Alpha-amino Acid Esters Research Articles

Recombinant alpha-amino ester acid hydrolase from Xanthomonas rubrilineans VKPM B-9915 is a highly efficient biocatalyst of cephalexin synthesis

Recombinant, as well as native alpha-amino acid ester hydrolase from Xanthomonas rubrilineans VKPM B-9915 (XrAEH, EC 3.1.1.43), was tested for synthesis of amino-beta-lactam antibiotic cephalexin. It was shown that the recombinant enzyme r-XrAEH produced by Escherichia coli VKPM B-11246 is more efficient in comparison with the native enzyme wt-XrAEH prepared from mutant strain Xanthomonas rubrilineans VKPM B-9915. When r-XrAEH was used as a biocatalyst, addition of ethylene glycol (33 vol %) to the reaction medium improved the yield from 70 to 95%. During synthesis of cephalexin under optimal conditions in the case of the native enzyme wt-XrAEH the cephalexin yield was 85%, in contrast to r-XrAEH where it was 95%. Furthermore, unlike native wt-XrAEH enzymes, preparations of recombinant r-XrAEH do not possess beta-lactamase side activity.

3D Structure Modeling of Alpha-Amino Acid Ester Hydrolase from Xanthomonas rubrilineans

Alpha-amino acid ester hydrolase (EC 3.1.1.43, AEH) is a promising biocatalyst for the production of semi-synthetic β-lactam antibiotics, penicillins and cephalosporins. The AEH gene from Xanthomonas rubrilineans (XrAEH) was recently cloned in this laboratory. The three-dimensional structure of XrAEH was simulated using the homology modeling method for rational design experiments. The analysis of the active site was performed, and its structure was specified. The key amino acid residues in the active site - the catalytic triad (Ser175, His341 and Asp308), oxyanion hole (Tyr83 and Tyr176), and carboxylate cluster (carboxylate groups of Asp209, Glu310 and Asp311) - were identified. It was shown that the optimal configuration of residues in the active site occurs with a negative net charge -1 in the carboxylate cluster. Docking of different substrates in the AEH active site was carried out, which allowed us to obtain structures of XrAEH complexes with the ampicillin, amoxicillin, cephalexin, D-phenylglycine, and 4-hydroxy-D-phenylglycine methyl ester. Modeling of XrAEH enzyme complexes with various substrates was used to show the structures for whose synthesis this enzyme will show the highest efficiency.

A New Binding Site Involving the C-terminal Domain to Design Specific Inhibitors of PepX

PepX is a X-prolyl dipeptidyl aminopeptidase of S15 family that cleaves dipeptides from the N-terminus of polypeptides having a proline or alanine residue at the second position. Involved in bacterial nutrition and in peptide maturation, this serine exopeptidase, counterpart of the mammalian DDP-4, has been proposed to play a role in pathogenicity for Streptococci and to be a promising target against trypanosomes. Searching for specific inhibitors, we undertook docking simulations on the whole surface of PepX from Lactococcus lactis, type example of the S15 family, which revealed a new putative binding site in connection with the active site and involving the C-terminal domain. Accordingly to the results of the computations, we synthesized two peptidomimetics of low molecular weight: the valinephenylpiperazine and the valine-isopropylpiperazine that can accommodate to this putative binding site. Experiments revealed that the valine-phenylpiperazine was an uncompetitive inhibitor whereas the valine-isopropylpiperazine showed to be an activator of the enzyme activity. The valine-phenylpiperazine is interacting with ASN 379, GLU 383, GLU 474, residues in connection with the specificity and active sites, and with the residues from the C-terminal domain LEU 693, GLU 710 and GLN 712. These results point out a role of the C-terminal domain in controlling access to the active site of enzymes of the S15 family, like PepX, the cocaine esterase or the alpha-amino acid ester hydrolase, and could have applications in human health giving new perspectives to struggle against streptococci or trypanosomes by designing inhibitors specific to the S15 family of enzymes.

