BINOl-Based Chiral Receptors as Fluorescent and Colorimetric Chemosensors for Amino Acids

Amino Acid-Based Molecular and Membranous Chiral Tools for Enantiomeric Recognition

Given the need, both academic and industrial, for new approaches and technologies for chiral discrimination of enantiomers, the present work demonstrates the development through rational design and integration of two new chiral platforms (molecular and membranous) for enantioselective recognition through visual as well as microscopic observation. The molecular platform (TPT) is based on the tryptophan derivative developed through the condensation of two tryptophan units with terepthaloyl chloride. While TPT based on l-tryptophan recognizes R-mandelic acid over the S-isomer, the host with reverse chirality (TPDT) recognizes S-mandelic acid over R-isomer. The role of chemical functionality in this sensitive recognition process was established experimentally by developing an analogue of TPT and by judiciously using different chiral analytes. Importantly, a detailed theoretical study at the molecular level revealed the U-shaped conformation of TPT, creating a cavity for accommodating a chiral guest with selective functional interaction resulting in the discrimination of enantiomers. Finally, a chiral polymeric mat of poly(methyl methacrylate) (PMMA)/polyacrylonitrile (PAN) (2:3) impregnated with TPT was developed via electrospinning. The resulting fibrous mat was successfully utilized for chiral recognition through microscopic and architectural observation. Hence, the present work reports simple chiral tools for enantiomeric recognition.

Chiral gelators for visual enantiomeric recognition

Introduction of chirality in supramolecular gels has allowed the effective translation and amplification of molecular chirality. Upon integrating the stimuli-responsive nature of these gels with supramolecular chirality, a new platform for the discrimination of the enantiomeric guests through naked eye can be developed. Over the past decade, several groups have reported the development of chiral supramolecular gels for enantioselective recognition through gel formation or collapse. However, to the best of our knowledge, we are yet to come across a review highlighting the utilization of chiral supramolecular gels for macroscopic discrimination of enantiomers. In this article, we have articulated the chiral gelators developed to date for the recognition of different enantiomeric analytes focusing on their mode of recognition with an in-depth analysis of the mechanism of interactions assisting the recognition process.

Recognition of Chiral Carboxylates by Synthetic Receptors

Recognition of anionic species plays a fundamental role in many essential chemical, biological, and environmental processes. Numerous monographs and review papers on molecular recognition of anions by synthetic receptors reflect the continuing and growing interest in this area of supramolecular chemistry. However, despite the enormous progress made over the last 20 years in the design of these molecules, the design of receptors for chiral anions is much less developed. Chiral recognition is one of the most subtle types of selectivity, and it requires very precise spatial organization of the receptor framework. At the same time, this phenomenon commonly occurs in many processes present in nature, often being their fundamental step. For these reasons, research directed toward understanding the chiral anion recognition phenomenon may lead to the identification of structural patterns that enable increasingly efficient receptor design. In this review, we present the recent progress made in the area of synthetic receptors for biologically relevant chiral carboxylates.

A novel Schiff base macrocycle based on 1,1'-binaphthyl for fluorescence recognition

A novel chiral polyimine macrocycle C-1 was designed and synthesized by the self-condensation of the dialdehyde of the chiral dinaphtho[2,1-d:1',2'-f][1,3]dioxepine derivative and o-phenylenediamine by Schiff base formation, and the corresponding polyamine macrocycle C-1H was obtained by the reduction of the polyimine macrocycle. The UV-vis and fluorescence spectral studies indicated that both C-1 and C-1H form the complex with metal ions in a 1:2 ratio. The fluorescence behaviour of C-1 upon the addition of Zn²⁺ or Cd²⁺ showed a 'turn-on' response accompanied by fluorescence enhancement at 510 nm six times for Cd²⁺ and 13 times for Zn²⁺ . In contrast, C-1H revealed a 'turn-off' response upon the addition of Co²⁺ , Ni²⁺ , and Cu²⁺ .

