Biocatalytic, Stereoselective Deuteration of α-Amino Acids and Methyl Esters

Binary Catalytic Hydrogen/Deuterium Exchange of Free α-Amino Acids and Derivatives

The increasing demand for deuterium-labeled amino acids and derivatives has heightened interest in direct hydrogen/deuterium exchange reactions of free amino acids. Existing methods, including biocatalysis and metal catalysis, typically require expensive deuterium sources or excessive use of deuterium reagents and often struggle with site selectivity. In contrast, this binary catalysis system, employing benzaldehyde and Cs2CO3 in the presence of inexpensive D2O with minimal stoichiometric quantities, facilitates efficient hydrogen/deuterium exchange at the α-position of amino acids without the need for protecting groups in the polar aprotic solvent DMSO. The process is highly compatible with most natural and non-natural α-amino acids and derivatives, even those with potentially reactive functionalities. This advancement not only addresses the cost and efficiency concerns of existing methods but also significantly broadens the applicability and precision of deuterium labeling in biochemical research.

Site-Selective Deuteration of α-Amino Esters with 2-Hydroxynicotinaldehyde as a Catalyst

An efficient method has been developed for the synthesis of α-deuterated α-amino esters via hydrogen isotope exchange of α-amino esters in D2O with 2-hydroxynicotinaldehyde as a catalyst under mild conditions. This methodology exhibits a wide range of substrate scopes, remarkable functional group tolerance, and affording the desired products in good yields with excellent deuterium incorporation. Notably, the ortho-hydroxyl group and the pyridine ring of the catalyst play a crucial role in the catalytic activity, which not only stabilizes the carbon-anion intermediates but also enhances the acidity of the amino esters' α-C-H bond.

Simple and Efficient Enantioselective α-Deuteration Method of α-Amino Acids without External Chiral Sources

Deuterium-labeled α-amino acids are useful in research related to drug discovery and biomedical science. However, a high degree of site selectivity and stereoselectivity in the deuterium incorporation process is still difficult to achieve. Herein, we report a new enantioselective deuteration method at the α-position of several amino acids without external chiral sources. The proposed deuteration methods (NaOEt and EtOD) are highly selective and simple. Additionally, we provide a mechanistic study for this enantioretentive deuteration.

Stereoselective Biocatalytic α-Deuteration of L-Amino Acids by a Pyridoxal 5'-Phosphate-Dependent Mannich Cyclase

α-Deuterated amino acids are valuable building blocks for developing deuterated drugs, and are important tools for studying biological systems. Biocatalytic deuteration represents an attractive strategy to directly access enantiopure α-deuterated amino acids. Here, we show that a PLP-dependent Mannich cyclase, LolT, involved in the biosynthesis of loline alkaloids, is capable of deuterating a diverse range of L-amino acids, including basic and acidic, nonpolar and polar, aliphatic and aromatic amino acids. Furthermore, complete deuteration of many amino acids can be achieved within minutes with exquisite control on the site- and stereoselectivity. During the course of this investigation, we also unexpectedly discovered that LolT exhibits β-elimination activity with L-cystine and O-acetyl-L-serine, confirming our previous hypothesis based on structural and phylogenetic analysis that LolT, a Cα-C bond forming enzyme, is evolved from a primordial Cβ-S lyase family. Overall, our study demonstrates that LolT is an extremely versatile biocatalyst, and can be used for not only heterocyclic quaternary amino acid biosynthesis, but also biocatalytic amino acid deuteration.

Biocatalytic reductive amination as a route to isotopically labelled amino acids suitable for analysis of large proteins by NMR

We demonstrate an atom-efficient and easy to use H2-driven biocatalytic platform for the enantioselective incorporation of 2H-atoms into amino acids. By combining the biocatalytic deuteration catalyst with amino acid dehydrogenase enzymes capable of reductive amination, we synthesised a library of multiply isotopically labelled amino acids from low-cost isotopic precursors, such as 2H2O and 15NH4+. The chosen approach avoids the use of pre-labeled 2H-reducing agents, and therefore vastly simplifies product cleanup. Notably, this strategy enables 2H, 15N, and an asymmetric centre to be introduced at a molecular site in a single step, with full selectivity, under benign conditions, and with near 100% atom economy. The method facilitates the preparation of amino acid isotopologues on a half-gram scale. These amino acids have wide applicability in the analytical life sciences, and in particular for NMR spectroscopic analysis of proteins. To demonstrate the benefits of the approach for enabling the workflow of protein NMR chemists, we prepared l-[α-2H,15N, β-13C]-alanine and integrated it into a large (>400 kDa) heat-shock protein oligomer, which was subsequently analysable by methyl-TROSY techniques, revealing new structural information.