Enantiomer separation of α-amino acid methyl esters and some aliphatic amines as 9-anthraldimine derivatives by high performance liquid chromatography

The liquid chromatographic enantiomer separation of several alpha-amino acid methyl esters as 9-anthraldehyde Schiff base derivatives on five polysaccharide-derived chiral stationary phases (CSPs) was described. The 9-anthraldehyde imine derivatives of a-amino acid esters were prepared by stirring 9-anthraldehyde with the alpha-amino acid ester hydrochloride salts. The chromatographic conditions were 3%-10% (v/v) 2-propanol-n-hexane as the mobile phases at the flow rate of 1.0 mL/min and detected at UV 254 nm. The performance of Chiralcel OD (or Chiralcel OD-H) among the five CSPs was superior to those of the other CSPs for the resolution of 9-anthraldimine derivatives of alpha-amino acid methyl esters. The enantiomers of all a-amino acid methyl esters as the 9-anthraldimine derivatives were base-line resolved on Chiralcel OD or Chiralcel OD-H (alpha = 1.24-5.47, Rs = 2.56-13.90). The L-enantiomers of all analytes were strongerly restrained than the respective D-enantiomers on the Chiralcel OD (or Chiralcel OD-H) CSPs. The method was also used for the enantiomer separation of aliphatic amines and good separation was obtained. It is expected that the analytical method will be useful for the enantiomer resolution of other alpha-amino acid esters and amine compounds as 9-anthraldimine derivatives.

First administration to man of Org 25435, an intravenous anaesthetic: A Phase 1 Clinical Trial

BackgroundOrg 25435 is a new water-soluble alpha-amino acid ester intravenous anaesthetic which proved satisfactory in animal studies. This study aimed to assess the safety, tolerability and efficacy of Org 25435 and to obtain preliminary pharmacodynamic and pharmacokinetic data.MethodsIn the Short Infusion study 8 healthy male volunteers received a 1 minute infusion of 0.25, 0.5, 1.0, or 2.0 mg/kg (n = 2 per group); a further 10 received 3.0 mg/kg (n = 5) or 4.0 mg/kg (n = 5). Following preliminary pharmacokinetic modelling 7 subjects received a titrated 30 minute Target Controlled Infusion (TCI), total dose 5.8-20 mg/kg.ResultsWithin the Short Infusion study, all subjects were successfully anaesthetised at 3 and 4 mg/kg. Within the TCI study 5 subjects were anaesthetised and 2 showed signs of sedation. Org 25435 caused hypotension and tachycardia at doses over 2 mg/kg. Recovery from anaesthesia after a 30 min administration of Org 25435 was slow (13.7 min). Pharmacokinetic modelling suggests that the context sensitive half-time of Org 25435 is slightly shorter than that of propofol in infusions up to 20 minutes but progressively longer thereafter.ConclusionsOrg 25435 is an effective intravenous anaesthetic in man at doses of 3 and 4 mg/kg given over 1 minute. Longer infusions can maintain anaesthesia but recovery is slow. Hypotension and tachycardia during anaesthesia and slow recovery of consciousness after cessation of drug administration suggest this compound has no advantages over currently available intravenous anaesthetics.

A Series of α-Amino Acid Ester Prodrugs of Camptothecin: In Vitro Hydrolysis and A549 Human Lung Carcinoma Cell Cytotoxicity

The objective of the present study was to identify a camptothecin (CPT) prodrug with optimal release and cytotoxicity properties for immobilization on a passively targeted microparticle delivery system. A series of alpha-amino acid ester prodrugs of CPT were synthesized, characterized, and evaluated. Four CPT prodrugs were synthesized with increasing aliphatic chain length (glycine (Gly) (2a), alanine (Ala) (2b), aminobutyric acid (Abu) (2c), and norvaline (Nva) (2d)). Prodrug reconversion was studied at pH 6.6, 7.0, and 7.4 corresponding to tumor, lung, and extracellular/physiological pH, respectively. Cytotoxicity was evaluated in A549 human lung carcinoma cells using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The hydrolytic reconversion rate to parent CPT increased with decreasing side chain length as well as increasing pH. The Hill slope of 2d was significantly less than CPT and the other prodrugs tested, indicating a higher cell death rate at lower concentrations. These results suggest that 2d is the best candidate for a passively targeted sustained release lung delivery system.

Racemic β-Sheets as Templates of Relevance to the Origin of Homochirality of Peptides: Lessons from Crystal Chemistry

