Bioinspired chemical inversion of L-amino acids to D-amino acids

Enantioselective Carbon Isotope Exchange

The synthesis of isotopically labeled organic molecules is vital for drug and agrochemical discovery and development. Carbon isotope exchange is emerging as a leading method to generate carbon-labeled targets, which are sought over hydrogen-based labels due to their enhanced stability in biological systems. While many bioactive small molecules bear carbon-containing stereocenters, direct enantioselective carbon isotope exchange reactions have not been established. We describe the first example of an enantioselective carbon isotope exchange reaction, where (radio)labeled α-amino acids can be generated from their unlabeled precursors using a stoichiometric chiral aldehyde receptor with isotopically labeled CO2 followed by imine hydrolysis. Many proteinogenic and non-natural derivatives undergo enantioselective labeling, including the late-stage radiolabeling of complex drug targets.

Remarkable enantioselectivity enhancement of the extractors with nonaxial chirality in liquid-liquid extraction of underivatized amino acids by introducing t-butyl group

A class of carbonyl extractors, (R)-3, (R)-4, and (R)-5, with nonaxial chirality containing asymmetric carbons has been synthesized and studied for their efficiencies in enantioselective liquid-liquid extraction for underivatized amino acids. The bulky t-butyl ketone extractors, (R)-4 and (R)-5, showed the stereoselectivities ranging 5.4-9.4 of l/d ratio much better than those of the aldehyde extractor, (R)-3, ranging 2.4-5.2. The imine formation rates and yields of the t-butyl ketones were not significantly affected by their bulkiness and even in the absence of resonance-assisted hydrogen bond. This work confirms that a bulky t-butyl ketone can be a good choice in the development of an extractor not only with axial chirality but also with nonaxial chirality for the enantioselective extraction of unprotected amino acids.

A selective electrochemical chiral interface based on a carboxymethyl-β-cyclodextrin/Pd@Au nanoparticles/3D reduced graphene oxide nanocomposite for tyrosine enantiomer recognition

Palladium@gold nanoparticle modified three-dimensional-reduced graphene oxide was coupled with carboxymethyl-β-cyclodextrin to form a novel nanocomposite, which served as an effective chiral sensing interface for electrochemical enantiorecognition.

Bioactive Plasma Coatings on Orthodontic Brackets: In Vitro Metal Ion Release and Cytotoxicity

The metal ion release characteristics and biocompatibility of meta-based materials are key factors that influence their use in orthodontics. Although stainless steel-based alloys have gained much interest and use due to their mechanical properties and cost, they are prone to localised attack after prolonged exposure to the hostile oral environment. Metal ions may induce cellular toxicity at high dosages. To circumvent these issues, orthodontic brackets were coated with a functional nano-thin layer of plasma polymer and further immobilised with enantiomers of tryptophan. Analysis of the physicochemical properties confirmed the presence of functional coatings on the surface of the brackets. The quantification of metal ion release using mass spectrometry proved that plasma functionalisation could minimise metal ion release from orthodontic brackets. Furthermore, the biocompatibility of the brackets has been improved after functionalisation. These findings demonstrate that plasma polymer facilitated surface functionalisation of orthodontic brackets is a promising approach to reducing metal toxicity without impacting their bulk properties.

Enantioselective extraction of unprotected amino acids coupled with racemization

Scalable and economical methods for the production of optically pure amino acids, both natural and unnatural, are essential for their use as synthetic building blocks. Currently, enzymatic dynamic kinetic resolution (DKR) underpins some of the most effective processes. Here we report the development of enantioselective extraction coupled with racemization (EECR) for the chirality conversion of underivatized amino acids. In this process, the catalytic racemization of amino acids in a basic aqueous solution is coupled with the selective extraction of one enantiomer into an organic layer. Back-extraction from the organic layer to an acidic aqueous solution then completes the deracemization of the amino acid. The automation of the EECR process in a recycling flow reactor is also demonstrated. Continuous EECR is made possible by the sterically hindered chiral ketone extractant 5, which prevents the coextraction of the copper racemization catalyst because of its nonplanar geometry. Furthermore, the extractant 5 unexpectedly forms imines with amino acids faster and with greater enantioselectivity than less bulky derivatives, even though 5 cannot participate in intramolecular resonance-assisted hydrogen bonding. These features may allow EECR to challenge the preponderance of enzymatic DKR in the production of enantiomerically enriched amino acids.

Dynamic kinetic resolution is a common approach for the preparation of optically pure amino acids using enzymes. Here the authors report an alternative method based on enantioselective extraction coupled with racemization, in which a bulky extractant affords hydrophobic extractable imines with amino acids rapidly, reversibly and enantioselectively.

Molecular chirality mediated amyloid formation on phospholipid surfaces

One of the neuropathological features of Alzheimer's disease (AD) is the misfolding of amyloid-β to form amyloid aggregates, a process highly associated with biological membranes. However, how molecular chirality affects the amyloid formation on phospholipid surfaces has seldom been reported. Here, l- and d-aspartic acid-modified 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (l-/d-Asp–DPPE) is synthesized to construct chiral phospholipid bilayers. We discover that the l-Asp–DPPE liposomes slightly inhibit the Aβ(1–40) nucleation process but cannot affect the oligomer elongation process. By contrast, the d-Asp–DPPE liposomes strongly inhibit both nucleation and elongation of the peptide. Notably, l- and d-Asp–DPPE liposomes not only have good biocompatibility but can also rescue Aβ(1–40)-aggregation induced cytotoxicity with significant chiral discrimination, in which the cell viability is higher in the presence of d-Asp–DPPE liposomes. Mechanism analysis and molecular dynamics simulation clearly demonstrate that differential electrostatic interactions of Lys16 in Aβ(1–40) with l- or d-Asp on the phospholipid contribute to the remarkable chiral discrimination. This study provides a deeper understanding of the crucial amyloidosis process from the perspective of the chiral interface and reveals that the convergence of d-amino acids with the liposomes might be a feasible route for AD prevention.