Synthesis of Metabolites and Metabolite-like Compounds Using Biocatalytic Systems

Methodologies for the synthesis and purification of metabolites, which have been developed following their discovery, analysis, and structural identification, have been involved in numerous life science milestones. The renewed focus on the small molecule domain of biological cells has also created an increasing awareness of the rising gap between the metabolites identified and the metabolites which have been prepared as pure compounds. The design and engineering of resource-efficient and straightforward synthetic methodologies for the production of the diverse and numerous metabolites and metabolite-like compounds have attracted much interest. The variety of metabolic pathways in biological cells provides a wonderful blueprint for designing simplified and resource-efficient synthetic routes to desired metabolites. Therefore, biocatalytic systems have become key enabling tools for the synthesis of an increasing number of metabolites, which can then be utilized as standards, enzyme substrates, inhibitors, or other products, or for the discovery of novel biological functions.

Enabling Broader Adoption of Biocatalysis in Organic Chemistry

Biocatalysis is becoming an increasingly impactful method in contemporary synthetic chemistry for target molecule synthesis. The selectivity imparted by enzymes has been leveraged to complete previously intractable chemical transformations and improve synthetic routes toward complex molecules. However, the implementation of biocatalysis in mainstream organic chemistry has been gradual to this point. This is partly due to a set of historical and technological barriers that have prevented chemists from using biocatalysis as a synthetic tool with utility that parallels alternative modes of catalysis. In this Perspective, we discuss these barriers and how they have hindered the adoption of enzyme catalysts into synthetic strategies. We also summarize tools and resources that already enable organic chemists to use biocatalysts. Furthermore, we discuss ways to further lower the barriers for the adoption of biocatalysis by the broader synthetic organic chemistry community through the dissemination of resources, demystifying biocatalytic reactions, and increasing collaboration across the field.

Mechanistic and Structural Insights into a Divergent PLP-Dependent l-Enduracididine Cyclase from a Toxic Cyanobacterium

Cyclic arginine noncanonical amino acids (ncAAs) are found in several actinobacterial peptide natural products with therapeutically useful antibacterial properties. The preparation of ncAAs like enduracididine and capreomycidine currently takes multiple biosynthetic or chemosynthetic steps, thus limiting the commercial availability and applicability of these cyclic guanidine-containing amino acids. We recently discovered and characterized the biosynthetic pathway of guanitoxin, a potent freshwater cyanobacterial neurotoxin, that contains an arginine-derived cyclic guanidine phosphate within its highly polar structure. The ncAA l-enduracididine is an early intermediate in guanitoxin biosynthesis and is produced by GntC, a unique pyridoxal-5'-phosphate (PLP)-dependent enzyme. GntC catalyzes a cyclodehydration from a stereoselectively γ-hydroxylated l-arginine precursor via a reaction that functionally and mechanistically diverges from previously established actinobacterial cyclic arginine ncAA pathways. Herein, we interrogate l-enduracididine biosynthesis from the cyanobacterium Sphaerospermopsis torques-reginae ITEP-024 using spectroscopy, stable isotope labeling techniques, and X-ray crystallography structure-guided site-directed mutagenesis. GntC initially facilitates the reversible deprotonations of the α- and β-positions of its substrate before catalyzing an irreversible diastereoselective dehydration and subsequent intramolecular cyclization. The comparison of holo- and substrate-bound GntC structures and activity assays on site-specific mutants further identified amino acid residues that contribute to the overall catalytic mechanism. These interdisciplinary efforts at structurally and functionally characterizing GntC enable an improved understanding of how nature divergently produces cyclic arginine ncAAs and generate additional tools for their biocatalytic production and downstream biological applications.

