The chemo- enzymatic synthesis of labeled l-amino acids and some of their derivatives

A practical guide for the preparation of C1-labeled α-amino acids using aldehyde catalysis with isotopically labeled CO2

Isotopically carbon-labeled α-amino acids are valuable synthetic targets that are increasingly needed in pharmacology and medical imaging. Existing preparations rely on early stage introduction of the isotopic label, which leads to prohibitive synthetic costs and time-intensive preparations. Here we describe a protocol for the preparation of C1-labeled α-amino acids using simple aldehyde catalysts in conjunction with [*C]CO2 (* = 14, 13, 11). This late-stage labeling strategy is enabled by the one-pot carboxylate exchange of unprotected α-amino acids with [*C]CO2. The protocol consists of three separate procedures, describing the syntheses of (±)-[1-13C]phenylalanine, (±)-[1-11C]phenylalanine and (±)-[1-14C]phenylalanine from unlabeled phenylalanine. Although the delivery of [*C]CO2 is operationally distinct for each experiment, each procedure relies on the same fundamental chemistry and can be executed by heating the reaction components at 50-90 °C under basic conditions in dimethylsulfoxide. Performed on scales of up to 0.5 mmol, this methodology is amenable to C1-labeling of many proteinogenic α-amino acids and nonnatural derivatives, which is a breakthrough from existing methods. The synthesis of (±)-[1-13C]phenylalanine requires ~2 d, with product typically obtained in a 60-80% isolated yield (n = 3, μ = 71, σ = 8.3) with an isotopic incorporation of 70-88% (n = 18, μ = 72, σ = 9.0). Starting from the preformed imino acid (~3 h preparation time), rapid synthesis of (±)-[1-11C]phenylalanine can be completed in ~1 h with an isolated radiochemical yield of 13%. Finally, (±)-[1-14C]phenylalanine can be accessed in ~2 d with a 51% isolated yield and 11% radiochemical yield.

Biomanufacture of L-homoserine lactone building block: A strategy for preparing γ-substituted L-amino acids by modular reaction

A strain high-performance of esterase producing bacteria was screened from soil, which could selectively hydrolyze D-homoserine lactone from its racemate to achieve the resolution of L- homoserine lactone with more than 99% e.e. in 48% yield. L-homoserine lactone building block was then converted to L-α-amino-γ-bromobutyronic acid chiral blocks, which reacted with various nucleophilic reagent modules could to be applied to prepare L-γ- substituted α-amino acids such as L-selenomethionine, L-methionine, L-glufosinate and L-selenocystine. Its advantages included high selectivity of biocatalytic resolution reactions, high optical purity of products, racemic recycle of D-substrates and modular reaction, which simplified the production process of these products and highlighted the power of biological manufacturing.

Aldehyde-catalysed carboxylate exchange in α-amino acids with isotopically labelled CO2

The isotopic labelling of small molecules is integral to drug development and for understanding biochemical processes. The preparation of carbon-labelled α-amino acids remains difficult and time consuming, with established methods involving label incorporation at an early stage of synthesis. This explains the high cost and scarcity of C-labelled products and presents a major challenge in 11C applications (11C t1/2 = 20 min). Here we report that aldehydes catalyse the isotopic carboxylate exchange of native α-amino acids with *CO2 (* = 14, 13, 11). Proteinogenic α-amino acids and many non-natural variants containing diverse functional groups undergo labelling. The reaction probably proceeds via the trapping of *CO2 by imine-carboxylate intermediates to generate iminomalonates that are prone to monodecarboxylation. Tempering catalyst electrophilicity was key to preventing irreversible aldehyde consumption. The pre-generation of the imine carboxylate intermediate allows for the rapid and late-stage 11C-radiolabelling of α-amino acids in the presence of [11C]CO2.

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.

Sustainable one-pot chemo-enzymatic synthesis of chiral furan amino acid from biomass via magnetic solid acid and threonine aldolase

Sustainable synthesis of valuable noncanonical amino acids from renewable feedstocks is of great importance. Here, a feasible chemo-enzymatic procedure was developed for the synthesis of chiral β-(2-furyl)serine from biomass catalyzed by a solid acid catalyst and immobilized E. coli whole-cell harboring l-threonine aldolase. A novel magnetic solid acid catalyst Fe₃O₄@MCM-41/SO₄^2- was successfully synthesized for conversion of corncob into furfural in an aqueous system. Under the optimum conditions, furfural yield of 63.6% was achieved in 40 min at 180 ℃ with 2.0% catalyst (w/w). Furthermore, biomass-derived furfural was converted into an aldol-addition product β-(2-furyl)serine with 73.6% yield, 99% ee and 20% de by immobilized cells in 6 h. The magnetic solid acid and biocatalyst can be readily recovered and efficiently reused for five consecutive cycles without significant loss on product yields. This chemo-enzymatic route can be attractive for producing noncanonical amino acids from biomass.

Synthesis of deuterium-labeled cinnamic acids: Understanding the volatile benzenoid pathway in the flowers of the Japanese loquat Eriobotrya japonica

Cinnamic acids are widely distributed in plants, including crops for human use, and exhibit a variety of activities that are beneficial to human health. They also occupy a pivotal position in the biosynthesis of phenylpropanoids such as lignins, anthocyanins, flavonoids, and coumarins. In this context, deuterium-labeled cinnamic acids have been used as tracers and internal standards in food and medicinal chemistry as well as plant biochemistry. Therefore, a concise synthesis of deuterium-labeled cinnamic acids would be highly desirable. In this study, we synthesized deuterium-labeled cinnamic acids using readily available deuterium sources. We also investigated a hydrogen-deuterium exchange reaction in an ethanol-d₁ /Et₃ N system. This method can introduce deuterium atoms at the ortho and para positions of the phenolic hydroxy groups as well as at the C-2 position of alkyl cinnamates and is applicable to various phenolic compounds. Using the synthesized labeled compounds, we demonstrated that the benzenoid volatiles, such as 4-methoxybenzaldehyde, in the scent of the flowers of the Japanese loquat Eriobotrya japonica are biosynthesized from phenylalanine via cinnamic and 4-coumaric acids. This study provides easy access to a variety of deuterium-labeled (poly)phenols, as well as to useful tools for studies of the metabolism of cinnamic acids in living systems.

