Biocatalysis as a profound tool in the preparation of highly enantiopure β-amino acids

Reshaping the Substrate Binding Pocket of β-Amino Acid Dehydrogenase for the Synthesis of Aromatic β-Amino Acids

By reshaping the substrate-binding pocket of β-amino acid dehydrogenase (β-AADH), some variants were obtained with up to 2560-fold enhanced activity toward the model substrates (S)-β-homophenylalanine and (R)-β-phenylalanine. A few aromatic β-amino acids were prepared with >99% ee and high isolated yields via either kinetic resolution of racemates or reductive amination of the corresponding β-keto acids. This work expands the catalytic capability of β-AADHs and highlights their practical application in the synthesis of pharmaceutically relevant β-amino acids.

In depth analysis of heterogeneous catalysts for the chemoenzymatic dynamic kinetic resolution of β-amino esters

The chemoenzymatic dynamic kinetic resolution of β-amino esters is established after detailed evaluation of metal-based heterogeneous catalysts for racemization and enzyme catalysts for kinetic resolution.

Sequential Selective C-H and C(sp3 )-+ P Bond Functionalizations: An Entry to Bioactive Arylated Scaffolds

Organophosphonium salts containing C(sp³ )-⁺ P bonds are among the most utilized reagents in organic synthesis for constructing C-C double bonds. However, their use as C-selective electrophilic groups is rare. Here, we explore an efficient and general transition-metal-free method for sequential chemo- and regioselective C-H and C(sp³ )-⁺ P bond functionalizations. In the present study, C-H alkylation resulting in the synthesis of benzhydryl triarylphosphonium salts was achieved by one-pot, four-component cross-coupling reactions of simple and commercially available starting materials. The utility of the resulting phosphonium salt building blocks was demonstrated by the chemoselective post-functionalization of benzylic C(sp³ )-⁺ PPh₃ groups to achieve aminations, thiolations, and arylations. In this way, benzhydrylamines, benzhydrylthioethers, and triarylmethanes, structural motifs that are present in many pharmaceuticals and agrochemicals, are readily accessed. These include the synthesis of two anticancer agents from simple materials in only two to three steps. Additionally, a protocol for late-stage functionalization of bioactive drugs has been developed using benzhydrylphosphonium salts. This new approach should provide novel transformations for application in both academic and pharmaceutical research.

Combination of high throughput and structural screening to assess protein stability - a screening perspective

High throughput screening to measure the stability of industrially relevant proteins and their variants is necessary for quality assessment in the development process. Advances in automation, measurement time and sample consumption for many techniques allow rapid measurements with minimal amount of protein. However, many methods include automated data analysis, potentially neglecting important aspects of the proteińs behavior in certain conditions. In this study we implement small angle X-ray scattering (SAXS), typically not used to assess protein behavior in industrial screening, in a high throughput screening workflow to address problems of contradicting results and reproducibility among different high throughput methods. As a case study we use the lipases of Thermomyces lanuginosus and Rhizomucor miehei, widely used industrial biocatalysts. We show that even the initial analysis of the SAXS data without performing any time-consuming modelling provide valuable information on interparticle interactions. We conclude that recent advances in automation and data processing, have enabled SAXS to be used more widely as a tool to gain in-depth knowledge highly useful for protein formulation development. This is especially relevant in light of increasing accessibility to SAXS due to the commercial availability of benchtop instruments.

Crystal Structures and Catalytic Mechanism of l-erythro-3,5-Diaminohexanoate Dehydrogenase and Rational Engineering for Asymmetric Synthesis of β-Amino Acids

Amino acid dehydrogenases (AADHs) have shown considerable potential as biocatalysts in the asymmetric synthesis of chiral amino acids. However, compared to the widely studied α-AADHs, limited knowledge is available about β-AADHs that enable the synthesis of β-amino acids. Herein, we report the crystal structures of a l-erythro-3,5-diaminohexanoate dehydrogenase and its variants, the only known member of β-AADH family. Crystal structure analysis, site-directed mutagenesis studies and quantum chemical calculations revealed the differences in the substrate binding and catalytic mechanism from α-AADHs. A number of rationally engineered variants were then obtained with improved activity (by 110-800 times) toward various aliphatic β-amino acids without an enantioselectivity trade-off. Two β-amino acids were prepared by using the outstanding variants with excellent enantioselectivity (>99 % ee) and high isolated yields (86-87 %). These results provide important insights into the molecular mechanism of 3,5-DAHDH, and establish a solid foundation for further design of β-AADHs for the asymmetric synthesis of β-amino acids.

