Chiral Sensing Platform Based on the Self-Assemblies of Diphenylalanine and Oxalic Acid

Enantioselective Target Transport-Mediated Nanozyme Decomposition for the Identification of Reducing Enantiomers in Asymmetric Nanochannel Arrays

Enantioselective identification of chiral molecules is regarded as one of the key issues in biological and medical sciences because of their configuration-dependent effects on biological systems. In this study, we developed an electrochemical platform based on a tandem recognition-reaction zone design in TiO2 nanochannels for the specific recognition of reducing enantiomers. In this system, MIL-125(Ti) Ti-metal-organic frameworks, in situ grown in TiO2 nanochannels, provided a homochiral recognition environment via postmodification with l-tartaric acid (l-TA); MnO2 nanosheets possessing both glucose oxidase (GOD)- and peroxidase (POD)-mimicking activities served as the target-reactive zone at the end of the nanochannels. The use of penicillamine (Pen) enantiomers as model-reducing targets facilitated the passage of d-Pen through the homochiral recognition zone, owing to its lower affinity with l-TA. The passed Pen molecules reached the responsive zone and induced a target concentration-dependent MnO2 disassembly. Such target recognition event impaired the cascade GOD- and POD-like activities of MnO2. Combining the enantioselectivity of the recognition nanochannels with the cascade enzyme-like activity of MnO2 toward glucose and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate), the quantitative identification of l- and d-Pen was achieved through the changes in transmembrane ionic current induced by the generated charged products. This recognition-reaction zone design paves an effective way for developing a promising electrochemical platform for the identification of reducing enantiomers with improved selectivity and sensitivity.

Chiral Materials: Progress, Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the "codebook" of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l-amino acids and d-sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.

Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal-Organic Frameworks

Chiral materials capable of electrochemical enantiomeric recognition are highly desirable for many applications, but it is still very challenging to achieve high recognition efficiency for lack of the knowledge of structure-property relationships. Here, we report the completely distinct enantiomeric recognition related to slightly different achiral colinkers in isomorphic homochiral metal-organic frameworks with the same chiral linker. Cu-TBPBe, for which the achiral colinker has two pyridyl rings connected by ─CH═CH─, shows excellent enantioselectivity and sensitivity for electrochemical recognition of l-tryptophan (Trp) with a detection limit of 3.16 nM. The l-to-d ratio of differential pulse voltammetric (DPV) currents reaches 53, which is much higher than the values (2-14) reported for previous electrochemical sensors. By contrast, Cu-TBPBa, in which the achiral colinker has -CH₂-CH₂- between pyridyl rings, is incapable of discrimination between l-Trp and d-Trp. Structural and spectral analyses suggest that the achiral conjugated colinker and the chiral moieties around it cooperate to produce a chiral pocket in favor of enantioselective adsorption through multiple hydrogen-bonding and π-π stacking interactions. The work demonstrated that Cu-TBPBe can be used to fabricate reliable electrochemical sensors for ultrasensitive quantification of Trp enantiomers in racemic mixtures and in complex biological systems such as urine. The work also highlights that an achiral coligand can be of vital importance in determining enantiomeric discrimination, opening up a new avenue for the design of chiral sensing materials.

Chiral recognition of tryptophan enantiomer based on the electrode modified by polyaniline adsorption bovine serum albumin complex

A chiral sensing platform was constructed via adsorptive functionalization of ammonium persulfate doped polyaniline (APS-DPANI) with bovine serum albumin (BSA). The novelty of this work is the construction of such chiral interface with adsorption principle. The material has been characterized by scanning electron microscope, Fourier transform infrared and X-ray photoelectron spectroscopy, and thermogravimetric and water contact angle analyses. It displayed considerable stability in multi-run cyclic voltammetric scanning. Moreover, the superior conductivity of APS-DPANI and the decent binding ability of BSA endowed this sensing platform with an excellent recognition effect for tryptophan (Trp) enantiomers in the differential pulse voltammetry (DPV) test. The recognition was highly reproducible, and the detection limits for L- and D-isomer were 0.071 and 0.0478 mM, respectively.

