Anion binding with urea and thiourea derivatives

Carbazole Framework as Functional Scaffold for the Design of Synthetic Receptors

Carbazole serves as a prominent framework in the design of synthetic receptors, being a valuable scaffold for supramolecular chemistry, thanks to its planarity, fluorescence and versatility. This review provides a comprehensive analysis of notable examples of carbazole-based receptors, highlighting the impact of structural modifications on binding affinity and selectivity toward different guests.

Microenvironmental engineering of active sites for selective catalytic hydrolysis of acetals

Rational design of synthetic catalysts that mimic enzymes in catalysis and substrate selectivity is a long-standing goal of chemists. We report bottom-up synthesis of artificial acetal hydrolase that hydrolyzes its substrate with high selectivity under otherwise impossible neutral and basic conditions. Our synthetic method allows facile modification of the active site, including introduction of a local water pool near the acetal group of the bound substrate to alter the catalytic mechanism or installment of a secondary catalytic group to enhance the catalytic activity.

Anion recognition properties of acridine-urea conjugate in bulk and silica nanopore systems

This study investigated the anion recognition properties of acridine-urea conjugate (AcU) in bulk DMSO and silica nanopore. The free AcU in bulk DMSO was in tautomeric equilibrium between fluorescent amino form (a-AcU) and non-fluorescent imino form (i-AcU). Owing to this tautomerization of AcU, the free AcU worked as fluorescence enhancement sensing by hydrogen bonds mediated complexation between urea unit and anion. The estimated dissociation constants were 1.4 ± 0.2 mM for CH3COO- and 3.1 ± 1.0 mM for H2PO4-, whereas those for Cl-, ClO4-, and HSO4-were quite large. The hydrogen bond between urea unit and anion was also available for the anion recognition by AcU immobilized at the pore surface of mesoporous silica when the anion concentration is above 0.2 mM. In addition, we found that fluorescence of protonated a-AcU (a-AcHU+) could also be utilized for the recognition of weak acid anions over strong acid anions when the anion concentration is below 0.1 mM. The AcU@MPS with two recognition system has potential application for the anion recognition.

Fluorescent calix[4]triazole for selective fluoride anion sensing

Fluoride ions (F-) play an important role in preventing cavities and treating osteoporosis, but excessive exposure can lead to serious health problems such as fluorosis and kidney damage. These dual characteristics highlight the need for selective and sensitive methods to detect fluoride ions for health monitoring. Accordingly, in this study, we investigated the anion-binding ability of Py-CT4, a fluorescent chemosensor in which pyrene is linked to calix[4]triazole via an ester linker. Notably, Py-CT4 exhibited significant fluorescence quenching for F- compared to other anions, and its fluorescence intensity gradually decreased with increasing F- concentration. This phenomenon is driven by electron transfer from calix[4]triazole to pyrene, initiated by hydrogen bonding with F- and followed by F--induced deprotonation of calix[4]triazole. The selectivity of Py-CT4 for F- appears to stem from its relatively flexible structure and low acidity compared to the previously reported Py-CT4+. Py-CT4 thus represents the first macrocyclic receptor based on charge-neutral 1,2,3-triazole that selectively recognizes F- through fluorescence quenching. Compared to traditional detection methods, Py-CT4 utilizes the advantages of fluorescent detection, such as higher sensitivity, faster response times, and ease of use, for fluoride ion detection. Py-CT4 also demonstrates excellent selectivity for F- even in the presence of competing anions. These features make Py-CT4 a promising tool for monitoring fluoride ions in biological and environmental systems, providing valuable insights into public health and safety.

Amide/urea-based simple fluorometric receptors for iodide and Hg2+ ions in aqueous medium: Aggregation induced emission and DFT studies