Versatile Chemo-Biocatalytic Cascade Driven by a Thermophilic and Irreversible C-C Bond-Forming α-Oxoamine Synthase

We report a chemo-biocatalytic cascade for the synthesis of substituted pyrroles, driven by the action of an irreversible, thermostable, pyridoxal 5'-phosphate (PLP)-dependent, C-C bond-forming biocatalyst (ThAOS). The ThAOS catalyzes the Claisen-like condensation between various amino acids and acyl-CoA substrates to generate a range of α-aminoketones. These products are reacted with β-keto esters in an irreversible Knorr pyrrole reaction. The determination of the 1.6 Å resolution crystal structure of the PLP-bound form of ThAOS lays the foundation for future engineering and directed evolution. This report establishes the AOS family as useful and versatile C-C bond-forming biocatalysts.

Mechanistic and structural insights into a divergent PLP-dependent L-enduracididine cyclase from a toxic cyanobacterium

Cyclic arginine noncanonical amino acids (ncAAs) are found in several actinobacterial peptide natural products with therapeutically useful antibacterial properties. The preparation of ncAAs like enduracididine and capreomycidine currently takes multiple biosynthetic or chemosynthetic steps, thus limiting the commercial availability and applicability of these cyclic guanidine-containing amino acids. We recently discovered and characterized the biosynthetic pathway of guanitoxin, a potent freshwater cya-nobacterial neurotoxin, that contains an arginine-derived cyclic guanidine phosphate within its highly polar structure. The ncAA L-enduracididine is an early intermediate in guanitoxin biosynthesis and is produced by GntC, a unique pyridoxal-5'-phosphate (PLP)-dependent enzyme. GntC catalyzes a cyclodehydration from a stereoselectively γ-hydroxylated L-arginine precursor via a reaction that functionally and mechanistically diverges from previously established actinobacterial cyclic arginine ncAA pathways. Herein, we interrogate L-enduracididine biosynthesis from the cyanobacterium Sphaerospermopsis torques-reginae ITEP-024 using spectroscopic, stable isotope labeling techniques, and X-ray crystal structure-guided site-directed mutagenesis. GntC initially facilitates the reversible deprotonations of the α- and β-positions of its substrate prior to catalyzing an irreversible diastereoselective dehydration and subsequent intramolecular cyclization. The comparison of holo- and substrate bound GntC structures and activity assays on sitespecific mutants further identified amino acid residues that contribute to the overall catalytic mechanism. These interdisciplinary efforts at structurally and functionally characterizing GntC enables an improved understanding of how Nature divergently produces cyclic arginine ncAAs and generates additional tools for their biocatalytic production and downstream biological applications.

Catalytic asymmetric deuterosilylation of exocyclic olefins with mannose-derived thiols and deuterium oxide

Metal-free, photoinduced asymmetric deuterosilylation of exocyclic olefins has been achieved using a mannose-derived thiol catalyst.

Synthesis of β-Deuterated Amino Acids via Palladium-Catalyzed H/D Exchange

Despite several synthetic approaches that have been developed for α-deuterated amino acids, the synthesis of β-deuterated amino acids has remained a challenge. Herein, we disclose a palladium catalyzed H/D exchange protocol for a β-deuterated N-protected amino amide, which can be converted to a β-deuterated amino acid simply by removal of protecting groups. This protocol is highly efficient, simply manipulated, and appliable for deuterium-labeling of many amino amides. In addition, deuterium labeling of phenylalanine derivatives was also successful when pivalic acid served as an additive to promote the H/D exchange process.

New Additions to the Arsenal of Biocatalysts for Noncanonical Amino Acid Synthesis

Noncanonical amino acids (ncAAs) merge the conformational behavior and native interactions of proteinogenic amino acids with nonnative chemical motifs and have proven invaluable in developing modern therapeutics. This blending of native and nonnative characteristics has resulted in essential drugs like nirmatrelvir, which comprises three ncAAs and is used to treat COVID-19. Enzymes are appearing prominently in recent syntheses of ncAAs, where they demonstrate impressive control over the stereocenters and functional groups found therein. Here we review recent efforts to expand the biocatalyst arsenal for synthesizing ncAAs with natural enzymes. We also discuss how new-to-nature enzymes can contribute to this effort by catalyzing reactions inspired by the vast repertoire of chemical catalysis and acting on substrates that would otherwise not be used in synthesizing ncAAs. Abiotic enzyme-catalyzed reactions exploit the selectivity afforded by a macromolecular catalyst to access molecules not available to natural enzymes and perhaps not even chemical catalysis.