The origin of life is a historical event that has left no relevant fossils; therefore, it is unrealistic to reconstruct the chronology of its occurrence. Instead, by performing laboratory experiments under conditions that resemble the prebiotic world, one might validate feasible reaction pathways and reconstruct model systems of artificial life. Creating such life in a test tube should go a long way toward removing the shroud of mystery over how it began naturally. The riddle of the appearance of natural proteins and nucleic acids--that is, biopolymers wholly consisting of homochiral subunits (L-amino acids and D-sugars, respectively)--from the unanimated racemic prebiotic world is still unsolved. There are two hypotheses concerning the sequence of their emergence: one maintains that long homochiral (isotactic) peptides must have been formed after the appearance of the first living systems, whereas the other presumes that such biopolymers preceded the primeval enzymes. The latter scenario necessitates, however, the operation of nonlinear synthetic routes, because the polymerization of racemates in ideal solutions yields chains composed of residues of either handedness. In this Account, we suggest applying lessons learned from crystal chemistry, in which molecules from isotropic media are converted into crystals with three-dimensional (3D) periodic order, to understand how the generation of homochiral peptides from racemic alpha-amino acids might be achieved, despite its seemingly overwhelming complexity. We describe systems that include the self-assembly of activated alpha-amino acids either in two-dimensional (2D) or in 3D crystals, followed by a partial lattice-controlled polymerization at the crystal-aqueous solution interface. We also discuss the polymerization of mixtures of activated hydrophobic racemic alpha-amino acids in aqueous solutions, as initiated by primary amines or thiols. The distribution of the diastereomeric oligopeptides was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MS/MS with monomers enantioselectively tagged with deuterium. The reaction performed in aqueous solutions encompasses the following sequential steps: (i) formation of a library of short racemic peptides enriched with isotactic diastereoisomers during the early stages of the polymerization, and (ii) self-assembly of oligopeptides into racemic beta-sheet colloidal-like aggregates that are delineated by enantiotopic sites or rims; these operate as templates (nuclei) for regio-enantioselective growth in the ensuing steps of chain elongation. Desymmetrization of the racemic mixtures of peptides was achieved with enantiopure alpha-amino acid esters as initiators. The enantiomeric excess of the isotactic peptides, not including the initiator, varies with chain length, the result of a cross-enantiomeric impeding mechanism. Our results suggest a feasible scenario in which primitive homochiral peptides might have emerged early in the prebiotic world.

Palladium-catalysed synthesis of 1-isoindolecarboxylic acid esters and sequential Diels–Alder reactions: access to bridged- and fused-ring heterocycles

The Pd-catalysed intramolecular alpha-arylation of alpha-amino acid esters provides a useful methodology for the synthesis of substituted isoindole derivatives, which have been used in Diels-Alder reactions to access diverse skeletal frameworks.

Ψ[CH(CF3)NH]Gly-peptides: synthesis and conformation analysis

Psi[CH(CF(3))NH]Gly peptides, a conceptually new class of peptidomimetics having a stereogenic trifluoroethylamine group as a natural peptide-bond surrogate, have been synthesized by stereoselective addition of alpha-amino acid esters to trans-3,3,3-trifluoro-1-nitropropene. Long range nuclear Overhauser effects, detected via ROESY experiments, provided evidence that model Psi[CH(CF(3))NH]Gly-tetrapeptides incorporating a trifluoroethylamine mimic of the dipeptide loop Pro-Gly can be represented by an ensemble of unfolded and folded conformations. The latter are driven by the formation of a hydrogen bond between a carbonyl group and the aminic proton of the trifluoroethylamine unit. MD calculations indicate a population of conformers which adopt folded beta turn structures for all the L-peptides; on the other hand, a D-stereochemistry at the Pro residue induces a natural folding towards a beta hairpin conformation driven by the formation of a second hydrogen bond, regardless of the stereochemistry of the stereogenic peptide bond surrogate.

Imidazole-based Potential Bi- and Tridentate Nitrogen Ligands: Synthesis, Characterization and Application in Asymmetric Catalysis

Twelve new imidazole-based potential bi- and tridentate ligands were synthesized and characterized. Whereas in the first series the α-amino acid and imidazole moieties were linked by an amino bond, in the second series the tridentate ligands, containing two imidazole groups, were separated by an amide bond. The first series was obtained by the reductive amination of 2-phenylimidazole-4-carboxaldehyde with α‑amino acid esters. The tridentate ligands were prepared from 2‑phenylimidazole-4-carboxylic acid and chiral amines. In the Henry reaction, the amines were revealed as a more reactive species than the less nucleophilic amides, however the enantiomeric excesses were generally poor.

Catalytic Hydrolysis of Esters of 2-Hydroxypyridine Derivatives for Cu2+ Detection

Hydrolysis of activated esters, e.g., picolinic acid and alpha-amino acid esters, in the presence of Cu(2+) salts is a stoichiometric process because products of this reaction bind the catalytic metal ion substantially stronger than starting materials do. Herein we report improved ester substrates, which are cleaved in the presence of catalytic amounts of Cu(2+); 55 turnovers of hydrolysis are observed for the best substrate, acetic acid 2-hydroxypyridine ester. We demonstrate that this reaction can be used for sensitive and selective detection of Cu(2+) by using both absorption and fluorescence spectroscopy.