Herein, we report pyrene-tagged amide and urea-based sugar derivatives 1 and 2 in a simple synthetic pathway to recognize I- and Hg2+ ions. Both molecules showed absorbance and fluorescence selectivity towards iodide ions in THF/H2O (7/3, v/v) medium. The selectivity and sensitivity of 2 for iodide ions are superior to 1 due to more H-bond donors in 2. Interestingly, fluorometric receptor 2 exhibited aggregation-induced emission (AIE) at higher pH with a remarkable fluorometric color change. The AIE phenomenon might be explained by the self-association of 2 after forming imine functionality in the alkali medium. The Stern-Volmer plot showed the fluorescence quenching constant of each receptor with an iodide ion and indicated the quenching pathway. The LODs of 1 and 2 for iodide ions were evaluated as 0.84 and 0.17 µM, respectively. The 1:1 binding stoichiometry of 1 or 2 with iodide was found from the Job plot and verified by measuring the complex mass. Further, the complexes of each receptor with I- ions can detect Hg2+ ions selectively by fluorescence turn-on method with low sensitivities (LODs: 0.008 µM for 1 and 0.01 µM for 2). DFT results were used to understand the binding mode of receptors 1 and 2 with iodide ions and the quenching process in the aqueous THF medium. The real application of the receptors was established for the recovery of iodide and Hg2+ ions from natural water samples.

Substituent-Driven Anion-Binding Selectivity in Aliphatic Chain-Substituted 1,2-Phenylene Urea Macrocycles and Optimized Synthetic Methodology

1,2-Phenylene tetraurea macrocycles recently attracted attention as self-assembled channel-making compounds with high selectivity to chlorides. Here, we report on the introduction of aliphatic chains in the periphery of the 1,2-phenylene tetraurea macrocycle, which led to deterioration in the ability of the macrocycle to form channels and to a reversal of anion binding preferences in favour of dihydrogen phosphate. In addition, we have developed a new method of synthesis of 1,2-phenylene tetraurea macrocycle, using a direct click of two diamino ureido derivatives by triphosgene in the presence of chloride template. This approach saves time and eliminates demanding isolation of the non-cyclic tetrameric intermediates.

An Isoniazid Based Schiff Base Sensor for Selective Detection of Pd2+ Ions

Here, we developed a novel isoniazid based fluorescent probe (E)-N'-(thiophen-2-ylmethylene)isonicotinohydrazide (TINH) through simple condensation reaction and employed for selective detection of Pd2+ ions with a low detection limit of 4.102 × 10-11 M. Among the many existing cations, TINH bound Pd2+ ions with an association affinity of 9.794 × 105 M-1. Adding Pd2+ ions to ligand solution increased the absorption intensity in UV-Visible and quenched the emission intensity in fluorescence spectroscopic experiments. More importantly, this TINH complexed to Pd2+ ions in 1:1 stoichiometric ratio. To evaluate the stability of complexed system, pH experiments has been performed. The binding insights among the ligand and Pd2+ ions has been confirmed by IR spectroscopic and MASS spectrometric methods. Additionally, TINH also applied to real water samples for the identification and measurement of Pd2+ ions. Hence, this system could be highly applicable for detection of Pd2+ ions in environmental and industrial samples with in low detection range.

Conformational Switching of a Nano-Size Urea-Bridged Zn(II)Porphyrin Dimer by External Stimuli

For the first time, explicit stabilization of all the three conformers, viz. (cis,cis), (cis,trans) and (trans,trans), of a 'nano-sized' highly-flexible urea-bridged Zn(II)porphyrin dimer have been achieved via careful manipulations of external stimuli such as solvent dielectrics, temperature, anionic interactions, axial ligation and surface-induced stabilization. The conformers differ widely in their structures, chemical and photophysical properties and thus have vast potential applicability. X-ray structural characterizations have been reported for the (cis,cis) and (cis,trans)-conformers. While (cis,cis) conformer stabilized exclusively in dichloromethane, more polar solvents resulted in the stabilization of (cis,trans) and (trans,trans)-conformers. Low temperature promotes the stabilization of (cis,trans)-conformer while rise in temperature facilitates flipping to the (cis,cis) one. Significantly, exclusive stabilization of the (trans,trans)-isomer has been illustrated using acetate anion which facilitates H-bonding with the two amide linkages of the urea spacer. Remarkably, HOPG surface facilitates stabilization of the energetically challenging (trans,trans)-conformer via CH⋅⋅⋅π and π⋅⋅⋅π interactions with the solid surface to the porphyrinic cores. DFT calculations demonstrate that the relative stability of the conformers can be modulated upon slight external perturbations as also observed in the experiment. Several factors contributing towards the conformational landscape for the highly flexible urea-bridged porphyrin dimers have been mapped.