Understanding and Circumventing the Requirement for Native Thioester Substrates for α-Oxoamine Synthase Reactions

Many enzyme classes require thioester electrophiles such as acyl-carrier proteins and acyl-coenzyme A substrates. For in vitro applications, these substrates can render these chemical transformations impractical. To address this challenge, we have investigated the mechanism of coenzyme A in gating catalysis of one α-oxoamine synthase, SxtA AOS. Through investigating the reactivity of SxtA AOS and corresponding enzyme variants against a panel of substrates and coenzyme A mimics, we determined that activity is gated through the binding of the pantetheine arm and a phosphate group that hydrogen bonds to residue Lys154 that is predicted by an AlphaFold2 model to be located in a tunnel leading to the active site. To provide an economical solution for preparative-scale reactions, in situ transthioesterification was used with pantetheine and simple thioester substrate precursors, resulting in productive reactions. These findings outline a strategy for employing ACP- and CoA-dependent enzymes that are inaccessible through other means without the need for cost-prohibitive coenzyme A or carrier protein-activated substrates.

Stereoselective α-Deuteration of Serine, Cysteine, Selenocysteine, and 2,3-Diaminopropanoic Acid Derivatives

Efficient methodologies for synthesizing enantiopure α-deuterated derivatives of serine, cysteine, selenocysteine, and 2,3-diaminopropanoic acid have been developed. H/D exchange was achieved by deprotonation of a chiral bicyclic serine equivalent followed by selective deuteration. Additionally, diastereoselective additions of thiols, selenols, and amines to a chiral bicyclic dehydroalanine in deuterated alcohols allowed site-selective deuteration at the Cα atom of cysteine, selenocysteine, and 2,3-diaminopropanoic acid derivatives. A deuterated analogue of carbocysteine, a drug for the treatment of bronchiectasis, was synthesized.

Stereochemical course of cobalamin-dependent radical SAM methylation by TokK and ThnK

Complex carbapenems are important clinical antibiotics for difficult-to-treat infections. An essential step in the biosyntheses of these natural products is stereospecific methylation at C6 and subsequent alkylations by cobalamin-dependent radical SAM methylases such as TokK and ThnK. We have prepared isotopically labeled substrates in a stereospecific manner and found that both homologous enzymes selectively abstract the 6-pro-S hydrogen, followed by methyl transfer to the opposite face to give the (6R)-methyl carbapenam product proceeding, therefore, by inversion of absolute configuration at C6. These data clarify an unexpected ambiguity in the recently solved substrate-bound crystal structure of TokK and have led to a stereochemically complete mechanistic proposal for both TokK and ThnK.

Stereodivergent Synthesis of Enantioenriched α-Deuterated α-Amino Acids via Cascade Cu(I)-Catalyzed H-D Exchange and Dual Cu- and Ir-Catalyzed Allylation

A one-pot Cu-mediated H-D exchange with inexpensive heavy water as the deuterium source, followed by Cu- and Ir-catalyzed stereodivergent allylic alkylation, has been developed, providing efficient access to enantioenriched α-deuterium-labeled α-amino acids from readily available glycine imine esters in a high yield with excellent stereoselectivity. High deuterium enrichment, exquisite regioselectivity, precise stereoselectivity control, and operationally convenient procedures make this protocol appealing for the preparation of highly synthetically useful α-deuterated α-amino acids.