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.

Methylated derivatives of l-tyrosine in reaction catalyzed by l-amino acid oxidase: isotope and inhibitory effects

l-Amino acid oxidase (LAAO) is widely distributed in nature and shows important biological activity. It induces cell apoptosis and has antibacterial properties. This study was designed to investigate the effect of methyl substituent on its activity as methylated derivatives of l-tyrosine, labelled with short-lived B+ emitters, have been used in oncological diagnostics. To study isotope effects in the oxidative deamination of O-methyl-l-tyrosine, the deuterated isotopomer, i.e. O-methyl-[2-2H]-l-tyrosine, was synthesized by isotope exchange, catalyzed enzymatically by tryptophanase. Isotope effects were determined using the spectrophotometric non-competitive method. The values of isotope effects indicate that the α-C-H bond cleavage occurs in the rate determining step of the investigated reaction and α-hydrogen plays a role in the substrate binding process at the enzyme active site. The inhibitory effect on LAAO activity was studied with α-methyl-l-tyrosine and N-methyl-l-tyrosine. The mode of inhibition was determined based on Lineweavear-Burk plots intersections. α-Methyl-l-tyrosine has been found a mixed type inhibitor of the investigated enzyme, whereas N-methyl-l-tyrosine is a non-competitive inhibitor of LAAO.

Microwave enhanced synthesis of halogenated derivatives of L-tyrosine labeled with deuterium in aromatic ring

Three halogenated derivatives of L-tyrosine, selectively labeled with deuterium in aromatic ring, i.e., 3′-fluoro-[5′-2H]-, 3′-chloro-[5′-2H]-, and 3′-iodo-[2′,5′-2H2]-L-tyrosine, were synthesized using microwave assisted acid-catalyzed isotope exchange between 3′-fluoro-, 3′-chloro- and 3′-iodo-L-tyrosine and heavy water. The degree of deuterium incorporation was confirmed by 1H NMR spectroscopy. The spectroscopic data indicate that isotope exchange depends on the method of heating and the power of microwaves. The deuterium enrichment of 3′-fluoro-[5′-2H]- and 3′-chloro-[5′-2H]-L-tyrosine amounted to 70% and 60%, respectively, while for 3′-iodo-[2′,5′-2H2]-L-tyrosine this value was about 50% and 95% for the 2′- and 5′-position. The isotopomers were obtained in good chemical yields of 50–70%.

Overview on Multienzymatic Cascades for the Production of Non-canonical α-Amino Acids

The 22 genetically encoded amino acids (AAs) present in proteins (the 20 standard AAs together with selenocysteine and pyrrolysine), are commonly referred as proteinogenic AAs in the literature due to their appearance in ribosome-synthetized polypeptides. Beyond the borders of this key set of compounds, the rest of AAs are generally named imprecisely as non-proteinogenic AAs, even when they can also appear in polypeptide chains as a result of post-transductional machinery. Besides their importance as metabolites in life, many of D-α- and L-α-"non-canonical" amino acids (NcAAs) are of interest in the biotechnological and biomedical fields. They have found numerous applications in the discovery of new medicines and antibiotics, drug synthesis, cosmetic, and nutritional compounds, or in the improvement of protein and peptide pharmaceuticals. In addition to the numerous studies dealing with the asymmetric synthesis of NcAAs, many different enzymatic pathways have been reported in the literature allowing for the biosynthesis of NcAAs. Due to the huge heterogeneity of this group of molecules, this review is devoted to provide an overview on different established multienzymatic cascades for the production of non-canonical D-α- and L-α-AAs, supplying neophyte and experienced professionals in this field with different illustrative examples in the literature. Whereas the discovery of new or newly designed enzymes is of great interest, dusting off previous enzymatic methodologies by a "back and to the future" strategy might accelerate the implementation of new or improved multienzymatic cascades.

Bringing biocatalytic deuteration into the toolbox of asymmetric isotopic labelling techniques

Enzymes dependent on nicotinamide cofactors are important components of the expanding range of asymmetric synthetic techniques. New challenges in asymmetric catalysis are arising in the field of deuterium labelling, where compounds bearing deuterium (²H) atoms at chiral centres are becoming increasingly desirable targets for pharmaceutical and analytical chemists. However, utilisation of NADH-dependent enzymes for ²H-labelling is not straightforward, owing to difficulties in supplying a suitably isotopically-labelled cofactor ([4-²H]-NADH). Here we report on a strategy that combines a clean reductant (H₂) with a cheap source of ²H-atoms (²H₂O) to generate and recycle [4-²H]-NADH. By coupling [4-²H]-NADH-recycling to an array of C=O, C=N, and C=C bond reductases, we demonstrate asymmetric deuteration across a range of organic molecules under ambient conditions with near-perfect chemo-, stereo- and isotopic selectivity. We demonstrate the synthetic utility of the system by applying it in the isolation of the heavy drug (1S,3'R)-[2',2',3'-²H₃]-solifenacin fumarate on a preparative scale.