Candida antarctica Lipase A-Based Enantiorecognition of a Highly Strained 4-Dibenzocyclooctynol (DIBO) Used for PET Imaging

The enantiomers of aromatic 4-dibenzocyclooctynol (DIBO), used for radiolabeling and subsequent conjugation of biomolecules to form radioligands for positron emission tomography (PET), were separated by kinetic resolution using lipase A from Candida antarctica (CAL-A). In optimized conditions, (R)-DIBO [(R)-1, ee 95%] and its acetylated (S)-ester [(S)-2, ee 96%] were isolated. In silico docking results explained the ability of CAL-A to differentiate the enantiomers of DIBO and to accommodate various acyl donors. Anhydrous MgCl₂ was used for binding water from the reaction medium and, thus, for obtaining higher conversion by preventing hydrolysis of the product (S)-2 into the starting material. Since the presence of hydrated MgCl₂·6H₂O also allowed high conversion or effect on enantioselectivity, Mg²⁺ ion was suspected to interact with the enzyme. Binding site predictions indicated at least two sites of interest; one in the lid domain at the bottom of the acyl binding pocket and another at the interface of the hydrolase and flap domains, just above the active site.

Biodiesel Production Catalyzed by a Methanol-Tolerant Lipase A from Candida antarctica in the Presence of Excess Water

In this article, biodiesel was prepared using a novel free liquid lipase A from Candida antarctica (CALA) as a catalyst in the presence of excess water. The methanol tolerance of CALA was investigated. The effect of reaction conditions, including the molar ratio of soybean oil to methanol, water load, CALA load, reaction temperature, and reaction time, was evaluated. Reaction thermodynamics and kinetics were also analyzed. Results showed that free liquid lipase CALA showed excellent methanol tolerance in the reaction system using one-step addition of methanol and can be used to prepare biodiesel with water load of 12-14%. The influence of three transesterification variables on biodiesel yield was water load > temperature > time. The transesterification conditions were optimized by response surface methodology as follows: CALA load 5%, substrate molar ratio (soybean oil/methanol) 1:7, water load 14%, reaction time 26 h, and temperature 38 °C. The maximum biodiesel yield (92.4 ± 0.8%) was obtained under optimal conditions. The activation energy for biodiesel formation was 52.58 kJ/mol. Kinetic parameters K _m ^' and V _max were 4.84 × 10^-1 mol/L and 6.85 × 10^-2 mol/(L·min), respectively. The mechanism of CALA-catalyzed transesterification was also proposed.

Regioselective β-Csp3-Arylation of β-Alanine: An Approach for the Exclusive Synthesis of Diverse β-Aryl-β-amino Acids

An approach for the synthesis of a variety of new β-aryl-β-amino acids has been developed via a palladium-catalyzed auxiliary-directed regioselective Csp³-H arylation of the unactivated β-methylene bond of β-alanine. The use of 8-aminoquinoline amide as an auxiliary efficiently directs the desired regioselective β-Csp³-H functionalization. The developed protocol enables the easy and straightforward access to several high-value β-aryl-β-amino acids useful for peptide engineering, starting from inexpensive and readily available β-alanine precursors in moderate to excellent yields.

Access to β2-Amino Acids via Enantioselective 1,4-Arylation of β-Nitroacrylates Catalyzed by Chiral Rhodium Catalysts

The highly enantioselective conjugate addition of a variety of arylboronic acids to β-nitroacrylates is reported to provide optically active α-aryl β-nitropropionates in up to 70% yields and >99.5% ee's, which are useful building blocks for preparing chiral β²-amino acids. The applicability of this transformation is demonstrated by converting 3aa into the β²-amino acid 5 and transforming 3ap to β-amino ester 7 via reduction and reductive N-alkylation. The latter compound is a precursor for preparing ent-ipatasertib.

Computational redesign of enzymes for regio- and enantioselective hydroamination

Introduction of innovative biocatalytic processes offers great promise for applications in green chemistry. However, owing to limited catalytic performance, the enzymes harvested from nature's biodiversity often need to be improved for their desired functions by time-consuming iterative rounds of laboratory evolution. Here we describe the use of structure-based computational enzyme design to convert Bacillus sp. YM55-1 aspartase, an enzyme with a very narrow substrate scope, to a set of complementary hydroamination biocatalysts. The redesigned enzymes catalyze asymmetric addition of ammonia to substituted acrylates, affording enantiopure aliphatic, polar and aromatic β-amino acids that are valuable building blocks for the synthesis of pharmaceuticals and bioactive compounds. Without a requirement for further optimization by laboratory evolution, the redesigned enzymes exhibit substrate tolerance up to a concentration of 300 g/L, conversion up to 99%, β-regioselectivity >99% and product enantiomeric excess >99%. The results highlight the use of computational design to rapidly adapt an enzyme to industrially viable reactions.

