Binding of Flavylium Ions to Sulfonatocalix[4]arene and Implication in the Photorelease of Biologically Relevant Guests in Water

Supramolecular Handshakes: Characterization of Urea-Carboxylate Interactions Within Calixarene Frameworks

The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido-derivatives of p-tert-butylcalix[4]arene comprising phenylureido-moieties and diacetate-calix[4]arenes were prepared. The binding of acetate by ureido-derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido-calixarenes for carboxylates. This suggested high potential for uniting the complementary calix[4]arenes via H-bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure-stability relationship. The tetraureido-calixarene derivative formed the most stable heterodimers with diacetate-calix[4]arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re-forming the heterodimer by addition of an acid and base to the solution containing urea- and carboxylate-derivative calix[4]arene was demonstrated using NMR spectroscopy.

Host-Guest Chemosensor Ensembles based on Water-Soluble Sulfonated Calix[n]arenes and a Pyranoflavylium Dye for the Optical Detection of Biogenic Amines

Biogenic amines (BAs) are biologically active nitrogen-containing compounds formed during the food spoilage process and are often related as key markers of food quality, safety, and freshness. Because their presence in foods at high levels can cause significant health problems, researchers have been focused on developing novel strategies and methods for early detection and capture of these analytes. Herein, water-soluble sulfonated calix[n]arene macrocycles (SC4, SC6, and SC8) and a pH-sensitive dye (4'-hydroxy-10-methylpyranoflavylium) were investigated as host-guest systems for BA sensing. The hosts were able to bind the flavylium cation of the dye with association constants of 103 to 104 M-1. The dye complexation also allowed tuning its pKa from 6.72 (free) toward high values: 7.68 (SC4), 7.79 (SC6), and 8.45 (SC8). These data were crucial to optimize the host-guest complexes as optical sensing systems for putrescine/tyramine (pH 7.2-7.6), yielding a colorimetric redshift from yellow to red. The BA sensing was also demonstrated by fluorescence quenching for the calix[n]arene/dye complexes and fluorescence recovery after the addition of BAs. 1H NMR spectroscopy was used to demonstrate the interaction mode, confirming an encapsulation-driven mechanism. Overall, these host-guest systems demonstrated great potential for the detection of BAs, one of the main key markers of food spoilage.

Visible-light-switchable Chalcone-Flavylium Photochromic Systems in Aqueous Media

The chalcone-flavylium photochromic system switches in aqueous media. However, the chalcone→flavylium conversion requires detrimental ultra-violet (UV) light for the switching which deters their applications in the biological domain. To address this issue, we have synthesized strategically modified chalcone scaffolds that can be reversibly switched to the flavylium forms with visible light ranging from 456 nm (blue) to 640 nm (red).

A Tutorial Review on the Fluorescent Probes as a Molecular Logic Circuit-Digital Comparator

The rapid progress in the field of fluorescent probes and fluorescent sensing material extended this research area toward more complex molecular logic gates capable of carrying out a variety of sensing functions simultaneously. These molecules are able to calculate a composite result in which the analysis is not performed by a man but by the molecular device itself. Since the first report by de Silva of AND molecular logic gate, all possible logic gates have been achieved at the molecular level, and currently, utilization of more complicated molecular logic circuits is a major task in this field. Comparison between two digits is the simplest logic operation, which could be realized with the simplest logic circuit. That is why the right understanding of the applied principles during the implementation of molecular digital comparators could play a critical role in obtaining logic circuits that are more complicated. Herein, all possible ways for the construction of comparators on the molecular level were discussed, and recent achievements connected with these devices were presented.

A Novel Fluorescent Probe for Determination of pH and Viscosity Based on a Highly Water-Soluble 1,8-Naphthalimide Rotor

A novel highly water-soluble 1,8-naphthalimide with pH and viscosity-sensing fluorescence was synthesized and investigated. The synthesized compound was designed as a molecular device in which a molecular rotor and molecular “off-on” switcher were integrated. In order to obtain a TICT driven molecular motion at C-4 position of the 1,8-naphthalimide fluorophore, a 4-methylpiperazinyl fragment was introduced. The molecular motion was confirmed after photophysical investigation in solvents with different viscosity; furthermore, the fluorescence-sensing properties of the examined compound were investigated in 100% aqueous medium and it was found that it could be used as an efficient fluorescent probe for pH. Due to the non-emissive deexcitation nature of the TICT fluorophore, the novel system showed low yellow–green emission, which represented “power-on”/“rotor-on” state. The protonation of the methylpiperazine amine destabilized the TICT process, which was accompanied by fluorescence enhancement indicating a “power-on”/“rotor-off” state of the system. The results obtained clearly illustrated the great potential of the synthesized compound to serve as pH- and viscosity-sensing material in aqueous solution.