Quinoline based reversible fluorescent probe for Pb2+; applications in milk, bioimaging and INHIBIT molecular logic gate

We report the modular design and synthesis of an amine dangled Schiff base quinoline-morpholine conjugate (QMC) for highly selective detection of Pb²⁺ ions via fluorimetry. The sensing strategy of QMC towards Pb²⁺ ion exhibits a large blue shift with fluorescent enhancement via the intramolecular charge transfer (ICT) process. At the same time, QMC coordination with Pb²⁺, the CN single bond rotation between quinoline and morpholine rings and the CN isomerization process were blocked. Best of our knowledge, this is the first blue shifted turn-on fluorescent chemosensor for Pb²⁺ ion via the ICT process. Furthermore, QMC selectively detects Pb²⁺ ion without any interference with alkali, alkaline earth, and transition metal ions, and limit of detection (LOD) downs to 13 μM, which is a permissible level of Pb²⁺ ion in drinking water reported by WHO. The 1:2 binding stoichiometry between QMC and Pb²⁺ was confirmed by fluorimetric, ¹H NMR titration, mass spectrometry, and theoretical studies. Finally, QMC was potentially applied for the sensing of Pb²⁺ ions in milk, red wine, live cells and an INHIBIT molecular logic function was constructed by using Pb²⁺ and EDTA as chemical inputs.

Optical sensing of anions by macrocyclic and interlocked hosts

This review summarises recent developments in the use of macrocyclic and mechanically-interlocked host molecules as optical sensors for anions.

Recent advances of BINOL-based sensors for enantioselective fluorescence recognition

Enantioselective fluorescent sensors show large potential for fast, real-time, and highly sensitive measurement of the concentration and enantiomeric composition of chiral molecules. Among all of the sensors, BINOL-based sensors have been actively investigated and extensively used to carry out highly enantioselective, sensitive recognition of chiral α-hydroxycarboxylic acids, amino acids, amino acid derivatives, amino alcohols and amines. In this manuscript, the recent progress of chiral BINOL-based sensors for enantioselective fluorescence recognition of different substrates is reviewed and discussed. The structure of BINOL is tuned by introducing various groups or molecules which systematically changed its fluorescence properties and offered potential for rapid assays of chiral organic molecules. From the development of this area, we gain fresh insight into the challenges and chances of BINOL-based sensors.

Pyrene-phenylglycinol linked reversible ratiometric fluorescent chemosensor for the detection of aluminium in nanomolar range and its bio-imaging

Pyrene-phenylglycinol tangled ratiometric sensor (R)-1 was developed for the detection of Al³⁺ ion over other metal ions. Ratiometric behaviour of (R)-1 for Al³⁺ ion explained through monomer emission and excimer quenching leads to avoiding the π-π interactions of bis-pyrene rings. Pull-push to push-pull binding mechanism is successfully explained by DFT and sensing of Al³⁺-ions demonstrated in living cells. The LOD of (R)-1 for Al³⁺ downs to nanomolar concentrations which is lower than the allowed concentration of drinking water set by the (World Health Organization) WHO.

Determination of enantiomeric excess of some amino acids by second-order calibration of kinetic-fluorescence data

In this investigation a new non-separative kinetic-spectroflourimetric method is proposed for the determination of lysine (lys), leucine (leu) and phenylalanine (phe) enantiomers as their o-phthaldialdehyde (OPA) derivatives in the presence of an optically active chiral thiol compound, 1-mercapto-2-propanol (MP). At ambient temperature and in the borate buffer media of pH 9.6, MP, OPA, as highly selective fluorogenic reagents, and amino acid (AA) enantiomers reacts with each other to yield two fluorescent diasteriomers of D and L-AA with maximum difference in fluorescence intensity at about 450 nm. To achieve information from the small spectral changes, the data are analyzed by Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) method. Linear calibration curves are achieved to distinct D and L-lys, leu and phe in different mole ratios by applying appropriate constraints in MCR-ALS procedures. This is the first application of MCR-ALS in determination of enantiomeric excess (ee) using OPA/MP adduct as chiral reagent, which benefits from direct time dependent-fluorescence spectral analysis and does not require prior separation of chiral analytes. Both the cross-validated correlation coefficient (Q²) and root mean squares error of prediction (RMSEP) indicated satisfactory prediction ability of this method.