New Entry for Synthesis of N-Acylhydrazones, Pyridazinones, and 1,3,4-Oxadiazin-6-ones from .ALPHA.-Amino Acid Esters

Versatile electrophiles N-acylhydrazones are synthesized via diazotization, reduction, and acylation of alpha-amino acid esters. Reduction of diazo esters with L-selectride or tributylphosphine affords the corresponding hydrazones in good yields. Both reducing agents give anti-hydrazones as the major product although the reactivity of each reductant is slightly different. The resulting hydrazones are acylated to give N-acylhydrazones, which are subjected to further reactions to give 1,3,4-oxadiazin-6-ones that serve as useful synthetic intermediates for the Diels-Alder reaction.

Extended application of a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid to the resolution of N‐(substituted benzoyl)‐α‐amino acid amides

A chiral stationary phase (CSP 1) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of N-(substituted benzoyl)-alpha-amino acid amides and esters. N-(Substituted benzoyl)-alpha-amino acid amides were well resolved using a mixture of acetic acid-triethylamine-acetonitrile (0.01:0.05:100, v/v/v) as an optimum mobile phase while N-(substituted benzoyl)-alpha-amino acid esters were not resolved at all. In contrast, both N-(substituted benzoyl)-alpha-amino acid amides and esters were not resolved at all or resolved very poorly on another CSP (CSP 2), which lacks the two N-H hydrogens of the amide tethers of CSP 1. Among the substituents on the benzoyl group of analytes, the nitro group was the best for good resolution of analytes on CSP 1. From these results, the two N-H hydrogens of the amide tethers of CSP 1, the carbonyl oxygen of the amide group of analytes, and the nitro group on the benzoyl group of analytes were concluded to play significant roles in chiral recognition. In addition, various N-(3,5-dinitrobenzoyl)leucine amides with different lengths of N-alkylamide chains were resolved on CSP 1 and N-(3,5-dinitrobenzoyl) leucine N-propylamide was found to show the best chiral recognition in terms of the separation (alpha = 1.30) and the resolution factor (Rs= 3.17).

Kinetics and Mechanisms of Activation of α-Amino Acid Ester Prodrugs of Camptothecins

The alpha-amino acid ester prodrugs of the antitumor agent camptothecin and a more potent, lipophilic silatecan analogue, DB-67, have been shown by NMR spectroscopy and quantitative kinetic analyses to undergo quantitative conversion to their pharmacologically active lactones via a nonenzymatic mechanism that at pH 7.4 is favored over direct hydrolysis. The alternate pathway involves the reversible intramolecular nucleophilic amine attack at the camptothecin E-ring carbonyl to generate a lactam (I) followed by a second intramolecular reaction to produce a bicyclic hemiortho ester (I'). The intermediates were isolated and shown to exist in an apparent equilibrium dominated by the hemiortho ester in DMSO using NMR spectroscopy. The conversion of prodrugs of camptothecin or DB-67 containing either alpha-NH(2) or alpha-NHCH(3) and their corresponding hemiortho esters were monitored versus time in aqueous buffer (pH 3.0 and 7.4) at 37 degrees C, and the kinetic data were fit to a model based on the proposed mechanism. The results indicated that while the prodrugs are relatively stable at pH 3, facile lactone release occurs from both the prodrugs and their corresponding hemiortho ester intermediates under physiological conditions (pH 7.4). The glycinate esters and their hemiortho esters were found to be more cytotoxic than the N-methylglycinates or their corresponding hemiortho ester intermediates in vitro using a human breast cancer cell line (MDA-MB-435S), consistent with their more rapid conversion to active lactone. The pH dependence of the nonenzymatic pathway for conversion of these alpha-amino acid ester prodrugs suggests that they may be useful for tumor-targeting via liposomes, as they can be stabilized in an acidic environment in the core of liposomes and readily convert to the active lactone following their intratumoral release.