2-Hydroxychalcone-β-Cyclodextrin Conjugate with pH-Modulated Photoresponsive Binding Properties

Stimuli-responsive supramolecular receptors are important building blocks for the construction of self-assembled functional materials. We report the design and synthesis of a pH- and light-responsive 2-hydroxychalcone-β-cyclodextrin conjugate (1-Ct) and its characterization by spectroscopic and computational methods. 1-Ct follows the typical reaction network of trans-chalcone-flavylium photoswitches. Upon light irradiation, 1-Ct can be photochemically converted into the cis-chalcone/hemiketal forms (1-Cc/1-B) under neutral pH conditions or to the flavylium cation (1-AH⁺) at acidic pH values. This stimuli-responsive β-cyclodextrin host, 1-Ct, was found to form stronger intramolecular self-inclusion complexes (K_intra = 14) than 1-AH⁺ (K_intra = 3) and weaker than 1-Cc/1-B (overall K_intra = 179), allowing control over their stability and binding properties by combinations of pH and light stimuli.

pH-regulated interaction modes between cyanidin-3-glucoside and phenylboronic acid-modified alginate

Stabilizing mechanisms through covalent and non-covalent interactions have been studied along the years for color stabilization of anthocyanin dyes. In this work, the chemical functionalization of a natural and biocompatible marine-based polysaccharide (alginic acid) with 3-aminophenylboronic acid via carbodiimide coupling chemistry was carried out in order to create a bifunctional material for pH-dependent selective interaction with colored cyanidin-3-glucoside chemical species. The interaction studies were performed by UV-Vis, ¹H, and ¹¹B NMR spectroscopy. Overall, the apparent acidic and hydration constants are more stabilized in the presence of phenylboronic acid-modified alginate. For more acidic pH values the red cationic flavylium cation of cyanidin-3-glucoside mainly interacts through non-covalent electrostatic interactions with the carboxylate groups of the biopolymer derivative with association constant around 0.5 mM^-1 while at higher pH values boronate-catechol covalent bonds are favored, promoting the stabilization of the colored neutral and anionic quinoidal bases of anthocyanins instead of colorless hemiketal and chalcones.

Photomodulation of ultrastable host–guest complexes in water and their application in light-controlled steroid release

Host–guest complexation of dithienylethene photoswitches with cucurbit[8]uril leads to photoresponsive binding pairs with picomolar affinity in water.

Supramolecular capture of highly polar amidosquaraine dye in water with nanomolar affinity and large turn-on fluorescence

A supramolecular dye-capture system comprising anionic amidosquaraine guest and macrocyclic tetralactam host exhibits nanomolar affinity and "turn on" visible fluorescence. Utility is demonstrated with a new fluorescent assay for liposome leakage induced by the biomedically important enzyme phospholipase A₂.

Natural and Synthetic Flavylium-Based Dyes: The Chemistry Behind the Color

Flavylium compounds are a well-known family of pigments because they are prevalent in the plant kingdom, contributing to colors over a wide range from shades of yellow-red to blue in fruits, flowers, leaves, and other plant parts. Flavylium compounds include a large variety of natural compound classes, namely, anthocyanins, 3-deoxyanthocyanidins, auronidins, and their respective aglycones as well as anthocyanin-derived pigments (e.g., pyranoanthocyanins, anthocyanin-flavan-3-ol dimers). During the past few decades, there has been increasing interest among chemists in synthesizing different flavylium compounds that mimic natural structures but with different substitution patterns that present a variety of spectroscopic characteristics in view of their applications in different industrial fields. This Review provides an overview of the chemistry of flavylium-based compounds, in particular, the synthetic and enzymatic approaches and mechanisms reported in the literature for obtaining different classes of pigments, their physical-chemical properties in relation to their pH-dependent equilibria network, and their chemical and enzymatic degradation. The development of flavylium-based systems is also described throughout this Review for emergent applications to explore some of the physical-chemical properties of the multistate of species generated by these compounds.