A photo-stable fluorescent chiral thiourea probe for enantioselective discrimination of chiral guests

Herein, a chiral thiourea Schiff base derived from (1R,2R)-1,2-cyclohexanediamine and tetraphenylethylene (TPE) was applied as a highly effective chiral sensor for the enantioselective discrimination of various acids and aminesviaion-pair and hydrogen-bond interaction.

Differentiation of enantiomeric anions by NMR spectroscopy with chiral bisurea receptors

Enantiomeric ¹H NMR signals of chiral anions are separated by forming 1 : 1 and 1 : 2 host–guest complexes with chiral bisurea.

Salen-based enantiomeric polymers for enantioselective recognition

In a simple way, the spatial arrangement of the building blocks in a main chain polymer determines its recognition properties.

A simple chalcone-based ratiometric chemosensor for silver ion

Herein, we report the selective binding of Ag(+) ion by the anthracene-based chalcone receptor 1. Receptor 1 behaves as a selective and sensitive chemosensor for the recognition of Ag(+) over other heavy and transition metal ions without any interference and is capable of detecting the metal ion down to 0.15 × 10(-6) M. Receptor 1 on binding with Ag(+) ions exhibits a ratiometric fluorescence enhancement, which is due to the inhibition of photoinduced electron transfer along with the intramolecular charge transfer mechanism.

Chiral recognition of carboxylates by a static library of thiourea receptors with amino acid arms

Chiral recognition is based on a large network of very subtle interactions whose outcome is difficult to predict. A combinatorial approach is therefore the most suitable to search for the most efficient receptor and obtain a structure-enantioselectivity correlation. We synthesized a set of 12 receptors constructed with 1,9-diaminoantracene and α-amino acid esters, linked via thiourea groups. The association constants and enantioselectivities for the complexes with mandelate and N-acetylphenylalanine were determined by competitive NMR titrations. Association constants quite regularly depend on the substituents in the receptor structure, but the distribution of enantioselectivities across the library could not easily be rationalized.

Specific fluorescent sensing of aluminium using naphthalene benzimidazole derivative in aqueous media

Naphthalene benzimidazole conjugate bearing a hydroxyl group was synthesized. Its binding properties towards various metal ions were examined and it showed a high selectivity and sensitivity towards Al(3+) ions in aqueous media. The recognition processes follows a photo induced electron transfer (PET) mechanism assisted with the restricted intramolecular C-C single bond rotation and are scarcely influenced by other coexisting metal ions. In addition, determination of Al(3+) in a variety of sewage water samples was also determined.

Application of optically active chiral bis(imidazolium) salts as potential receptors of chiral dicarboxylate salts of biological relevance

New chiral ionic liquids as receptors for dicarboxylic acid derivatives.

Static and dynamic properties of 1,1'-bi-2-naphthol and its conjugated acids and bases

Several convergent techniques were used to characterize 1,1'-bi-2-naphthol (BINOL) and some of its properties. Its acidity in the gas-phase, from neutral species to monoanion, was measured by mass spectrometry. The conformation and structure of BINOL in the gas phase was determined by microwave rotational spectroscopy. NMR experiments in fluorosulfonic acid established that BINOL was monoprotonated on one of the hydroxyl oxygen atoms. The enantiomerization barriers reported in the literature for BINOL under neutral, basic, and acid conditions were analyzed with regard to the species involved. Finally, DFT calculations allowed all of these results to be gathered in a coherent picture of the BINOL structure.