Anion recognition using meta-substituted ureidocalix[4]arene receptors

Calix[4]arenes bearing urea units at the meta position(s) of the upper rim of the macrocyclic skeleton were prepared by the reaction of the corresponding amines with aryl isocyanates. As shown by the 1H NMR and UV/vis titration experiments, these systems are capable of effectively complexing selected anions even in a highly competitive environment (such as DMSO-d6). While the monoureido derivatives showed approximately the same complexation ability irrespective of the substitution (para vs. meta isomers), the bisureas at the upper rim demonstrated interesting differences in complexation. The meta,meta and para,para isomers were shown to prefer 2 : 1 complexes (anion : receptor) regardless of the anion tested, while the analogous meta,para isomer formed 1 : 1 complexes with strongly coordinated anions (e.g. H2PO4-) based on synchronous complexation by both ureido groups. This suggests that the regioselective introduction of urea units into the upper rim of calix[4]arene brings with it the possibility of "tuning" the complexation properties depending on the substitution pattern of the functional groups.

Synthesis, in vitro, and in silico study of novel pyridine based 1,3-diphenylurea derivatives as tyrosinase inhibitors

Tyrosinase inhibitors are studied in the cosmetics and pharmaceutical sectors as tyrosinase enzyme is involved in the biosynthesis and regulation of melanin, hence these inhibitors are beneficial for the management of melanogenesis and hyperpigmentation-related disorders. In the current work, a novel series of diphenyl urea derivatives containing a halo-pyridine moiety (5a-t) was synthesized via a multi-step synthesis. In vitro, tyrosinase inhibitory assay results showed that, except for two compounds, the derivatives were excellent inhibitors of human tyrosinase. The average IC50 value of the inhibitors (15.78 μM) is lower than that of kojic acid (17.3 μM) used as the reference compound, indicating that, on average, these molecules are more potent than the reference. Derivative 5a was identified as the most potent human tyrosinase inhibitor of the series, with an IC50 value of 3.5 ± 1.2  μM, approximately 5 times more potent than kojic acid. To get further insights into the nature of binding site interactions, molecular docking and molecular dynamics simulation studies were carried out. Moreover, the evaluation of in silico ADME properties showed a highly favorable profile for the synthesized compounds. These findings suggested that the further development of this class of compounds could be useful to get potent drug-like compounds that can target hyperpigmentation-related disorders.

Artificial Channels Based on Bottlebrush Polymers: Enhanced Ion Transport Through Polymer Topology Control

Synthetic structures mimicking the transport function of natural ion channel proteins have a wide range of applications, including therapeutic treatments, separation membranes, sensing, and biotechnologies. However, the development of polymer-based artificial channels has been hampered due to the limitation on available models. In this study, we demonstrate the great potential of bottlebrush polymers as accessible and versatile molecular scaffolds for developing efficient artificial ion channels. Adopting the bottlebrush configuration enhanced ion transport activity of the channels compared to their linear analogs. Matching the structure of lipid bilayers, the bottlebrush channel with a hydrophilic-hydrophobic-hydrophilic triblock architecture exhibited the highest activity among the series. Functionalized with urea groups, these channels displayed high anion selectivity. Additionally, we illustrated that the transport properties could be fine-tuned by modifying the chemistry of ion binding sites. This work not only highlights the importance of polymer topology control in channel design, but also reveals the great potential for further developing bottlebrush channels with customized features and diverse functionalities.

π-Conjugation effects on excited-state intermolecular proton-transfer reactions of anthracene-urea derivatives in the presence of acetate anions

This study investigated emissive urea compounds with an anthryl moiety on one side and a substituent group (biphenyl, naphthyl, benzyl, or cyclohexyl) on the other side across from the urea group. This was performed to determine the contribution of π-conjugation on a substituent group to excited-state intermolecular proton-transfer (ESPT) reactions in the presence of acetate anions. Fluorescence lifetime measurements revealed that the rate constant of the ESPT reaction from the normal form to the tautomer form increased with the length of the π-conjugation. Considering that there were a few differences among the wavelengths of the fluorescence maxima for the anthracene-urea derivatives in the presence of acetate anions, we observed that the extension of π-conjugation promoted tautomer formation. This maintained the energy levels of the normal and tautomer forms in the excited state. Furthermore, an anthracene-urea derivative without π-conjugation did not undergo a reverse ESPT reaction, implying that π-conjugation is considerably involved in the reverse ESPT reaction from the tautomer form to the normal form.

