Amine Donor Research Articles

Chemo-Enzymatic Synthesis of Pyrazines and Pyrroles.

Herein we report the biocatalytic synthesis of substituted pyrazines and pyrroles using a transaminase (ATA) to mediate the key amination step of the ketone precursors. Treatment of α‐diketones with ATA‐113 in the presence of a suitable amine donor yielded the corresponding α‐amino ketones which underwent oxidative dimerization to the pyrazines. Selective amination of α‐diketones in the presence of β‐keto esters afforded substituted pyrroles in a biocatalytic equivalent of the classical Knorr pyrrole synthesis. Finally we have shown that pyrroles can be prepared by internal amine transfer catalyzed by a transaminase in which no external amine donor is required.

Iron(II) complexes of dimethyltriazacyclophane.

Treatment of the ortho-triazacyclophane 1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-triene [(C6H5)3(NH)(NCH3)2, L1] with Fe[N(SiMe3)2]2 yields the dimeric iron(II) complex bis(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido)bis[(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido)iron(II)], [Fe(C20H18N3)4] or Fe2(L1)4 (9). Dissolution of 9 in tetrahydrofuran (THF) results in solvation by two THF ligands and the formation of a simpler monoiron complex, namely bis(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido-κN7)bis(tetrahydrofuran-κO)iron(II), [Fe(C20H18N3)2(C4H8O)2] or (L1)2Fe(THF)2 (10). The reaction is reversible and 10 reverts in vacuo to diiron complex 9. In the structures of both 9 and 10, the monoanionic triazacyclophane ligand L1- is observed in only the less-symmetric saddle conformation. No bowl-shaped crown conformers are observed in the solid state, thus preventing chelating κ3-coordination to the metal as had been proposed earlier based on density functional theory (DFT) calculations. Instead, the L1- ligands are bound in either a η2-chelating fashion through the amide and one amine donor (for one of the four ligands of 9), or solely through their amide N atoms in an even simpler monodentate η1-coordination mode. Density functional calculations on dimer 9 revealed nearly full cationic charges on each Fe atom and no bonding interaction between the two metal centers, consistent with the relatively long Fe...Fe distance of 2.912 (1) Å observed in the solid state.

Resonance Raman and UV–vis characterization of charge transfer complexes of TCNQ and aromatic amines

Although the molecular charge transfer complexes formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and several different donors have been widely investigated in the past, in this paper it is shown for the first time the vibrational spectroscopic characterization of the complexes formed by TCNQ and the simplest series of substituted anilines in solution. The UV–vis spectra indicate the formation of TCNQ complexes stabilized by two aromatic amines in a sandwich π type complex. It was observed that the charge transfer transition energies of the complexes follow a linear correlation with the ionization potential of the amines. The resonance Raman spectra revealed, by the analysis of the ν(CC) and ν(CN) stretching modes of TCNQ, that the complexes keep their neutral character in the ground electronic state, and not as a TCNQ2− dianion species as reported before. The observed enhancement of the TCNQ bands as the amine donor capacity increases, confirmed the charge transfer nature of the electronic transitions. The DFT and TDDFT results were obtained and the theoretical Raman and electronic spectra data supported the experimental findings. The results of the model systems presented herein can contribute to a deep understanding of the photoinduced charge transfer process in complexes of TCNQ, which is a key step in the designing of organic molecular devices.

Application of “Smart” Amine Donors for Rapid Screening and Scale‐Up of Transaminase‐Mediated Biotransformations

The “smart” amine donors o‐xylylenediamine and cadaverine were employed for the rapid screening of a large ketone library and subsequent preparative‐scale synthesis of selected compounds using a commercially available amine transaminase, ATA256. The methodology enables both screening and preparative‐scale biotransformations to be performed with a single enzyme and simplifies the generation of sp3‐rich small‐molecule libraries.

Controlling Dark Equilibria and Enhancing Donor-Acceptor Stenhouse Adduct Photoswitching Properties through Carbon Acid Design.

A novel library of tunable negative photochromic compounds, donor-acceptor Stenhouse adducts (DASAs), is reported. Tailoring the electron deficient "acceptor" moiety yielded DASAs that can be activated with mild visible and far red light. The effect of acceptor composition on reactivity, absorption, equilibrium, and cyclability is exploited for the design of high performance photoswitches. The structural changes to the carbon acid acceptor also provide access to new, more structurally diverse DASA derivatives by facilitating the ring-opening reaction with electron deficient amine donors.