Acetobacter turbidans α-Amino Acid Ester Hydrolase: HOW A SINGLE MUTATION IMPROVES AN ANTIBIOTIC-PRODUCING ENZYME

The alpha-amino acid ester hydrolase (AEH) from Acetobacter turbidans is a bacterial enzyme catalyzing the hydrolysis and synthesis of beta-lactam antibiotics. The crystal structures of the native enzyme, both unliganded and in complex with the hydrolysis product D-phenylglycine are reported, as well as the structures of an inactive mutant (S205A) complexed with the substrate ampicillin, and an active site mutant (Y206A) with an increased tendency to catalyze antibiotic production rather than hydrolysis. The structure of the native enzyme shows an acyl binding pocket, in which D-phenylglycine binds, and an additional space that is large enough to accommodate the beta-lactam moiety of an antibiotic. In the S205A mutant, ampicillin binds in this pocket in a non-productive manner, making extensive contacts with the side chain of Tyr(112), which also participates in oxyanion hole formation. In the Y206A mutant, the Tyr(112) side chain has moved with its hydroxyl group toward the catalytic serine. Because this changes the properties of the beta-lactam binding site, this could explain the increased beta-lactam transferase activity of this mutant.

One‐Step Synthesis of O‐Benzyl Hydroxamates from Unactivated Aliphatic and Aromatic Esters.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Simple, High-Yield Methods for the Synthesis of Aldehydes Directly from o-, m-, and p-Carborane and Their Further Conversions

Aldehydes have long served as important building blocks for synthetic chemists, and carboranyl aldehydes are no exception. Recent literature reports, for example, illustrate their application as intermediates in biomedicine, materials science, and basic organic chemistry. We report here new methods for the single-step preparation of C-monoformyl and C,C-diformyl derivatives directly from o-, m-, and p-carborane, as well as improved synthetic routes to homologated carboranyl aldehydes. Additionally, reductive amination is used to transform these aldehydes into a series of 2 degrees amines of alpha-amino acid esters.

Hydrogen-Mediated Reductive Coupling of Conjugated Alkynes with Ethyl (N-Sulfinyl)iminoacetates: Synthesis of Unnatural α-Amino Acids via Rhodium-Catalyzed C−C Bond Forming Hydrogenation

Rhodium-catalyzed hydrogenation of 1,3-enynes 1a-8a and 1,3-diynes 9a-13a at ambient temperature and pressure in the presence of ethyl (N-tert-butanesulfinyl)iminoacetate and ethyl (N-2,4,6-triisopropylbenzenesulfinyl)iminoacetates, respectively, results in reductive coupling to afford unsaturated alpha-amino acid esters 1b-13b in good to excellent yields with exceptional levels of regio- and stereocontrol. Further hydrogenation of the diene containing alpha-amino acid esters 1b-8b using Wilkinson's catalyst at ambient temperature and pressure results in regioselective reduction to afford the beta,gamma-unsaturated alpha-amino acid esters 1c-8c in good to excellent yields. Exhaustive hydrogenation of the unsaturated side chains of the Boc- and Fmoc-protected derivatives of enyne and diyne coupling products 14b-16b occurs in excellent yield using Crabtree's catalyst at ambient temperature and pressure providing the alpha-amino acid esters 14d-16d, which possess saturated side chains. Finally, cross-metathesis of the Boc-protected reductive coupling product 14b with cis-1,4-diacetoxy-2-butene proceeds readily to afford the allylic acetate 14e. Isotopic labeling studies that involve reductive coupling of enyne 1a and diyne 9a under an atmosphere of elemental deuterium corroborate a catalytic mechanism in which oxidative coupling of the alkyne and imine residues is followed by hydrogenolytic cleavage of the resulting metallacycle. A stereochemical model accounting for the observed sense of asymmetric induction is provided. These studies represent the first use of imines as electrophilic partners in hydrogen-mediated reductive carbon-carbon bond formation.

One-Step Synthesis of O-Benzyl Hydroxamates from Unactivated Aliphatic and Aromatic Esters

We have developed a simple and high yielding one-step method for the synthesis of hydroxamate derivatives directly from a broad range of unactivated esters and the anion of O-benzyl-hydroxylamine generated in situ. The reaction takes place in minutes at -78 degrees C. Very importantly, the method was successfully employed with enolizable esters, including chiral alpha-amino acid esters and peptides, with no trace of racemization/epimerization at the alpha carbon detected.

Wang-aldehyde resin as a recyclable support for the synthesis of α,α-disubstituted amino acid derivatives

Merrifield resin was functionalised with hydroxybenzaldehyde under microwave irradiation. The resultant resin was used as a means for immobilisation and activation of alpha-amino acid esters for alkylation reactions. Alpha,alpha-disubstituted and cyclic amino acid esters were prepared in good yields.