Substituent effects of halogens on the excited-state intermolecular proton transfer reactions

Fluorescent aromatic urea compounds undergo excited-state intermolecular proton transfer (ESPT) in the presence of acetate anions to produce an excited state of the tautomer (T*) from the excited state of the complex (N*), resulting in dual fluorescence. Herein, we performed spectroscopic measurements of anthracen-1-yl-3-phenylurea derivatives with substituents, -CF3, -F, or -Cl, at the p-position of the phenyl group in the presence of acetate to investigate the substituent effects on the ESPT reaction and the deactivation processes of N* and T*. Kinetic analysis showed that the reverse ESPT rate constant (k-PT) depended on the respective substituents, suggesting that each substituent may influence the reverse ESPT process differently. In particular, since the electron-withdrawing properties of -F are estimated by the - I and + Iπ effects, it is plausible that -F has a slight electron-donating property and influences the reverse process from T* to N* in the excited state. This study shows that it is possible to control emission by selecting specific substituents in the ESPT system.

Recent Advances in Thiourea Based Colorimetric and Fluorescent Chemosensors for Detection of Anions and Neutral Analytes: A Review

Thioureas and their derivatives are organosulfur compounds having excellent biological and non-biological applications. These compounds contain S- and N-, which are nucleophilic and allow for establishing inter-and intramolecular hydrogen bonding. These characteristics make thiourea moiety a very important chemosensor to detect various environmental pollutants. This article covers a broad range of thioureas and their derivatives that are used for highly sensitive, selective, and simple fluorimetric (turn-off and turn-on), and colorimetric chemosensors for the detection and determination of different types of anions, such as CN-, AcO-, F-, ClO- and citrate ions, etc., and neutral analytes such as ATP, DCP, and Amlodipine, etc., in biological, environmental, and agriculture samples. Further, the sensing performances of thioureas-based chemosensors have been compared and discussed, which could help the readers for the future design of organic fluorescent and colorimetric sensors to detect anions and neutral analytes. We hope this study will support the new thoughts to design highly efficient, selective, and sensitive chemosensors to detect different analytes in biological, environmental, and agricultural samples.

Nitrophenyl Thiourea-Modified Polyethylenimine Colorimetric Sensor for Sulfate, Fluorine, and Acetate

A thiourea-based colorimetric sensor incorporating polyethyleneimine (PEI) and chromophoric nitrophenyl groups was synthesized and utilized for detecting various anions. Structural characterization of the sensor was accomplished using FTIR and 1H-NMR spectroscopy. The sensor's interactions and colorimetric recognition capabilities with different anions, including CI-, Br-, I-, F-, NO3-, PF6-, AcO-, H2PO4-, PO43-, and SO42-, were investigated via visual observation and UV/vis spectroscopy. Upon adding SO42-, F-, and AcO- anions, the sensor exhibited distinct color changes from colorless to yellow and yellowish, while other anions did not induce significant color alterations. UV/vis spectroscopic titration experiments conducted in a DMSO/H2O solution (9:1 volume ratio) demonstrated the sensor's selectivity toward SO42-, F-, and AcO-. The data revealed that the formation of the main compounds and anion complexes was mediated by hydrogen bonding, leading to signal changes in the nitrophenyl thiourea-modified PEI spectrum.

Antioxidant, antibacterial, enzyme inhibition and fluorescence characteristics of unsymmetrical thiourea derivatives