Copper(II) complexes of 7-amino-2-methylchromone and 7-aminoflavone: Magneto-structural, spectroscopic and DFT characterization

Four new complexes of copper(II) consisting of two different chromone derivatives with carbonyl and amine donor atoms: 7-aminoflavone (L1) and 7-amino-2-methylchromone (L2), were synthesized and characterized by X-ray diffraction, UV–Vis, FT-IR, EPR spectroscopy, MS and elemental analysis. The complexes were synthesized using 2:1 ligand-to-metal ratio, but while three products [Cu(L2)2Cl2]2 (1b), Cu(L1)2(ClO4)2(H2O)2 (2a), Cu(L2)2(ClO4)2(H2O)2 (2b) maintained the applied stoichiometry, the fourth compound showed an unexpected 1:1 ratio and was identified as [Cu(L1)Cl2]n (1a). In all four compounds, the chromone derivatives were found to act as monodentate O- or N-ligands. Interestingly, in a doubly chloro-bridged dinuclear complex 1b each Cu(II) cation interacts with one L2 ligand through the carbonyl O-donor, and with another L2 molecule through the amine N-donor, so the complex adopts a square pyramid geometry. According to a single-crystal X-ray diffraction analysis the complex 2b is a distorted mononuclear octahedron with the central Cu(II) cation interacting only with oxygen donors (two perchlorate anions, two L2 ligands, and two water molecules). The structural characteristics of 1a and 2a complexes were elucidated from EPR measurements, where their g-tensor components were compared to those observed for 1b and 2b, the structures of which were determined by X-ray diffraction method. DFT calculations were performed to estimate the most favored mode of coordination, whereas comparative Hirshfeld surface studies were used to characterize intermolecular interactions in the studied complexes. Magnetic interactions of 1a and 2a were also analyzed.

Process Development for Synthesizing the Cephalosporin Antibiotic Cefotaxime in Batch and Flow Mode

The pharmaceutically active substance cefotaxime, a commercial cephalosporin-type antibiotic, is accessible in an amide-bond-forming reaction from 7-aminocephalosporanic acid as the amine donor and nonactivated (Z)-(2-aminothiazol-4-yl)-methoxyiminoacetic acid as the acid component with 4-toluenesulfonyl chloride as a coupling reagent, leading to only toluenesulfonic acid as an easy-to-separate byproduct. In this work, optimization of a batch process for this reaction is described as well as the extension toward a continuous process in a tube reactor with a diameter in the millimeter range. An opportunity to avoid the utilization of a chlorinated solvent system has been identified, thus contributing to the development of an ecologically friendly process. It was further shown that a higher reaction temperature of up to −10 °C is possible for the reaction when the process is conducted in a continuously operating fashion, which is an advantage from the perspective of energy demand. Thus, compared with the ba...

Isopropylamine as Amine Donor in Transaminase‐Catalyzed Reactions: Better Acceptance through Reaction and Enzyme Engineering

AbstractAmine transaminases (ATA) have now become frequently used biocatalysts in chemo‐enzymatic syntheses including industrial applications. They catalyze the transfer of an amine group from a donor to an acceptor leading to an amine product with high enantiopurity. Hence, they represent an environmentally benign alternative for waste intensive chemical amine synthesis. Isopropylamine (IPA) is probably one of the most favored amine donors since it is cheap and achiral, but nevertheless there is no consistency in literature concerning reaction conditions when IPA is best to be used. At the same time there is still a poor understanding which structural properties in ATA are responsible for IPA acceptance. Herein, we demonstrate, on the basis of the 3FCR enzyme scaffold, a substantial improvement in catalytic activity towards IPA as the amine donor. The asymmetric synthesis of industrial relevant amines was used as model reaction. A systematic investigation of the pH‐value as well as concentration effects using common benchmark substrates and several ATA indicates the necessity of a substrate‐ and ATA‐dependent reaction engineering.