Importance of Solvent-Bridged Structures of Fluorinated Diphenylalanines: Synthesis, Detailed NMR Analysis, and Rotational Profiles of Phe(2-F)-Phe(2-F), Phe(2-F)-Phe, and Phe-Phe(2-F)

The crystal structure of l-phenylalanyl l-phenylalanine (Phe-Phe, FF, a.k.a. diphenylalanine) is not merely noncentrosymmetric, but it is highly dipole parallel aligned. It is for this reason that FF is a nonlinear optical (NLO) material and exhibits strong second harmonic generation (SHG). Enhancement of the SHG response by ortho fluorination was demonstrated. Crystallization is nontrivial, and learning about the zwitterion structures in solution is important for the rational improvement of the crystallization process. Here, we present an NMR study of di-fluorinated FF (Phe(2-F)-Phe(2-F)) and mono-fluorinated FF isomers (Phe(2-F)-Phe and Phe-Phe(2-F)). The dipeptides were prepared by solid-phase synthesis and purified by high-performance liquid chromatography (HPLC). Their ¹H and ¹³C NMR spectra were recorded in partially deuterated water (10% D₂O), and two-dimensional (2D) NMR techniques were employed for signal assignments. The unambiguous assignments are reported of all chemical shifts for the aliphatic H and C atoms and of the C atoms of the carboxylate, the amide carbonyl, the CF carbons, and of every arene C atom in each phenyl ring. The dipeptides are trans amides and intramolecular hydrogen bonding between the ammonium group and the amide carbonyl restricts the H₃N-CH-C(O) geometry. We explored the rotational profile of the diphenylalanines as a function of the τ = ∠(C-N-C-CO₂) dihedral angle at the SMD(B3LYP/6-31G*) level without and with specific hydration and report the associated Karplus curves J(θ) vs θ = ∠(H-N-C-H). The rotational profiles show a maximum of three stationary structures, and relative conformer stabilities of the free diphenylalanines show that the conformation found in the crystal M1 is the least stable among the three, M3 > M2 ≫ M1. Specific water solvation makes all of the difference and adds a large competitive advantage to the water-bridged ion pair M1a. In fact, M1a becomes the most stable and dominant conformation for the parent diphenylalanine and mono1 F-FF and M1a becomes competitive with M3c for mono2 F-FF and di F-FF. Implications are discussed regarding the importance of the conformational preorganization of diphenylalanines in solution and the facility for their crystallization.

TiO2 Nanotubes Decorated with CdSe Quantum Dots: A Bifunctional Electrochemiluminescent Platform for Chiral Discrimination and Chiral Sensing

Chiral analysis is of significant importance for living organisms since chirality is the fundamental phenomenon in nature. In this work, a bifunctional electrochemiluminescent (ECL) platform is constructed for chiral discrimination and chiral sensing. 3-Mercaptopropionic acid-functionalized CdSe quantum dots (CdSe QDs) are combined with aminated TiO₂ nanotubes (NH₂-TiNTs) via amidation. The resultant CdSe QDs/TiNTs display significantly enhanced ECL signals due to the synergistic effect between CdSe QDs and TiNTs, which are then used for the chiral discrimination of the isomers of nine chiral amino acids (AAs) in the presence of d-AA oxidase (DAAO). DAAO can selectively catalyze the oxidation of d-AAs to generate H₂O₂, which acts as the coreaction reagent and triggers the ECL signals of CdSe QDs/TiNTs, and thus, the isomers of the nine chiral AAs can be effectively discriminated. In addition, the as-constructed ECL platform can also be used for the sensitive detection of d-AAs in the presence of DAAO with a wide linear range and a low limit of detection. These findings suggest that the CdSe QDs/TiNTs can work as a bifunctional ECL platform (chiral discrimination and chiral sensing), which might be an advanced ECL platform for biomedical applications.