Efficient Enzymatic Routes for the Synthesis of New Eight-membered Cyclic β-Amino Acid and β-Lactam Enantiomers

Efficient enzymatic resolutions are reported for the preparation of new eight-membered ring-fused enantiomeric β-amino acids [(1R,2S)-9 and (1S,2R)-9] and β-lactams [(1S,8R)-3, (1R,8S)-3 (1S,8R)-4 and (1R,8S)-7], through asymmetric acylation of (±)-4 (E > 100) or enantioselective hydrolysis (E > 200) of the corresponding inactivated (±)-3 or activated (±)-4 β-lactams, catalyzed by PSIM or CAL-B in an organic solvent. CAL-B-catalyzed ring cleavage of (±)-6 (E > 200) resulted in the unreacted (1S,8R)-6, potential intermediate for the synthesis of enantiomeric anatoxin-a. The best strategies, in view of E, reaction rate and product yields, which underline the importance of substrate engineering, are highlighted.

One-Pot Lipase-Catalyzed Enantioselective Synthesis of (R)-(-)-N-Benzyl-3-(benzylamino)butanamide: The Effect of Solvent Polarity on Enantioselectivity

The use of the solvent engineering has been applied for controlling the resolution of lipase-catalyzed synthesis of β-aminoacids via Michael addition reactions. The strategy consisted of the thermodynamic control of products at equilibrium using the lipase CalB as a catalyst. The enzymatic chemo- and enantioselective synthesis of (R)-(−)-N-benzyl-3-(benzylamino)butanamide is reported, showing the influence of the solvent on the chemoselectivity of the aza-Michael addition and the subsequent kinetic resolution of the Michael adduct; both processes are catalyzed by CalB and both are influenced by the nature of the solvent medium. This approach allowed us to propose a novel one-pot strategy for the enzymatic synthesis of enantiomerically enriched β-aminoesters and β-aminoacids.

Tandem Tetrahydroisoquinoline-4-carboxylic Acid/β-Alanine as a New Construct Able To Induce a Flexible Turn

Tetrahydroisoquinoline-4-carboxylic acid, a constrained β² -amino acid named β-TIC, was synthesised for the first time in enantiopure form. The biocatalytic route applied herein represents one of the few successful examples of enzymatic resolution of β² -amino acids. Model tetrapeptides, namely, Fmoc-l-Ala-β-TIC-β-Ala-l-Val-OBn (Fmoc=fluorenylmethyloxycarbonyl, Bn=benzyl), containing both isomers of β-TIC, were prepared. Both computational and NMR spectroscopy studies were performed. A reverse-turn conformation was observed in the case of (R)-β-TIC enantiomer that was obtained in 99 % enantiomeric excess by enzymatic resolution. The β-TIC/β-Ala construct represents the first example of a flexible turn mimetic containing a cyclic and an acyclic β-amino acid. Furthermore, the presence of an aromatic ring of β-TIC could facilitate non-covalent interactions to increase the potential of this scaffold for the preparation of protein-protein interaction modulators.

Statistical Evaluation of HTS Assays for Enzymatic Hydrolysis of β-Keto Esters

β-keto esters are used as precursors for the synthesis of β-amino acids, which are building blocks for some classes of pharmaceuticals. Here we describe the comparison of screening procedures for hydrolases to be used for the hydrolysis of β-keto esters, the first step in the preparation of β-amino acids. Two of the tested high throughput screening (HTS) assays depend on coupled enzymatic reactions which detect the alcohol released during ester hydrolysis by luminescence or absorption. The third assay detects the pH shift due to acid formation using an indicator dye. To choose the most efficient approach for screening, we assessed these assays with different statistical methods—namely, the classical Z’-factor, standardized mean difference (SSMD), the Kolmogorov-Smirnov-test, and t-statistics. This revealed that all three assays are suitable for HTS, the pH assay performing best. Based on our data we discuss the explanatory power of different statistical measures. Finally, we successfully employed the pH assay to identify a very fast hydrolase in an enzyme-substrate screening.