Abstract 752: CovisoLinK: New bacterial transglutaminase Q-Tag substrate for the development of site specific antibody-drug conjugates

Abstract CovIsoLink™ (Covalently Isopeptide crosslinking) relates to methods for enzymatic covalently coupling drugs and other compounds through transglutaminase site specific generated in the targeted proteins including, polypeptides, proteins and immunoglobulins (patent pending1). Transglutaminases (TGases) catalyze covalent cross- linking of specific glutamine residues to the primary amine of peptide-bound lysine residues or primary amines of other compounds such as polyamines2. Using an in house peptide library and the transglutaminase colorometric assay3, we identified several amino acid sequences that were recognized as glutamine donor substrates. The optimum peptide sequences were selected and we further confirmed that these sequences have improved affinity compared with the conventional small peptides Z-QG. In different experiments we engineered Fc-containing polypeptide at the C-terminal domain and showed that mTG incorporates with high efficiency several types of amine donors to the engineered antibodies. CovIsoLink™ is now used to develop new antibody drug conjugates (ADCs) since the major advantage of this method is to obtain a homogenous immunoconjugate with uniform stoichiometry. We developed and characterized different recombinant anti Her2 IgG1 Mab carrying optimized enzymatic substrates (tag) by genetic insertion in the coding sequence of MAb. We then evaluated the best linkers and conformation to incorporate different compounds through bacterial Transglutaminase (mTG) enzymatic reaction. We set up experimental conditions, production, purification, HPLC/MS analysis and control of the immunoreactivity of CovIsoLink™ Her2-ADC. Using mTG, we obtained site specific conjugation of different modified drugs with optimized linker on the anti Her2 IgG1 antibody. By HIC analysis, we validated a specific and reproducible DAR reaching DAR2 depending on drugs and experimental conditions. In vitro and In vivo characterization and dose response studies of CovIsolink-ADC specificity and reactivity are currently performed in Her2 positive models demonstrating an improved efficacy by comparison with Kadcyla (T-DM1).1) PCT/EP2014/0792278, 2) El Alaoui S et al, Int J Cancer, 1991; 3) Perez AM et al, Anal Biochem, 2009. Citation Format: Eva Sivado, Vincent Thomas, Meddy El Alaoui, Anne-Catherine Jallas, Mike R. Dyson, John McCafferty, Said El Alaoui, Sandrine Valsesia-Wittmann. CovisoLinK: New bacterial transglutaminase Q-Tag substrate for the development of site specific antibody-drug conjugates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 752.

Optimization of Hexadentate Bispidine Ligands as Chelators for 64 CuII PET Imaging.

The coordination chemistry of the hexadentate bispidine ligand L1 with three pyridine, one pyridazine, and two tertiary amine donors with CuII and ZnII is reported. The substitution of one of the two cis-disposed pyridine donors in-plane with the two tertiary amines of the bispidine backbone by a pyridazine group was predicted to substantially reduce distortion from planarity of the tetradentate subunit and therefore was assumed to lead to higher CuII complex stability. The X-ray single-crystal structures of the ZnII and CuII complexes, which are in excellent agreement with the DFT- and MM-optimized structures, confirmed this prediction but indicated that deviation from planarity is appreciable. This also emerges from solution spectroscopy (UV/Vis/NIR and EPR of the CuII complex), which indicated that the in-plane ligand field is only slightly increased. The geometric parameters together with the lower basicity of pyridazine versus pyridine (pKa =2.33 vs. 5.23) are also in agreement with an only slightly more negative redox potential of the CuII/I couple (Eo =-780 vs. -760 mV, MeCN, vs. Fc/Fc+ ), and the potentiometrically determined CuII stability constant with L1 is slightly lower than that with the parent ligand L2 (log K=12.7 vs. 14.5). Therefore, modification of the donor groups is a more promising approach to increasing the stabilities of these complexes and yields 64 CuII chelators that outperform known hexadentate bispidine ligands.

Synthesis, characterization and antimicrobial studies of metal complexes of mixed ligands : citric acid and amodiaquine

A range of d-block transition metals complexes of mixed amodiaquine and citric acid have been synthesized and characterised using microanalytical technique, elemental analysis, FT-IR spectroscopy and magnetic measurement. The results of the analytical and spectroscopic data revealed that both amodiaquine and citric acid acted as bidentate ligands in their mode of coordination with the metal ions.The citric acid bound through the carbonyl (C=O) and hydroxyl groups to the metal ions while, amodiaquine coordinated through the nitrogen atom of the amine and oxygen donor atom of hydroxyl group; resulted in structures with octahedral geometry. Percentage composition and magnetic Bohr moment data indicated that all metal complexes were paramagnetic. Molar conductance measurement indicates that the complexes are non-electrolytes in nature. The antimicrobial activity of the ligands and their mixed complexes was carried out against g(+)Pseudomonas aeruginosa, g(+) E. coli , g(+) Klebsiella pneumonia , g(+) Staphylococcus aureus and g(+)Candida using Muller Hinton diffusion method. The citric acid and amodiaquine result showed low or moderate value from (0.6-0.8) for all the metal complexes with the exception of the Cu (II) complex having inhibitory zones ranging from 1-0-1.3, 0.7-0.9 at 200 ppm respectively. Moreover, the free ligands were found being less active compared with their mixed complexes. Keywords: Citric acid, Amodiaquine, Complexes, Antimicrobial studies

