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.

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.

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.

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.

Chirality dependent inverse-melting and re-entrant gelation in α-cyclodextrin/1-phenylethylamine mixtures

Solutions of cyclohexakis-(1→4)-α-d-glucopyranosyl, α-cyclodextrin, αCD, in R-(+)-1-phenylethylamine, αCD/R-PEA, and S-(−)-1-phenylethylamine, αCD/S-PEA, display abnormal phase transitions that strongly depend on supramolecular diastereomeric interactions. While αCD/R-PEA mixtures show one sol–gel inverse-melting phase transition, αCD/S-PEA mixtures show temperature dependent gel–sol–gel re-entrant behavior. NMR, Raman spectroscopy, microscopy and X-ray scattering measurements reveal that hydrogen bond weakening in solution, as well as changes in crystal composition are responsible for entropy increase and gel formation upon heating.

Solutions of α-cyclodextrin in chiral 1-phenylethylamine display abnormal phase transitions. Depending on supramolecular diastereomeric interactions, inverse-melting and re-entrant gels are formed.

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.

Preferential intermolecular interactions lead to chiral recognition: enantioselective gel formation and collapse

Enantioselective recognition of chiral amines through gel formation and collapse.

A chiral BINOL-based Gemini amphiphilic gelator and its specific discrimination of native arginine by gelation in water

A novel axially chiral cationic Gemini amphiphile gelator (S1) derived from (S)-BINOL has been synthesized and characterized by ¹H NMR, ¹³C NMR, ESI-MS and FT-IR analyses. The critical micelle concentration (CMC) of S1 was determined to be 0.21 mM in water at room temperature. A transparent hydrogel with S1 at 43 mM was obtained at room temperature and characterized using various methods including SEM, CD, fluorescence, ¹H NMR, FT-IR, and XRD. The results indicate that the hydrophobic effect of long alkyl chains, π-π stacking of naphthalene rings, and intermolecular hydrogen-bonding of the amide groups of S1 should be responsible for the hydrogel formation. Moreover, an 8.5 mM aqueous solution of S1 could gel by the addition of l-arginine, whereas it failed to gel in the presence of other 15 amino acids, respectively. It is suggested that S1 could discriminate native arginine by hydrogel formation, mainly due to the electrostatic interaction and hydrogen bonding effects between S1 and l-arginine molecules.

Preparation and investigation of temperature-responsive calix[4]arene-based molecular gels

Higher temperature enhances the strength and the toughness of the gel comprised of kerosene and a tetracholesteryl derivative based on calix[4]arene.

Calix[4]arene-based low molecular mass gelators to form gels in organoalkoxysilanes

Organoalkoxysilanes-based molecular gels for melting-free deposition molding, potential smart materials for 3D printing.

A novel calix[4]arene-based dimeric-cholesteryl derivative: synthesis, gelation and unusual properties

The exceptionally high thermo-stability and superior thixotropic property may make the C2N2C/benzene gel find important real-life applications.

Two-component supramolecular gels derived from amphiphilic shape-persistent cyclo[6]aramides for specific recognition of native arginine

A unique supramolecular two-component gelation system was constructed from amphiphilic shape-persistent cyclo[6]aramides and diethylammonium chloride (or triethylammonium chloride). This system has the ability to discriminate native arginine from 19 other amino acids in a specific fashion. Cyclo[6]aramides show preferential binding for the guanidinium residue over ammonium groups. This specificity was confirmed by both experimental results and theoretical simulations. These results demonstrated a new modular displacement strategy, exploring the use of species-binding hydrogen-bonded macrocyclic foldamers for the construction of two-component gelation systems for selective recognition of native amino acids by competitive host-guest interactions. This strategy may be amenable to developing a variety of functional two-component gelators for specific recognition of various targeted organic molecular species.

Hydrogels formed by enantioselective self-assembly of histidine-derived amphiphiles with tartaric acid

Two chiral enantiomers of histidine-derived amphiphilic gelators, (4R,6S)-UIPCA and (4S,6R)-UIPCA, were synthesized through Pictet-Spengler reaction and their gelation behaviors with different organic acids were investigated. Interestingly, the chiral enantiomers of UIPCA showed smart enantioselectivity for gelating tartaric acid enantiomers to form hydrogels with excellent mechanical strength. The TEM and SEM images demonstrated that the hydrogels were composed of networks by physical entanglement of nanofibers with high aspect ratios. The formation of nanofibers was considered to be driven by the interplay of hydrogen bonding, electrostatic attraction, and hydrophobic interaction, which was supported by XRD and FT-IR spectra. The hydrogels exhibited sensitive response to a series of external stimuli, such as temperature, metal ions, and host-guest interactions, to realize the reversible gel-sol transition. The property of the gelation was elaborated and the gelators were expected to find their applications in chiral discrimination.

Supramolecular hydrogel based on amphiphilic calix[4]arene and its application in the synthesis of silica nanotubes

A hydrogel could be formed by leaving a suspension of novel amphiphilic calix[4]arene in aqueous media to stand at room temperature through a dissolution-reassembly process.

Enantiomeric organogelators from d-/l-arabinose for phase selective gelation of crude oil and their gel as a photochemical micro-reactor

Easy access via single steps to each enantiomer of a simple chiral organogelator, their gelation abilities and applications are reported.

Visual-size molecular recognition based on gels

Since their discovery, stimuli-responsive organogels have garnered considerable and increasing attention from a broad range of research fields. In consideration of an one-dimensional ordered relay in anisotropic phase, the assembled gel networks can amplify various properties of the functional moieties possessed by the gelator molecules. Recently, substantial efforts have been focused on the development of facile, straightforward, and low-cost molecular recognition approaches by using nanostructured gel matrices as visual sensing platforms. In this research news, the recent progresses in macroscopic or visual-size molecular recognition for a number of homologues, isomers, and anions, as well as extremely challenging chiral enantiomers, using polymer and molecular gels are reviewed. Several strategies--including guest molecular competition, hydrogen-bonding blocking, and metal-coordination--for visual discrimination are included. Finally, the future trends and potential application in facile visual-size molecular recognition based on organogel matrices are highlighted.