Purification and characterization of enantioselective N-acetyl-β-Phe acylases from Burkholderia sp. AJ110349

For the production of enantiopure β-amino acids, enantioselective resolution of N-acyl β-amino acids using acylases, especially those recognizing N-acetyl-β-amino acids, is one of the most attractive methods. Burkholderia sp. AJ110349 had been reported to exhibit either (R)- or (S)-enantiomer selective N-acetyl-β-Phe amidohydrolyzing activity, and in this study, both (R)- and (S)-enantioselective N-acetyl-β-Phe acylases were purified to be electrophoretically pure and determined the sequences, respectively. They were quite different in terms of enantioselectivities and in their amino acids sequences and molecular weights. Although both the purified acylases were confirmed to catalyze N-acetyl hydrolyzing activities, neither of them show sequence similarities to the N-acetyl-α-amino acid acylases reported thus far. Both (R)- and (S)-enantioselective N-acetyl-β-Phe acylase were expressed in Escherichia coli. Using these recombinant strains, enantiomerically pure (R)-β-Phe (>99% ee) and (S)-β-Phe (>99% ee) were obtained from the racemic substrate.

Remarkable magnitude of the self-disproportionation of enantiomers (SDE) via achiral chromatography: application to the practical-scale enantiopurification of β-amino acid esters

We report the best performance yet for the self-disproportionation of enantiomers (SDE) via achiral chromatography as typically used in laboratories for the isolated yield of the excess enantiomer using N-acetyl β-amino acid ethyl esters. The results are the most convincing ever demonstration of the capability of the SDE for practical-scale enantiopurification as comparable, or even superior for some systems, to that of recrystallization. For example, from a sample of 94.4 % ee, a yield of 71 % of enantiopure material was isolated in a single chromatographic run. Moreover, the lack of an esoteric structural entity, e.g. strongly polarizing groups, such as, for instance CF3, highlights the fact that the phenomenon is not dependent on the presence of such and thus the process is relevant to any usual-type structure. In contrast to recrystallization, the procedure is predictable, general, and dependable, boding well for its widespread application in routine laboratory settings.

Chemical synthesis and enzymatic, stereoselective hydrolysis of a functionalized dihydropyrimidine for the synthesis of β-amino acids

A novel substrate, 6-(4-nitrophenyl)dihydropyrimidine-2,4(1H,3H)-dione (pNO₂PheDU), was chemically synthesized and analytically verified for the potential biocatalytic synthesis of enantiopure β-amino acids. The hydantoinase (EC 3.5.2.2) from Arthrobacter crystallopoietes DSM20117 was chosen to prove the enzymatic hydrolysis of this substrate, since previous investigations showed activities of this enzyme toward 6-monosubstituted dihydrouracils. Whole cell biotransformations with recombinant Escherichia coli expressing the hydantoinase showed degradation of pNO₂PheDU. Additionally, the corresponding N-carbamoyl-β-amino acid (NCarbpNO₂βPhe) was chemically synthesized, an HPLC-method with chiral stationary phases for detection of this product was established and thus (S)-enantioselectivity toward pNO₂PheDU has been shown. Consequently this novel substrate is a potential precursor for the enantiopure β-amino acid para-nitro-β-phenylalanine (pNO₂βPhe).

A practical route to β(2,3)-amino acids with alkyl side chains

Enantiopure N(Boc)-β³-amino nitriles, valuable synthetic intermediates in the multistep homologation of α-amino acids, were alkylated using n-BuLi as base. Alkylations afforded easily separable, almost equimolecular mixtures of diastereomeric N(Boc)-protected syn and anti β^2,3-amino nitriles. Suitable manipulations of both cyano and amino groups eventually led to enantiopure N- and/or C-protected β^2,3-amino acids. For example, methanolysis using conc. HCl gas in MeOH, provides C-protected β^2,3 amino acids in excellent yields. This methodology is applied to the synthesis of a series N(Boc)-β^2,3-dialkyl amino nitriles derived from l-phenylalanine, d-phenylalanine, l-valine and one C-protected β^2,3 amino acid. We demonstrate an efficient procedure for the preparation of anti and syn β^2,3-amino acids with alkyl side chains, from α-amino acids in reasonable yields.

High-performance liquid chromatographic separation of paclitaxel intermediate phenylisoserine derivatives on macrocyclic glycopeptide and cyclofructan-based chiral stationary phases

High-performance liquid chromatographic methods were developed for the separation of enantiomers of four unnatural paclitaxel precursor phenylisoserine analogs on chiral stationary phases containing macrocyclic glycopeptides and cyclofructans as chiral selectors. The effects of the mobile phase composition, the nature and concentration of different mobile phase additives (alcohols, amines and acids) in different chromatographic modes, temperature and the structures of the analytes on the separations were investigated. Separations were carried out at constant mobile phase compositions in the temperature range 10-50°C on macrocyclic antibiotic-based and 5-35°C on cyclofructan-based columns and the changes in enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated. The elution sequence was determined in most cases; no general rule could be observed.