Anthocyanin Color Stabilization by Host-Guest Complexation with p-Sulfonatocalix[n]arenes

Flavylium-based compounds in their acidic and cationic form bring color to aqueous solutions, while under slightly acidic or neutral conditions they commonly bring discoloration. Selective host-guest complexation between water-soluble p-sulfonatocalix[n]arenes (SCn) macrocycles and the flavylium cationic species can increase the stability of the colored form, expanding its domain over the pH scale. The association constants between SCn and the cationic (acid) and neutral basic forms of flavylium-based compounds were determined through UV-Vis host-guest titrations at different pH values. The affinity of the hosts for synthetic chromophore was found to be higher than for a natural anthocyanin (Oenin). The higher affinity of SC4 for the synthetic flavylium was confirmed by ¹H NMR showing a preferential interaction of the flavylium phenyl ring with the host cavity. In contrast with its synthetic counterpart, the flavylium substitution pattern in the anthocyanin seems to limit the inclusion of the guest in the host’s binding pocket. In this case, the higher affinity was observed for the octamer (SC8) likely due to its larger cavity and higher number of negatively charged sulfonate groups.

Exploring the pH-dependent kinetics, thermodynamics and photochemistry of a flavylium-based pseudorotaxane

Flavylium-based molecular switches are attractive molecular components to devise stimuli-responsive host-guest systems such as rotaxanes and pseudorotaxanes. These compounds display a pH-dependent reaction network of several species that reversibly interconvert within different time scales. Therefore, to explore and take profit of exceptional stimuli-responsive properties of these systems, detailed kinetic and thermodynamic characterizations are often required. In this work, we present the results of such characterization for a new flavylium compound decorated with a trimethylalkylammonium substituent designed to form a pseudorotaxane with cucurbit[7]uril (CB7). The formation of the pseudorotaxane was characterized in detail, and the thermodynamic and kinetic aspects of the flavylium interconversion reactions in the assembly were investigated and compared with the free molecular switch.

Synthesis and Photoswitching Properties of Bioinspired Dissymmetric γ-Pyrone, an Analogue of Cyclocurcumin

Cyclocurcumin (CC), a turmeric curcuminoid with potential therapeutic properties, is also a natural photoswitch that may undergo E/Z photoisomerization under UV light. To be further exploited in relevant biological applications, photoactivation under near-infrared (NIR) irradiation is required. Such requirement can be met through opportune chemical modifications, by favoring two-photon absorption (TPA) probability. Herein, a general and efficient synthesis of a biomimetic 2,6-disubstituted-γ-pyrone analogue of CC is described, motivated by the fact that molecular modeling previews an order of magnitude increase of its NIR TPA compared to CC. Three retrosynthetic pathways have been identified (i) via an aryl-oxazole intermediate or via aryl-diynone through (ii) a bottom-up or (iii) a top-down approach. While avoiding the passage through unstable synthons or low-yield intermediate reactions, only the latest approach could conveniently afford the 2,6-disubstituted-γ-pyrone analogue of CC, in ten steps and with an overall yield of 18%. The photophysical properties of our biomimetic analogue have also been characterized showing an improved photoisomerization yield over the parent natural compound. The potentially improved nonlinear optical properties, as well as enhanced stability, may be correlated to the enforcement of the planarity of the pyrone moiety leading to a quadrupolar D-π-A-π-D system.

Screening of Xanthine Oxidase Inhibitors by Liquid Crystal-Based Assay Assisted with Enzyme Catalysis-Induced Aptamer Release

Small-molecule drugs play an important role in the treatment of various diseases. The screening of enzyme inhibitors is one of the most important means in developing therapeutic drugs. Herein, we demonstrate a liquid crystal (LC)-based screening assay assisted with enzyme catalysis-induced aptamer release for screening xanthine oxidase (XOD) inhibitors. The oxidation of xanthine by XOD prevents the specific binding of xanthine and its aptamer, which induces a bright image of LCs. However, when XOD is inhibited, xanthine specifically binds to the aptamer. Correspondingly, LCs display a dark image. Three compounds are identified as potent XOD inhibitors by screening a small library of triazole derivatives using this method. Molecular docking verifies the occupation of the active site by the inhibitor, which also exhibits excellent biocompatibility to HEK293 cells and HeLa cells. This strategy takes advantages of the unique aptamer-target binding, specific enzymatic reaction, and simple LC-based screening assay, which allows high-throughput and label-free screening of inhibitors with high sensitivity and remarkable accuracy. Overall, this study provides a competent and promising approach to facilitate the screening of enzyme inhibitors using the LC-based assay assisted with the enzyme catalysis-induced aptamer release.

Revising the formation and electronic properties in flavylium derivatives. A theoretical tandem towards optimized DSSCs

A study of benzopyrylium derivatives obtained from a luminescent precursor, including the optical pathway, bonding analysis, and transmission properties, toward green energy applications.

