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.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Development of Low-Molecular-Weight Organogelators from Cyclic β-Amino Acid: Effect of Stereochemistry and their Application on Visual Chiral Recognition of Amines

This study reports the formation of low-molecular-weight gelators based on carboxylic acids derived from chiral cyclicβ-amino acids. The effect of their stereochemistry on the gelation of organic solvents was investigated, and their assemblies with the intermolecular interactions in the xerogels were proposed via infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and crystallographic details of the related model compounds. The effect of the alkyl chain length on the gelators was studied, and they were applied to the chiral recognition of amines. Only one diastereomeric salt with amines afforded gels, whereas the others resulted in precipitates. Chiral recognition was also achieved in the gel state, and the appearance of the as-prepared gel changed upon the addition of each amine enantiomer, thus enabling the visual detection of their chirality.

A chiral BINOL-based supramolecular gel enabling sensitive enantioselective and chemoselective collapse toward histidine

A chiral small molecule gelator (R)-H₃L based on 1,1'-bi-2,2'-naphthol (BINOL)-phosphoric acid was designed and prepared, which spontaneously forms a stable water-induced gel. The gelation mechanism was revealed by single crystal X-ray diffraction analysis and a number of spectroscopic methods. Addition of Cu²⁺ improved the gelation ability, and the resultant metal organic gel realized visual enantioselective and chemoselective recognition toward L-histidine from enantiomers of 19 amino acids via gel collapse. The gel showed a highly sensitive response to L-histidine, and as low as 0.01 equiv. of L-histidine relative to the critical gelation concentration of (R)-H₃L-Cu caused the gel to collapse. This strategy of regulating the assembly behavior through the interaction of amino acids and metal ions not only provides a simple and direct way to distinguish enantiomers, but also provides insight into how metal ions regulate the organization of biological supramolecular systems.

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.

Chirality-directed hydrogel assembly and interactions with enantiomers of an active pharmaceutical ingredient

Enantiomers of the low-molecular-weight gelator (LMWG) DBS-CONHNH₂, based on D- or L- 1,3 : 2,4-dibenzylidenesorbitol (DBS), were synthesised. Enantiomeric gels are equivalent, but when mixtures of enantiomers are used, although gels still form, they are weaker than homochiral gels. Nanoscale chirality is lost on adding even a small proportion of the opposite enantiomer - homochiral assembly underpins effective gelation. Enantiomeric gels encapsulate the two enantiomers of anti-inflammatory drug naproxen, with thermal & mechanical differences between diastereomeric systems. We hence demonstrate the importance of chirality in DBS assembly and its interactions with chiral additives.

Molecular-level insights into the self-assembly driven enantioselective recognition process

The importance of the orientation of functional groups in a chiral environment on enantioselective recognition has been demonstrated. Orientation controlled interactions of functional groups in (R)/(S)-MA lead to a visually differentiable morphology with an arginine-based gelator. The crucial role of various molecular-level interactions discriminating the enantioselective self-assembly has been established using different analytical techniques, crystal structure analysis, and DFT calculations.

Novel self-healing and multi-stimuli-responsive supramolecular gel based on d-sorbitol diacetal for multifunctional applications

A simple-structured super gelator with self-healability and multi-stimuli responses was reported herein, which exhibited multiple visual molecular recognition abilities. Multifunctional applications such as effective lubricants, safe fuels, high-efficient propellants, dyes adsorbents, enhanced fluorescence emission and separation of aldehydes from aqueous solutions are integrated into a single organogelator, which was rarely reported.

A simple-structured super gelator with self-healability and multi-stimuli responses was reported herein, which exhibited multiple visual molecular recognition abilities.

Preferential intermolecular interactions in a racemic mixture of amino acid Schiff base, conformational structures in solid state, and DFT studies

In this work, we report the green synthesis of a Shiff base [N-(2-hydroxy-1-naphthylidene)-l-phenylalanine (1)] derived from an α-amino acid through an ultrasound-assisted synthesis method with excellent chemical yield in a short period of time.