A series of six unsymmetrical thiourea derivatives, namely 1-cyclohexyl-3-(pyridin-2-yl) thiourea (1), 1-cyclohexyl-3-(3-methylpyridin-2-yl)thiourea (2), 1-cyclohexyl-3-(2,4-dimethylphenyl) thiourea (3), 1-(4-chlorophenyl)-3-cyclohexylthiourea (4), 1-(3-methylpyridin-2-yl)-3-phenylthiourea (5), and 1-(3-chlorophenyl)-3-phenylthiourea (6), were successfully synthesized via reaction between different amines with isothiocyanates under a non-catalytic environment. Structural elucidation of compounds (1-6) was performed using FT-IR and NMR (1H and 13C) spectroscopy. The infrared spectra displayed characteristic stretching vibrations, while the 13C NMR chemical shifts of the thiourea moiety (C[bond, double bond]S) were observed in the range of 179.1-181.4 ppm. The antioxidative and antimicrobial properties of the compounds were assessed, as well as their inhibitory effects on acetylcholinesterase and butyrylcholinesterase were evaluated. In order to analyze the fluorescence characteristics of each compound (1-6), the excitation (λex) and emission (λem) wavelengths were scanned within the range of 250-750 nm, with the solvent blank serving as a standard. It was observed that when dissolved in acetone, toluene, tetrahydrofuran, and ethyl acetate, these compounds exhibited emission peaks ranging from 367 to 581 nm and absorption peaks ranging from 275 to 432 nm.

Exploring the latest trends in chemistry, structure, coordination, and diverse applications of 1-acyl-3-substituted thioureas: a comprehensive review

Acyl thioureas represent a privileged moiety with vast potential applicability across diverse fields, making them the subject of extensive research efforts. The inherent flexibility of thiourea facilitates the synthesis of a wide range of core structures with diverse functionalities and properties. The distinctive presence of hard and soft donor sites renders acyl thioureas inclined to act as versatile ligands, thereby engendering a diverse array of metal complexes incorporating acyl thiourea as a pivotal ligand. Extensive investigations into the synthesized acyl thioureas and their derivatives have culminated in the elucidation of their substantial potential across a spectrum of applications, spanning biological activities, materials chemistry, catalysis, and beyond. This literature review represents a continuation of our ongoing endeavor to compile comprehensive data on research endeavors concerning acyl thioureas over the past two years.

Recognition of Amino Acid Salts by Temperature-Dependent Allosteric Binding with Stereodynamic Urea Receptors

Positive homotropic artificial allosteric systems are important for the regulation of cooperativity, selectivity and nonlinear amplification. Stereodynamic homotropic allosteric receptors can transmit and amplify induced chirality by the first ligand binding to axial chirality between two chromophores. We herein report stereodynamic allosteric urea receptors consisting of a rotational shaft as the axial chirality unit, terphenyl units as structural transmission sites and four urea units as binding sites. NMR titration experiments revealed that the receptor can bind two carboxylate guests in a positive homotropic allosteric manner attributed to the inactivation by intramolecular hydrogen-bonding between urea units within the receptor. In addition, the VT-CD spectra observed upon binding of the urea receptor with l- or D-amino acid salts in MeCN showed interesting temperature-dependent Cotton effects, based on the differences of the receptor shaft unit and the guest structure. The successful discrimination of hydrocarbon-based side chains of amino acid salts indicated that the input of chiral and steric information for the guest was amplified as outputs of the Cotton effect and the temperature-dependence of VT-CD spectra through cooperativity of positive allosteric binding.

Cycloglycolurils: Hybrid Glycoluril-Cyclobenzil Macrocycles

Two novel glycoluril macrocycles have been synthesized from cyclotetrabenzil and cyclotribenzoin precursors using solvent-free condensations with urea. The crystal structure of the cyclotetra(p-phenylene)glycoluril macrocycle shows a twisted ring conformation, while that of the cyclotri(m-phenylene)glycoluril hybrid exhibits a distinct tubular supramolecular packing. These structures establish a potentially broad new class of macrocycles with intriguing guest binding properties owing to their available N-H motifs.

Thiourea-Based Receptors for Anion Recognition and Signaling

This work reports on two thiourea-based receptors with pyridine and amine units including 1-naphthyl (MT1N) and 4-nytrophenyl (MT4N) as signaling units. For both compounds, their affinity and signaling ability toward various anions of different geometry and basicity in DMSO were studied using UV-vis, fluorescence, and 1H NMR techniques. Anion recognition studies revealed that both MT1N and MT4N have, in general, high affinities toward basic anions. In this regard, a higher acidity of the MT4N receptor was demonstrated. Furthermore, MT4N has a higher affinity for fluoride (log K1 = 5.98) than for the other anions and can effectively detect it through colorimetric changes that can be monitored by the UV-vis technique. The interaction between receptors and anions mainly involves the hydrogens of the amino and thiourea groups of the former. Complementary single-crystal X-ray diffraction studies and molecular modeling at the DFT level were also performed.