Porphyrin Sensitizers Sensitizers Exceeding a World Champion Porphyrin Dye for Dye-Sensitized Solar Cells and Their Tandem Solar Cells

Push–pull structured (D–π–A) Zn(II)–porphyrin sensitizers, containing various electron donor groups for the enhancement of the light-harvesting ability within the expanded absorption region and efficient prevention of undesirable charge recombination of the sensitising dyes in dye-sensitized solar cells (DSSCs) were newly designed and synthesised through molecular engineering.1 We investigated the effect of bulky donor groups in the D-p-A structured porphyrins on retarded charge recombination between the electrolytes and TiO2 surface and enhanced power-conversion efficiency (PCE). The photovoltaic (PV) performance of the SGT sensitizer-based DSSCs, employing cobalt redox couples, were systematically evaluated in a thorough comparison with SM315 as a reference sensitizer. By introducing a more bulky fluorene unit containing alkoxyphenyl unit to the amine donor group in the world champion SM315 porphyrin dye, for efficient prevention of undesirable charge recombination, the PCE was enhanced. As a consequence, we observed a maximum 12.05% PCE for SGT-021 under simulated one-sun illumination (AM1.5G, 100 mWcm-2), whose PCE was much higher than the 11.67% PCE of SM315, which was known as the world champion dye with a world record PCE of 13%. To further improve the maximum efficiency of the DSSCs, the first parallel-connected (PC) tandem DSSCs employing cobalt electrolyte in the bottom cell with SGT-021 and the top cell with SGT-137 2 were demonstrated and an extremely high efficiency of 14.64% was found, which is currently the highest reported value for dye-sensitised tandem solar cells. In this presentation, Zn-Porphyrin sensitizers for superior performance dye-sensitized solar cells and their tandem solar cells will be described. References S. H. Kang and H. K. Kim, et.al., Adv. Energy Mater. 7(2017) 1602117.Y. K. Eom and H. K. Kim, et.al., J. Mater. Chem. A, 5(2017) 2297.

Protein-ligand interfaces are polarized: discovery of a strong trend for intermolecular hydrogen bonds to favor donors on the protein side with implications for predicting and designing ligand complexes.

Understanding how proteins encode ligand specificity is fascinating and similar in importance to deciphering the genetic code. For protein-ligand recognition, the combination of an almost infinite variety of interfacial shapes and patterns of chemical groups makes the problem especially challenging. Here we analyze data across non-homologous proteins in complex with small biological ligands to address observations made in our inhibitor discovery projects: that proteins favor donating H-bonds to ligands and avoid using groups with both H-bond donor and acceptor capacity. The resulting clear and significant chemical group matching preferences elucidate the code for protein-native ligand binding, similar to the dominant patterns found in nucleic acid base-pairing. On average, 90% of the keto and carboxylate oxygens occurring in the biological ligands formed direct H-bonds to the protein. A two-fold preference was found for protein atoms to act as H-bond donors and ligand atoms to act as acceptors, and 76% of all intermolecular H-bonds involved an amine donor. Together, the tight chemical and geometric constraints associated with satisfying donor groups generate a hydrogen-bonding lock that can be matched only by ligands bearing the right acceptor-rich key. Measuring an index of H-bond preference based on the observed chemical trends proved sufficient to predict other protein-ligand complexes and can be used to guide molecular design. The resulting Hbind and Protein Recognition Index software packages are being made available for rigorously defining intermolecular H-bonds and measuring the extent to which H-bonding patterns in a given complex match the preference key.