Selective Recognition of Amino Acids and Peptides by Small Supramolecular Receptors

To this day, the recognition and high affinity binding of biomolecules in water by synthetic receptors remains challenging, while the necessity for systems for their sensing, transport and modulation persists. This problematic is prevalent for the recognition of peptides, which not only have key roles in many biochemical pathways, as well as having pharmacological and biotechnological applications, but also frequently serve as models for the study of proteins. Taking inspiration in nature and on the interactions that occur between several receptors and peptide sequences, many researchers have developed and applied a variety of different synthetic receptors, as is the case of macrocyclic compounds, molecular imprinted polymers, organometallic cages, among others, to bind amino acids, small peptides and proteins. In this critical review, we present and discuss selected examples of synthetic receptors for amino acids and peptides, with a greater focus on supramolecular receptors, which show great promise for the selective recognition of these biomolecules in physiological conditions. We decided to focus preferentially on small synthetic receptors (leaving out of this review high molecular weight polymeric systems) for which more detailed and accurate molecular level information regarding the main structural and thermodynamic features of the receptor biomolecule assemblies is available.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Multimode binding and stimuli responsive displacement of acridine orange dye complexed with p-sulfonatocalix[4/6]arene macrocycles

Interaction of acridine orange (AOH+) dye with water soluble anionic p-sulfonatocalix[n]arene (SCXn) hosts, SCX4 and SCX6, having different cavity dimensions, has been investigated using multispectroscopic techniques. Intriguing modulation in the photophysical properties of AOH+ upon interaction with SCXn hosts indicate the formation of different host-guest complexes at different regions of the host concentrations. At lower host concentrations, AOH+ undergoes SCXn assisted aggregation, causing a drastic reduction in fluorescence intensity. At higher host concentrations, aggregated-AOH+-SCXn complexes disintegrate and monomeric-AOH+-SCXn exo and inclusion complexes are eventually formed, leading to a huge fluorescence enhancement finally. Observed effects are more pronounced with SCX6 as compared to SCX4 host. Time-resolved fluorescence studies indicate that at very high host concentrations, there is also a diffusion-controlled dynamic quenching for both monomeric-AOH+-SCXn exo and inclusion complexes, caused by the free SCXn present in the solution, a phenomenon not reported before for such host-guest systems. The aggregated-AOH+-SCXn complexes at lower host concentration were employed to investigate displacement study using an antiviral drug, 1-adamantanamine (AD) and a neurotransmitter, acetylcholine (AcCh), as the competitive binders cum external stimuli, which resulted in a drastic recovery of the fluorescence reduced initially due to aggregation process. Though both the AOH+-SCXn systems act as efficient supramolecular assemblies in sensing AD and AcCh as the analytes through fluorescence "OFF-ON" mechanism, the effect is more pronounced for AOH+-SCX4 system as compared to AOH+-SCX6. SCXn induced interesting modulation in the photophysical properties of AOH+ and the stimulus responsive dye displacement observed for aggregated-AOH+-SCXn systems can expectedly find applications in fluorescence OFF-ON sensing, supramolecular functional materials and similar others.

A route to virtually unlimited functionalization of water-soluble p-sulfonatocalix[4]arenes

The functionality of p-sulfonatocalix[4]arenes can be easily extended using the propargylation/CuAAC reaction sequence, which allows the introduction of up to four substituted triazole units to the narrow rims of the macrocycles while maintaining their cone shapes and water solubility and, thus, biomedical applicability.

Chemical signal cascading in a supramolecular network

A chemically-triggered signalling cascade between cucurbituril host-guest complexes by means of multi-step competitive displacement is demonstrated. The inter-complex communication of chemical information yields the release of bio-relevant cargo, reminiscent of cellular signalling pathways.

Computational NMR Spectra of o-Benzyne and Stable Guests and Their Hemicarceplexes

The incarceration of o-benzyne and 27 other guest molecules within hemicarcerand 1, as reported experimentally by Warmuth, and Cram and co-workers, has been studied by density functional theory (DFT). The ¹ H NMR chemical shifts, rotational mobility, and conformational preference of the guests within the supramolecular cage were determined, which showed intriguing correlations of the chemical shifts with structural parameters of the host-guest system. Furthermore, based on the computed chemical shifts reassignments of some NMR signals are proposed. This affects, in particular, the putative characterization of the volatile benzyne molecule inside a hemicarcerand, for which our CCSD(T) and KT2 results indicate that the experimentally observed signals are most likely not resulting from an isolated o-benzyne within the supramolecular host. Instead, it is shown that the guest reacted with an aromatic ring of the host, and this adduct is responsible for the experimentally observed signals.