A Bio-inspired Extended-Gate Metal-Oxide-Semiconductor Field-Effect-Transistor for Highly Sensitive Amino Acid Enantiodiscrimination

As the most important small molecules revealing the origins of life, amino acids (AAs) play essential roles in living organisms and their facile enantiodiscrimination has long been a great challenge for analytical chemists. Inspired by the specific stereomatching effect between biomolecules and AA enantiomers, herein, we first developed a bio-inspired highly sensitive platform based on an extended-gate metal-oxide-semiconductor field-effect-transistor (EG-MOSFET) for highly sensitive AA enantiodiscrimination. Bovine serum albumin (BSA) was self-assembled on deposited Au surfaces to afford the extended gate (EG) sensing unit, and its enantiorecognition ability was initially verified using common electrochemical techniques. The EG was thereafter installed to a MOSFET to build the desired BSA-EG-MOSFET highly sensitive chiral sensing platform, which realized the efficient enantiodiscrimination of essential AAs with high sensitivity, where effective chiral resolution was achieved at the femtomole level to phenylalanine (Phe). Combining molecular docking and circular dichroism spectroscopy, the weak intermolecular interactions between BSA and AAs enantiomers were investigated and the mechanism for signal amplification was proposed. Our results demonstrate that the as-fabricated biosensor has great potential in highly sensitive chiral sensing fields and can also afford a potential tool for biomolecular interaction investigations.

Electrochemiluminescent chiral discrimination with chiral Ag2S quantum dots/few-layer carbon nitride nanosheets

Well-dispersed chiral Ag₂S quantum dots (Ag₂S QDs) were facilely synthesized by using N-acetyl-L-cysteine (NALC) as the chiral ligand and loaded onto nanosheets of two-dimensional (2D) few-layer carbon nitride (C₃N₄). The resultant nanocomposite (Ag₂S QDs/few-layer C₃N₄) shows enhanced electrochemiluminescence (ECL) while maintaining the chirality of Ag₂S QDs, which can be used for the chiral discrimination of the enantiomers of tyrosine (Tyr). Due to the higher affinity of chiral Ag₂S QDs toward L-Tyr than toward its enantiomer, the ECL intensity of Ag₂S QDs/few-layer C₃N₄ is significantly decreased after its incubation with L-Tyr, and thus the Tyr enantiomers can be discriminated. The developed ECL chiral sensor exhibits high stability and reproducibility. The universality of the ECL chiral sensor for the discrimination of other chiral amino acids is also studied, and the results indicate that it can work only in the case of chiral aromatic amino acids.

Self-Assembly of Cyclic Dipeptides: Platforms for Functional Materials

Supramolecular self-assembled functional materials comprised of cyclic dipeptide building blocks have excellent prospects for biotechnology applications due to their exceptional structural rigidity, morphological flexibility, ease of preparation and modification. Although the pharmacological uses of many natural cyclic dipeptides have been studied in detail, relatively little is reported on the engineering of these supramolecular architectures for the fabrication of functional materials. In this review, we discuss the progress in the design, synthesis, and characterization of cyclic dipeptide supramolecular nanomaterials over the past few decades, highlighting applications in biotechnology and optoelectronics engineering.

Enantioselective Recognition of Chiral Tryptophan with Achiral Glycine through the Strategy of Chirality Transfer

Glycine (Gly), an achiral amino acid, has never been reported for enantioselective recognition owing to the absence of chiral sites. Herein, a facile strategy of chirality transfer is proposed to endow Gly with chirality. Optically active CuO, L-CuO, is first prepared, which can be used for the decoration of Gly through the formation of the Cu(Gly)₂ complex. Successful chirality transfer from L-CuO to Gly is confirmed by circular dichroism (CD) spectra. The formation of the Cu(Gly)₂ complex is further confirmed by Fourier transform infrared spectra and X-ray photoelectron spectroscopy. Next, the resultant L-CuO-Gly is used for chiral analysis of the isomers of tryptophan (Trp). Because of the higher affinity of L-CuO-Gly toward L-Trp than its isomer, the Trp isomers exhibit significant differences in their oxidation peak currents at the L-CuO-Gly-modified glassy carbon electrode (GCE) (I_L-Trp/I_D-Trp = 5.24). Finally, the practicability of the developed L-CuO-Gly/GCE is assessed, and the results indicate that it could be a reliable chiral sensor for the quantitative analysis of Trp isomers in nonracemic mixtures.