Asymmetric Synthesis of Chiral Halogenated Amines using Amine Transaminases

AbstractAmine transaminases (ATAs) are versatile and industrially relevant biocatalysts that catalyze the transfer of an amine group from a donor to an acceptor molecule. Asymmetric synthesis from a prochiral ketone is the most preferred route to the desired amine product, as it is obtainable in a theoretical yield of 100 %. In addition to the requirement of active and enantioselective ATAs, the choice of a suitable amine donor is also important to save costs and to avoid additional enzymes to shift the equilibrium and/or to recycle the cofactors. In this work, we identified suitable (R)‐ and (S)‐ATAs from Aspergillus fumigatus and Silicibacter pomeroyi, respectively, to afford a set of halogen‐substituted derivatives of brominated or chlorinated 1‐phenyl‐2‐propanamine, 4‐phenylbutan‐2‐amine, and 1‐(3‐pyridinyl)ethanamine. Optimization of the donor–acceptor ratio enabled application of isopropylamine as an amine donor, which resulted in high conversions and amines with 73–99 % ee.

Fast kinetics studies of the Lewis acid-base complexation of transient stannylenes with σ- and π-donors in solution.

The Lewis acid-base complexation chemistry of dimethyl- and diphenylstannylene (SnMe2 and SnPh2, respectively) in hexanes solution has been studied by laser photolysis methods. Complexation of the two stannylenes with a series of nine O-, S-, and N-donors (including cyclic and acyclic dialkyl ethers and sulfides, a primary, secondary, and tertiary amine, ethyl acetate and acetone), two alkenes, and an alkyne proceeds rapidly and reversibly to generate the corresponding stannylene-donor Lewis pairs, which have been detected in each case by time-resolved UV-vis absorption spectroscopy. The complexes exhibit UV-vis absorption maxima in the range of λmax ∼ 310-405 nm depending on the donor and substitution at tin. Bimolecular rate constants for complexation (kC), which could be determined for 14 of the 24 Lewis pairs that were studied, were found to fall within a factor of four of the diffusional limit in all cases, with SnMe2 showing consistently higher reactivity than SnPh2. Equilibrium constants for complexation (KC) could be measured for all but one of the stannylene-π- and O-donor pairs, the values corresponding to (gas phase) binding free energies in the range of +1.1 to -3.9 kcal mol-1. Comparison of the experimental equilibrium constants for complexation of SnMe2 and SnPh2 with methanol and diethyl ether to those measured previously for the homologous silylenes and germylenes indicates that Lewis acidity decreases in the order SiR2 > SnR2 > GeR2 for both the dimethyl- and diphenyltetrylene series, the diphenyl derivatives exhibiting significantly stronger Lewis acidity in all three cases. The experimental trends in the binding energies and UV-vis spectra of the complexes are reproduced well by density functional theory (DFT) calculations, which have been carried out at the (TD)ωB97XD/def2-TZVP level of theory. The experimental data also show evidence of a reaction between tetramethyldistannene (Me2Sn[double bond, length as m-dash]SnMe2, 4a) and amine donors, which is suggested to afford the corresponding amine-stabilized stannylidenestannylene structure. The mechanistic proposal is supported by DFT calculations of the complexation of 4a and SnMe2 with model O-, S- and N-donors.

Functional chararacterization of the enzymes TabB and TabD involved in tabtoxin biosynthesis by Pseudomonas syringae

Pseudomonas syringae pv. tabaci ATCC 11528 produces tabtoxin, a β-lactam-containing dipeptide phytotoxin. Tabtoxinine-β-lactam (TβL), one of tabtoxin's constituent amino acids, structurally mimics lysine, and many of the proteins encoded by the tabtoxin biosynthetic gene cluster are homologs of lysine biosynthetic enzymes, suggesting that the tabtoxin and lysine biosynthetic routes parallel one another. We cloned and expressed TabB and TabD, predicted homologs of tetrahydrodipicolinate (THDPA)-N-acyltransferase and N-acyl-THDPA aminotransferase, respectively, to determine their activities in vitro. We confirmed that TabB succinylates THDPA and that TabD is a PLP-dependent aminotransferase that utilizes glutamate as an amine donor. Surprisingly, we also found that though TabD could utilize the TabB product N-succinyl-THDPA as a substrate, THDPA itself was also recognized. These observations reveal that TabB functionally duplicates DapD, the THDPA-N-succinyltransferase involved in lysine biosynthesis, and reinforce the close relationship between the metabolic logics underpinning the respective biosynthetic pathways.

Benchmarking lithium amide versus amine bonding by charge density and energy decomposition analysis arguments.