Site-Selective Deuteration of Amino Acids through Dual-Protein Catalysis

Deuterated amino acids have been recognized for their utility in drug development, for facilitating nuclear magnetic resonance (NMR) analysis, and as probes for enzyme mechanism. Small molecule-based methods for the site-selective synthesis of deuterated amino acids typically involve de novo synthesis of the compound from deuterated precursors. In comparison, enzymatic methods for introducing deuterium offer improved efficiency, operating directly on free amino acids to achieve hydrogen-deuterium (H/D) exchange. However, site selectivity remains a significant challenge for enzyme-mediated deuteration, limiting access to desirable deuteration motifs. Here, we use enzyme-catalyzed deuteration, combined with steady-state kinetic analysis and ultraviolet (UV)-vis spectroscopy to probe the mechanism of a two-protein system responsible for the biosynthesis of l-allo-Ile. We show that an aminotransferase (DsaD) can pair with a small partner protein (DsaE) to catalyze Cα and Cβ H/D exchange of amino acids, while reactions without DsaE lead exclusively to Cα-deuteration. With conditions for improved catalysis, we evaluate the substrate scope for Cα/Cβ-deuteration and demonstrate the utility of this system for preparative-scale, selective labeling of amino acids.

Recent Developments for the Deuterium and Tritium Labeling of Organic Molecules

Organic compounds labeled with hydrogen isotopes play a crucial role in numerous areas, from materials science to medicinal chemistry. Indeed, while the replacement of hydrogen by deuterium gives rise to improved absorption, distribution, metabolism, and excretion (ADME) properties in drugs and enables the preparation of internal standards for analytical mass spectrometry, the use of tritium-labeled compounds is a key technique all along drug discovery and development in the pharmaceutical industry. For these reasons, the interest in new methodologies for the isotopic enrichment of organic molecules and the extent of their applications are equally rising. In this regard, this Review intends to comprehensively discuss the new developments in this area over the last years (2017-2021). Notably, besides the fundamental hydrogen isotope exchange (HIE) reactions and the use of isotopically labeled analogues of common organic reagents, a plethora of reductive and dehalogenative deuteration techniques and other transformations with isotope incorporation are emerging and are now part of the labeling toolkit.

Transamination-Like Reaction Catalyzed by Leucine Dehydrogenase for Efficient Co-Synthesis of α-Amino Acids and α-Keto Acids

α-Amino acids and α-keto acids are versatile building blocks for the synthesis of several commercially valuable products in the food, agricultural, and pharmaceutical industries. In this study, a novel transamination-like reaction catalyzed by leucine dehydrogenase was successfully constructed for the efficient enzymatic co-synthesis of α-amino acids and α-keto acids. In this reaction mode, the α-keto acid substrate was reduced and the α-amino acid substrate was oxidized simultaneously by the enzyme, without the need for an additional coenzyme regeneration system. The thermodynamically unfavorable oxidation reaction was driven by the reduction reaction. The efficiency of the biocatalytic reaction was evaluated using 12 different substrate combinations, and a significant variation was observed in substrate conversion, which was subsequently explained by the differences in enzyme kinetics parameters. The reaction with the selected model substrates 2-oxobutanoic acid and L-leucine reached 90.3% conversion with a high total turnover number of 9.0 × 10⁶ under the optimal reaction conditions. Furthermore, complete conversion was achieved by adjusting the ratio of addition of the two substrates. The constructed reaction mode can be applied to other amino acid dehydrogenases in future studies to synthesize a wider range of valuable products.

Computer-aided understanding and engineering of enzymatic selectivity

Enzymes offering chemo-, regio-, and stereoselectivity enable the asymmetric synthesis of high-value chiral molecules. Unfortunately, the drawback that naturally occurring enzymes are often inefficient or have undesired selectivity toward non-native substrates hinders the broadening of biocatalytic applications. To match the demands of specific selectivity in asymmetric synthesis, biochemists have implemented various computer-aided strategies in understanding and engineering enzymatic selectivity, diversifying the available repository of artificial enzymes. Here, given that the entire asymmetric catalytic cycle, involving precise interactions within the active pocket and substrate transport in the enzyme channel, could affect the enzymatic efficiency and selectivity, we presented a comprehensive overview of the computer-aided workflow for enzymatic selectivity. This review includes a mechanistic understanding of enzymatic selectivity based on quantum mechanical calculations, rational design of enzymatic selectivity guided by enzyme-substrate interactions, and enzymatic selectivity regulation via enzyme channel engineering. Finally, we discussed the computational paradigm for designing enzyme selectivity in silico to facilitate the advancement of asymmetric biosynthesis.