A remarkable inhibition effect and chiral discrimination are observed when the amyloid peptide aggregates on chiral phospholipid surfaces.

Hydrogen Bond Assisted l to d Conversion of α-Amino Acids

l to d conversion of unactivated α-amino acids was achieved by solubility-induced diastereomer transformation (SIDT). Ternary complexes of an α-amino acid with 3,5-dichlorosalicylaldehyde and a chiral guanidine (derived from corresponding chiral vicinal diamine) were obtained in good yield as diastereomerically pure imino acid salt complexes and were hydrolysed to obtain enantiopure α-amino acids. A combination of DFT computation, NMR spectroscopy, and crystal structure provide detailed insight into how two types of strong hydrogen bonds assist in rapid epimerization of the complexes that is essential for SIDT.

Recyclable and Stable α-Methylproline-Derived Chiral Ligands for the Chemical Dynamic Kinetic Resolution of free C,N-Unprotected α-Amino Acids

A novel special designed, stable, and recyclable chiral ligand bearing a quaternary carbon was developed for chemical dynamic kinetic resolution (DKR) of free C,N-unprotected racemic α-amino acids via Schiff base intermediates. This method furnishes high yields with excellent enantioselectivity, has a broad substrate scope, and uses operationally simple and convenient conditions. The present chemical DKR is a practical and useful method for the preparation of enantiopure α-amino acids.

Chiral Receptors for Lysine Based on Covalently Linked Bis- and Tris-binaphthylphosphoric Acids

The synthesis and application of three chiral receptors based on the covalent linkage of 1,1'-binaphthylphosphoric acids is reported. The binding of the lysine enantiomers to the chiral receptors was investigated by DOSY-NMR and NMR titrations, revealing that the bisphosphoric acid 1d acts as a highly stereoselective receptor for binding of d-lysine.

Resonance-Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

Resonance-assisted hydrogen bonding (RAHB), a concept introduced by Gilli and co-workers in 1989, concerns a kind of intramolecular H-bonding strengthened by a conjugated π-system, usually in 6-, 8-, or 10-membered rings. This Review highlights the involvement of RAHB as a driving force in the synthesis of organic, coordination, and organometallic compounds, as a handy tool in the activation of covalent bonds, and in starting moieties for synthetic transformations. The unique roles of RAHB in molecular recognition and switches, E/Z isomeric resolution, racemization and epimerization of amino acids and chiral amino alcohols, solvatochromism, liquid-crystalline compounds, and in synthons for crystal engineering and polymer materials are also discussed. The Review can provide practical guidance for synthetic chemists that are interested in exploring and further developing RAHB-assisted synthesis and design of materials.

Catalytic Stereoinversion of L-Alanine to Deuterated D-Alanine

A combination of an achiral pyridoxal analogue and a chiral base has been developed for catalytic deuteration of L-alanine with inversion of stereochemistry to give deuterated D-alanine under mild conditions (neutral pD and 25 °C) without the use of any protecting groups. This system can also be used for catalytic deuteration of D-alanine with retention of stereochemistry to give deuterated D-alanine. Thus a racemic mixture of alanine can be catalytically deuterated to give an enantiomeric excess of deuterated D-alanine. While catalytic deracemization of alanine is forbidden by the second law of thermodynamics, this system can be used for catalytic deracemization of alanine with deuteration. Such green and biomimetic approach to catalytic stereocontrol provides insights into efficient amino acid transformations.

Chemical dynamic kinetic resolution and S/R interconversion of unprotected α-amino acids

Reported herein is the first purely chemical method for the dynamic kinetic resolution (DKR) of unprotected racemic α-amino acids (α-AAs), a method which can rival the economic efficiency of the enzymatic reactions. The DKR reaction principle can be readily applied for S/R interconversions of α-AAs, the methodological versatility of which is unmatched by biocatalytic approaches. The presented process features a virtually complete stereochemical outcome, fully recyclable source of chirality, and operationally simple and convenient reaction conditions, thus allowing its ready scalability. A quite unique and novel mode of the thermodynamic control over the stereochemical outcome, including an exciting interplay between axial, helical, and central elements of chirality is proposed.

Synthesis and stereochemical assignments of diastereomeric Ni(II) complexes of glycine Schiff base with (R)-2-(N-{2-[N-alkyl-N-(1-phenylethyl)amino]acetyl}amino)benzophenone; a case of configurationally stable stereogenic nitrogen

A family of chiral ligands derived from α-phenylethylamine and 2-aminobenzophenone were prepared by alkylation of the nitrogen atom. Upon reaction with glycine and a Ni(II) salt, these ligands were transformed into diastereomeric complexes, as a result of the configurational stability of the stereogenic nitrogen atom. Different diastereomeric ratios were observed depending on the substituent R introduced in the starting ligand, and stereochemical assignments were based on X-ray analysis, along with NMR studies and optical rotation measurements.

Amino acid and water-driven tunable green protocol to access S–S/C–S bonds via aerobic oxidative coupling and hydrothiolation

Arginine in conjunction with water has been employed as an effective and recyclable organocatalyst for oxidative coupling of thiophenols and hydrothiolation of alkynes.

Zn(ii) promoted dramatic enhancement in the enantioselective fluorescent recognition of functional chiral amines by a chiral aldehyde

The addition of Zn²⁺ to the methanol solution of a BINOL-based chiral dialdehyde leads to dramatically enhanced enantioselective fluorescent responses toward various functional chiral amines, representing one of the first chiral aldehydes capable of such recognition.