Lithium amides are versatile C-H metallation reagents with vast industrial demand because of their high basicity combined with their weak nucleophilicity, and they are applied in kilotons worldwide annually. The nuclearity of lithium amides, however, modifies and steers reactivity, region- and stereo-selectivity and product diversification in organic syntheses. In this regard, it is vital to understand Li-N bonding as it causes the aggregation of lithium amides to form cubes or ladders from the polar Li-N covalent metal amide bond along the ring stacking and laddering principle. Deaggregation, however, is more governed by the Li←N donor bond to form amine adducts. The geometry of the solid state structures already suggests that there is σ- and π-contribution to the covalent bond. To quantify the mutual influence, we investigated [{(Me2NCH2)2(C4H2N)}Li]2 (1) by means of experimental charge density calculations based on the quantum theory of atoms in molecules (QTAIM) and DFT calculations using energy decomposition analysis (EDA). This new approach allows for the grading of electrostatic Li+N-, covalent Li-N and donating Li←N bonding, and provides a way to modify traditional widely-used heuristic concepts such as the -I and +I inductive effects. The electron density ρ(r) and its second derivative, the Laplacian ∇2ρ(r), mirror the various types of bonding. Most remarkably, from the topological descriptors, there is no clear separation of the lithium amide bonds from the lithium amine donor bonds. The computed natural partial charges for lithium are only +0.58, indicating an optimal density supply from the four nitrogen atoms, while the Wiberg bond orders of about 0.14 au suggest very weak bonding. The interaction energy between the two pincer molecules, (C4H2N)22-, with the Li22+ moiety is very strong (ca. -628 kcal mol-1), followed by the bond dissociation energy (-420.9 kcal mol-1). Partitioning the interaction energy into the Pauli (ΔEPauli), dispersion (ΔEdisp), electrostatic (ΔEelstat) and orbital (ΔEorb) terms gives a 71-72% ionic and 25-26% covalent character of the Li-N bond, different to the old dichotomy of 95 to 5%. In this regard, there is much more potential to steer the reactivity with various substituents and donor solvents than has been anticipated so far.

Conversion of γ‐ and δ‐Keto Esters into Optically Active Lactams. Transaminases in Cascade Processes

AbstractA one‐pot two‐step enzymatic strategy has been designed for the production of optically active γ‐ and δ‐lactams in aqueous medium under mild conditions. The approach is based on the biotransamination of ethyl or methyl keto esters bearing different alkyl or aryl substitution patterns at α‐position to the ketone functionality. In this manner, the keto esters were transformed into the corresponding amino esters with excellent conversions, which underwent spontaneous cyclisation in the reaction medium without addition of external reagents. Depending on the transaminase selectivity, both lactam enantiomers can be obtained, so initial enzyme screenings were performed using commercially available and made in house enzymes. Reaction conditions were optimised focusing on the substrate concentration, temperature and ratio of amine donor vs acceptor. Thus, ten γ‐ and δ‐lactams were obtained in good to high isolated yields (70–90%) and excellent selectivities (94–99%) after one or two days at 30 or 45 °C.magnified image

Amine transaminases in chiral amines synthesis: recent advances and challenges.

Transaminases, which catalyze the stereoselective transfer of an amino group between an amino donor and a prochiral ketone substrate, are interesting biocatalytic tools for the generation of optically pure chiral amines. In particular, amine transaminases (ATAs) are of industrial interest because they are capable of performing reductive amination reactions using a broad range of amine donors and acceptors. The most remarkable example of ATAs industrial application is in the production process of the anti-hyperglycaemic drug sitagliptin (Januvia®/Janumet®), which generated around 6billion U.S. dollars of revenue to Merck in 2016. In this review, an update about the availability of microbial ATAs, discovered by both screening and database-mining approaches, or obtained by protein engineering of wild-type enzymes, will be provided. Current challenges in ATAs application and possible solutions will be also discussed. In particular, innovative biocatalytic process strategies aimed at the improvement of ATAs performances in chiral amines synthesis, e.g., using in situ product removal process strategies or flow reactors, will be presented. The progress in the industrial exploitation of these enzymes will be highlighted by selected examples of large-scale ATA-catalyzed processes.

Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts.

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LXZn catalysts (LX = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO2, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (Pm = 0.72-0.78) polylactide (PLA) with low dispersities: Đ = 1.06-1.17. In contrast to previous work with aluminum catalysts for which electronic effects of ligand substituents were significant (Hammett ρ = +1.2-1.4), the LXZn systems exhibited much less of an effect (ρ = +0.3). Density functional calculations revealed details of the initiation and propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.