Anion-Dependent Hydrogen-Bond Polarity Switching in Ethylene-bridged Urea Oligomers

The reversible coordination of anions to an N,N'-disubstituted 3,5-bis(trifluoromethyl)phenylurea located at a terminus of a linear chain of ethylene-bridged hydrogen-bonded ureas triggers a cascade of conformational changes. A series of hydrogen-bond polarity reversals propagates along the oligomer, leading to a global switch of its hydrogen-bond directionality. The induced polarity switch, transmitted through four reversible urea groups, results in a change in emission and excitation wavelengths of a fluorophore located at the opposite terminus of the oligomer. The molecule thus behaves as a chemical sensor with a relayed remote spectroscopic response to variations in anion concentration. The polarity switch induced by anion concentration constitutes an artificial communication mechanism for conveying information through oligomeric structures.

Mechanosynthesis of Polyureas and Studies of Their Responses to Anions

Polyureas (PUs) have already found wide practical applications, and various methods of their synthesis have been reported. In this manuscript, we wished to report the very first mechanochemical approach towards aromatic PUs via reactions between isomeric 2,2'-, 3,3'-, and 4,4'-diaminobiphenyls and triphosgene under solvent-free conditions following ball-milling. By using this synthetic approach, both PUs and azomethine-capped Pus were obtained. The fluorescence response of the above-mentioned PUs towards various anions in solutions were studied and selective fluorescence responses towards the hydroxyl and fluoride anions were observed.

Gold nanoshells with magnetic cores and a urea-based receptor for SERS sensing of fluoride anions: experimental and computational study

The study demonstrates that a combination of plasmonic nanostructures and artificial receptors can be applied for sensing small molecular species. Gold nanoshells containing magnetic cores are used as the SERS-active substrates, which opens the way for the development of multimodal contrast agents with applicability extended to sensing or for the separation of analytes by magnetic solid-phase extraction. Disubstituted ureas forming hydrogen-bonded complexes with certain anions can be employed as molecular sensors. In this case study, gold nanoshells with silica-coated Mn-Zn ferrite cores were prepared by a multistep procedure. The nanoshells were co-functionalized with an N-(4-mercaptophenyl)-N'-(4-nitrophenyl)urea sensor synthesized directly on the gold surface, and with 4-nitrothiophenol, which is adopted as an internal standard. SERS measurements were carried out with acetonitrile solutions of tetrabutylammonium fluoride (Bu4NF) over a concentration range of 10-10-10-1 mol L-1. The spectral response of the sensor is dependent on the fluoride concentration in the range of 10-5-10-1 mol L-1. To investigate further the SERS mechanism, a model sensor, N-(4-bromophenyl)-N'-(4-nitrophenyl)urea, was synthesized and used in Raman spectroscopy with solutions of Bu4NF, up to a molar ratio of 1 : 20. The spectra and the interactions between the sensors and fluoride anions were also studied by extensive DFT computations.

Acid-Functionalized Artificial Enzymes with Tunable Selectivity for Vinyl Ether Hydrolysis

Enzymes often employ catalytic groups with a medium or low intrinsic activity for highly challenging catalytic tasks. In this work, we report nanoparticle catalysts with accurately positioned carboxylic acids through either a covalent or noncovalent imprinting technique. The rationally designed active site allows the catalysis to be highly selective or quite unselective with respect to the substrate. With the proper catalyst, the hydrolysis proceeds smoothly in neutral water or even a slightly basic solution at room temperature.

Stimulus-Controlled Anion Binding and Transport by Synthetic Receptors

Anionic species are omnipresent and involved in many important biological processes. A large number of artificial anion receptors has therefore been developed. Some of these are capable of mediating transmembrane transport. However, where transport proteins can respond to stimuli in their surroundings, creation of synthetic receptors with stimuli-responsive functions poses a major challenge. Herein, we give a full overview of the stimulus-controlled anion receptors that have been developed thus far, including their application in membrane transport. In addition to their potential operation as membrane carriers, the use of anion recognition motifs in forming responsive membrane-spanning channels is discussed. With this review article, we intend to increase interest in transmembrane transport among scientists working on host-guest complexes and dynamic functional systems in order to stimulate further developments.