Light-driven decarboxylative deuteration enabled by a divergently engineered photodecarboxylase

Despite the well-established chemical processes for C-D bond formation, the toolbox of enzymatic methodologies for deuterium incorporation has remained underdeveloped. Here we describe a photodecarboxylase from Chlorella variabilis NC64A (CvFAP)-catalyzed approach for the decarboxylative deuteration of various carboxylic acids by employing D2O as a cheap and readily available deuterium source. Divergent protein engineering of WT-CvFAP is implemented using Focused Rational Iterative Site-specific Mutagenesis (FRISM) as a strategy for expanding the substrate scope. Using specific mutants, several series of substrates including different chain length acids, racemic substrates as well as bulky cyclic acids are successfully converted into the deuterated products (>40 examples). In many cases WT-CvFAP fails completely. This approach also enables the enantiocomplementary kinetic resolution of racemic acids to afford chiral deuterated products, which can hardly be accomplished by existing methods. MD simulations explain the results of improved catalytic activity and stereoselectivity of WT CvFAP and mutants.

Chemoenzymatic Total Synthesis of Natural Products

The total synthesis of structurally complex natural products has challenged and inspired generations of chemists and remains an exciting area of active research. Despite their history as privileged bioactivity-rich scaffolds, the use of natural products in drug discovery has waned. This shift is driven by their relatively low abundance hindering isolation from natural sources and the challenges presented by their synthesis. Recent developments in biocatalysis have resulted in the application of enzymes for the construction of complex molecules. From the inception of the Narayan lab in 2015, we have focused on harnessing the exquisite selectivity of enzymes alongside contemporary small molecule-based approaches to enable concise chemoenzymatic routes to natural products.We have focused on enzymes from various families that perform selective oxidation reactions. For example, we have targeted xyloketal natural products through a strategy that relies on a chemo- and site-selective biocatalytic hydroxylation. Members of the xyloketal family are characterized by polycyclic ketal cores and demonstrate potent neurological activity. We envisioned assembling a representative xyloketal natural product (xyloketal D) involving a biocatalytically generated ortho-quinone methide intermediate. The non-heme iron (NHI) dependent monooxygenase ClaD was used to perform the benzylic hydroxylation of a resorcinol precursor, the product of which can undergo spontaneous loss of water to form an ortho-quinone methide under mild conditions. This intermediate was trapped using a chiral dienophile to complete the total synthesis of xyloketal D.A second class of biocatalytic oxidation that we have employed in synthesis is the hydroxylative dearomatization of resorcinol compounds using flavin-dependent monooxygenases (FDMOs). We anticipated that the catalyst-controlled site- and stereoselectivity of FDMOs would enable the total synthesis of azaphilone natural products. Azaphilones are bioactive compounds characterized by a pyranoquinone bicyclic core and a fully substituted chiral carbon atom. We leveraged the stereodivergent reactivity of FDMOs AzaH and AfoD to achieve the enantioselective synthesis of trichoflectin enantiomers, deflectin 1a, and lunatoic acid. We also leveraged FDMOs to construct tropolone and sorbicillinoid natural products. Tropolones are a structurally diverse class of bioactive molecules characterized by an aromatic cycloheptatriene core bearing an α-hydroxyketone moiety. We developed a two-step biocatalytic cascade to the tropolone natural product stipitatic aldehyde using the FDMO TropB and a NHI monooxygenase TropC. The FDMO SorbC obtained from the sorbicillin biosynthetic pathway was used in the concise total synthesis of a urea sorbicillinoid natural product.Our long-standing interest in using enzymes to carry out C-H hydroxylation reactions has also been channeled for the late-stage diversification of complex scaffolds. For example, we have used Rieske oxygenases to hydroxylate the tricyclic core common to paralytic shellfish toxins. The systemic toxicity of these compounds can be reduced by adding hydroxyl and sulfate groups, which improves their properties and potential as therapeutic agents. The enzymes SxtT, GxtA, SxtN, and SxtSUL were used to carry out selective C-H hydroxylation and O-sulfation in saxitoxin and related structures. We conclude this Account with a discussion of existing challenges in biocatalysis and ways we can currently address them.

