the generation of nucleophilic radical species, including primary, secondary, and tertiary alkyl radicals, giving various enantioenriched aza-heterocycle-based tertiary alcohols in good to excellent yields and enantioselectivities. The results of electron paramagnetic resonance (EPR) and high-resolution mass spectrum (HRMS) measurements provided favorable evidence for the stereocontrolled radical addition process involved in this reaction.