MOF-Based Supramolecule Helical Nanomaterials: Toward Highly Enantioselective Electrochemical Recognition of Penicillamine

For the first time, we report the formation of a chiral MOF-based helical nanomaterial (h-HDGA@ZIF-67) through arranging zeolitic imidazolate framework (ZIF-67) nanocrystals on helical l-glutamic acid terminated bolaamphiphile (h-HDGA) via a facile process at room temperature. The self-assembly leads to the chiral function of the ZIF-67 from an achiral ligand. The h-HDGA@ZIF-67 served as a new type of electrochemical sensing interface for recognizing and quantifying Pen enantiomers that realize significant enantioselectivity, satisfactory stability, and reproducibility. The synergetic effect from ZIF-67 nanocrystals on h-HDGA and stereoselectivity of h-HDGA@ZIF-67 lead to the excellent enantioselectivity. The present strategy showed the first example of a chiral MOF-based supramolecule helical nanomaterial, presenting high enantioselectivity for electrochemical enantiomeric determination.

Single-Template Molecularly Imprinted Chiral Sensor for Simultaneous Recognition of Alanine and Tyrosine Enantiomers

Chiral recognition of l-amino acids is of significant importance due to the crucial role of l-amino acids in life sciences and pharmaceutics. In this work, a chiral sensor with capability of probing two chiral amino acids by an attractive single-template molecular imprinting strategy is introduced and used in the simultaneous chiral recognition of d/l-alanine (d/l-Ala) and d/l-tyrosine (d/l-Tyr). The assay relies on the hydrolysis of l-alanyl-l-tyrosine dipeptide doped in silica/polypyrrole (SiO₂/PPy) under acidic conditions, resulting in l-Ala and l-Tyr coimprinted chiral sensor. This work opens up a new avenue for simultaneous chiral sensing of two or more chiral amino acids by incorporating only one template, circumventing the shortcomings encountered with multitemplate molecularly imprinted technology.

Covalent Functionalization of Bovine Serum Albumin with Graphene Quantum Dots for Stereospecific Molecular Recognition

Stereospecific molecular recognition with simple and easily available proteins is of significant importance in life science and biomaterial science. Herein, we report on a chiral sensing platform, graphene quantum dots (GQDs)-functionalized bovine serum albumin (BSA), for chiral recognition of tryptophan (Trp) isomers. Amidation reaction between BSA and GQDs was directly responsible for the introduction of GQDs to BSA, resulting in significant changes in the spatial configuration of BSA and the exposure of more chiral sites at the protein surface. The BSA-GQDs-based chiral sensor exhibited good biomolecular homochirality in the recognition of Trp isomers, and the higher affinity of BSA-GQDs toward l-Trp than its isomer, d-Trp, was also revealed by density functional theory (DFT) considering the possible hydrogen bonds between the Trp isomers and the solvent-accessible residues of BSA.

Phe-Phe Di-Peptide Nanostructure Self-Assembling Modulated by Luminescent Additives

In this work, supramolecular l-l-diphenylalanine (Phe-Phe) nanostructures were self-assembled in solvents of distinct polarity and in the presence of luminescent additives of distinct conjugation length that physically adhere to the nanostructures to provide growth environments of distinct properties. When the additive is poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], an electron donor polymer, and solvent is tetrahydrofuran (THF), Phe-Phe vesicle-like structures are obtained, whereas in water and in the presence of a similar additive in structure, poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene], nanotubes are formed. In contrast, when 9-vinyl-carbazole, an electron acceptor additive is used, nanotubes are formed even when THF is the solvent. The same structures are obtained when the additive is the macromolecule poly(vinyl carbazole). The morphologies of these self-assembled structures were observed by scanning electron microscopy, and their photophysical behavior was determined by steady-state fluorescence spectroscopy and time-resolved fluorescence spectroscopy. These data analyzed altogether inform about the formation mechanisms of such structures and about the influence that distinct interactions exert on self-assembling and charge-transfer processes through formation of complexes between the luminescent additives and the Phe-Phe nano- and microstructures.

Hydroxypropyl β-cyclodextrin cross-linked multiwalled carbon nanotube-based chiral nanocomposite electrochemical sensors for the discrimination of multichiral drug atorvastatin isomers

Drugs having multiple chiral centres pose a greater risk to the human health as their pharmacological effects on human organs, cells and systems due to more number of enantiomers as compared to that of a single enantiomeric drug.