Nanocatalyzed Hydrogen Isotope Exchange

Recently, hydrogen isotope exchange (HIE) reactions have experienced impressive development due to the growing importance of isotope containing compounds in various fields including materials and life sciences, in addition to their classical use for mechanistic studies in chemistry and biology. Tritium-labeled compounds are also of crucial interest to study the in vivo fate of a bioactive substance or in radioligand binding assays. Over the past few years, deuterium-labeled drugs have been extensively studied for the improvement of ADME (absorption, distribution, metabolism, excretion) properties of existing bioactive molecules as a consequence of the primary kinetic isotope effect. Furthermore, in the emergent "omic" fields, the need for new stable isotopically labeled internal standards (SILS) for quantitative GC- or LC-MS analyses is increasing. Because of their numerous applications, the development of powerful synthetic methods to access deuterated and tritiated molecules with either high isotope incorporation and/or selectivities is of paramount importance.HIE reactions allow a late-stage incorporation of hydrogen isotopes in a single synthetic step, thus representing an advantageous alternative to conventional multistep synthesis approaches which are time- and resource-consuming. Moreover, HIE reactions can be considered as the most fundamental C-H functionalization processes and are therefore of great interest for the chemists' community. Depending on the purpose, HIE reactions must either be highly regioselective or allow a maximal incorporation of hydrogen isotopes, sometimes both. In this context, metal-catalyzed HIE reactions are generally performed using either homogeneous or heterogeneous catalysis which may have considerable drawbacks including an insufficient isotope incorporation and a lack of chemo- and/or regioselectivity, respectively.Over the past 6 years, we have shown that nanocatalysis can be considered as a powerful tool to access complex labeled molecules (e.g., pharmaceuticals, peptides and oligonucleotides) via regio- and chemoselective or even enantiospecific labeling processes occurring at the surface of metallic nanoclusters (Ru or Ir). Numerous heterocyclic (both saturated and unsaturated) and acyclic scaffolds have been labeled with an impressive functional group tolerance, and highly deuterated compounds or high molar activity tritiated drugs have been obtained. An insight into mechanisms has also been provided by theoretical calculations to explain the regioselectivities of the isotope incorporation. Our studies have suggested that undisclosed key intermediates, including 4- and 5-membered dimetallacycles, account for the particular regioselectivities observed during the process, in contrast to the 5- or 6-membered metallacycle key intermediates usually encountered in homogeneous catalysis. These findings together with the important number of available coordination sites explain the compelling reactivity of metal nanoparticles, in between homogeneous and heterogeneous catalysis. They represent innovative tools combining the advantages of both methods for the isotopic labeling and activation of C-H bonds of complex molecules.

Hybrid Chemo-, Bio-, and Electrocatalysis for Atom-Efficient Deuteration of Cofactors in Heavy Water

Deuterium-labeled nicotinamide cofactors such as [4-2H]-NADH can be used as mechanistic probes in biological redox processes and offer a route to the synthesis of selectively [2H] labeled chemicals via biocatalytic reductive deuteration. Atom-efficient routes to the formation and recycling of [4-2H]-NADH are therefore highly desirable but require careful design in order to alleviate the requirement for [2H]-labeled reducing agents. In this work, we explore a suite of electrode or hydrogen gas driven catalyst systems for the generation of [4-2H]-NADH and consider their use for driving reductive deuteration reactions. Catalysts are evaluated for their chemoselectivity, stereoselectivity, and isotopic selectivity, and it is shown that inclusion of an electronically coupled NAD+-reducing enzyme delivers considerable advantages over purely metal based systems, yielding exclusively [4S-2H]-NADH. We further demonstrate the applicability of these types of [4S-2H]-NADH recycling systems for driving reductive deuteration reactions, regardless of the facioselectivity of the coupled enzyme.

Identification of an α-Oxoamine Synthase and a One-Pot Two-Step Enzymatic Synthesis of α-Amino Ketones

Alb29, an α-oxoamine synthase involved in albogrisin biosynthesis in Streptomyces albogriseolus MGR072, was characterized and responsible for the incorporation of l-glutamate to acyl-coenzyme A substrates. Combined with Alb29 and Mgr36 (an acyl-coenzyme A ligase), a one-pot enzymatic system was established to synthesize seven α-amino ketones. When these α-amino ketones were fed into the alb29 knockout strain Δalb29, respectively, the albogrisin analogs with different side chains were observed.