Reversible Chromatic Change of Supramolecular Gels for Visual and Selective Chiral Recognition of Histidine

In the present work, a chemical reaction has been performed for supramolecular gels to achieve multiple-stimuli-responsive smart soft materials. Simple chiral binaphthalene-based receptors can condense with histidine (His) to yield a Schiff base, which would help to achieve visual chiral recognition of unprotected l/d-His through gel formation along with specific selectivity toward 20 amino acids. Through intermolecular hydrogen bonds, the resultant Schiff base molecules assemble with excess His molecules to form three-dimensional (3D) networks of metastable cross-linked nanospheres and stable nanofibers in EtOH/water and MeOH/water, respectively. Significantly, this condensation reaction exhibits unique reversible and chromatic phenomena between sol-gel phase transitions in EtOH/water, which provide a way to design chemical reaction-based multivisual-change supramolecular gels for sensing and switching applications.

Ultralow-Molecular-Weight Stimuli-Responsive and Multifunctional Supramolecular Gels Based on Monomers and Trimers of Hydrazides

The simpler, the better. A series of simple, neutral and ultralow-molecular-weight (MW: 140-200) hydrazide-derived supramolecular gelators have been designed and synthesized in two straightforward steps. For non-conjugated cyclohexane-derived hydrazides, their monomers can self-assemble to form gels through intermolecular hydrogen bonds and dipole-dipole interactions. Significantly, conjugated phthalhydrazide can self-aggregate into planar and circular trimers through intermolecular hydrogen bonds and then self-assemble to form gels through intermolecular π-π stacking interactions. It is interesting that these simple gelators exhibit unusual properties, such as self-healing, multi-response fluorescence, and visual and selective recognition of chiral (R)/(S)-1,1'-binaphthalene-2,2'-diamine and S^2- through much different times of gel re-formation and blue-green color change, respectively. These results underline the importance of supramolecular gels and extend the scope of supramolecular gelators.

Self-assembly of pyrene-appended glucono gelators: spacer regulated morphological change and inversion of circularly polarized luminescence

Pyrene-appended glucono gelators with different spacer lengths (two and four methylene units) were designed and found to form supramolecular gels in organic aqueous solvents. The shorter spacer gelator 1 was prone to self-assemble into nanotubes due to well stacking multi-bilayer unit, while gelator 2 with the longer spacer formed nanofibers due to the relatively disordered packing structure. Both of the gels showed supramolecular chirality as well as circularly polarized luminescence (CPL) due to the chirality transfer from the glucose moiety to the assembly. Interestingly, the CD and CPL signals were opposite for the two gels. It was suggested that the packing of the pyrene unit in the gels were different due to the spacer and resulted in the inversed chiroptical properties. The work provided a deeper understanding of the origin of the supramolecular chirality and furthers the design of the CPL materials.

Pyrene-appended glucono gelators with different spacer lengths (two and four methylene units) were designed and found to form supramolecular gels in organic aqueous solvents.

A multi-stimuli-responsive metallohydrogel applied in chiral recognition, adsorption of poisonous anions, and construction of various chiral metal-organic frameworks

The design of stimuli-responsive hydrogels is attractive but challenging. A multi-stimuli-responsive chiral metallohydrogel was constructed using a rational approach to design a functional metallohydrogel using chiral organic gelators. The as-synthesized metallohydrogel reported here performed remarkably as a visual sensor for discriminating between (R)-phenylglycinol and (S)-phenylglycinol. Furthermore, the metallohydrogel showed a significant capacity for adsorption and enrichment of Cr(vi) in aqueous solution. Besides this, by utilizing anion-induced transformation, the metallohydrogel could be controllably directed to synthesize different types of chiral metal-organic frameworks.

Supramolecular Gel as the Template for Catalysis, Inorganic Superstructure, and Pharmaceutical Crystallization

A supramolecular gel is a fascinating combination of flexibility and orderliness. While the supramolecular nature of crosslinking contributes towards the adaptivity and the reversibility of the system, orderliness at the molecular level amplifies the functional output and induces extraordinary selectivity into the system. Therefore, use of supramolecular gels as the soft template is an emerging area of research, which includes but not limited to catalysis of a chemical or a photochemical process, transcription of gel property to a substrate, or even controlling the nucleation of drug molecules. This review aims to highlight the template effect of supramolecular gels in the above-mentioned areas relevant to novel fundamental chemistry, technology, and healthcare.

A simple and visual approach for enantioselective recognition through supramolecular gels with specific selectivity

(S)/(R)1 enantioselectively self-assemble to form a gel or solution with one enantiomer of (S)/(R)BINAM with specific selectivity.