Switching imidazole reactivity by dynamic control of tautomer state in an allosteric foldamer

Molecular biology achieves control over complex reaction networks by means of molecular systems that translate a chemical input (such as ligand binding) into an orthogonal chemical output (such as acylation or phosphorylation). We present an artificial molecular translation device that converts a chemical input - the presence of chloride ions - into an unrelated chemical output: modulation of the reactivity of an imidazole moiety, both as a Brønsted base and as a nucleophile. The modulation of reactivity operates through the allosteric remote control of imidazole tautomer states. The reversible coordination of chloride to a urea binding site triggers a cascade of conformational changes in a chain of ethylene-bridged hydrogen-bonded ureas, switching the chain's global polarity, that in turn modulates the tautomeric equilibrium of a distal imidazole, and hence its reactivity. Switching reactivities of active sites by dynamically controlling their tautomer states is an untapped strategy for building functional molecular devices with allosteric enzyme-like properties.

One-Pot Cyclization to Large Peptidomimetic Macrocycles by In Situ-Generated β-Turn-Enforced Folding

Macrocycles have been targets of extensive synthetic efforts for decades because of their potent molecular recognition and self-assembly capabilities. Yet, efficient syntheses of macrocyclic molecules via irreversible covalent bonds remain challenging. Here, we report an efficient approach to large peptidomimetic macrocycles by using the in situ-generated β-turn structural motifs afforded in the amidothiourea moieties from the early steps of the reaction of 2 molecules of bilateral amino acid-based acylhydrazine with 2 molecules of diisothiocyanate. Four chiral and achiral peptidomimetic large macrocycles were successfully synthesized in high yields of 45-63% in a feasible one-pot reaction under sub-molar concentration conditions and were purified by simple filtration. X-ray crystallographic characterization of three macrocycles reveals an important feature that their four β-turn structures, each maintained by four 10-membered intramolecular hydrogen bonds, alternatively network the four aromatic arms. This affords an interesting conformation switching mode upon anion binding. Binding of SO₄^2- to 1L or 1D that contains 4 alanine residues (with the lowest steric hinderance among the macrocycles) leads to an inside-out structural change of the host macrocycle, as confirmed by the X-ray crystal structure of 1L-SO₄^2- and 1D-SO₄^2- complexes, accompanied by an inversion of the CD signals. On the basis of the strong sulfate affinity of the macrocycles, we succeeded in the removal of sulfate anions from water via a macrocycle-mediated liquid-liquid extraction method. Our synthetic protocol can be easily extended to other macrocycles of varying arms and/or chiral amino acid residues; thus, a variety of structurally and functionally diverse macrocycles are expected to be readily made.

Diserinol Isophthalamide: A Novel Reagent for Complexation with Biomolecular Anions in Electrospray Ionization Mass Spectrometry

Transferring biomolecules from solution to vacuum facilitates a detailed analysis of molecular structure and dynamics by isolating molecules of interest from a complex environment. However, inherent in the ion desolvation process is the loss of solvent hydrogen bonding partners, which are critical for the stability of a condensed-phase structure. Thus, transfer of ions to vacuum can favor structural rearrangement, especially near solvent-accessible charge sites, which tend to adopt intramolecular hydrogen bonding motifs in the absence of solvent. Complexation of monoalkylammonium moieties (e.g., lysine side chains) with crown ethers such as 18-crown-6 can disfavor structural rearrangement of protonated sites, but no equivalent ligand has been investigated for deprotonated groups. Herein we describe diserinol isophthalamide (DIP), a novel reagent for the gas-phase complexation of anionic moieties within biomolecules. Complexation is observed to the C-terminus or side chains of the small model peptides GD, GE, GG, DF-OMe, VYV, YGGFL, and EYMPME in electrospray ionization mass spectrometry (ESI-MS) studies. In addition, complexation is observed with the phosphate and carboxylate moieities of phosphoserine and phosphotyrosine. DIP performs favorably in comparison to an existing anion recognition reagent, 1,1'-(1,2-phenylene)bis(3-phenylurea), that exhibits moderate carboxylate binding in organic solvent. This improved performance in ESI-MS experiments is attributed to reduced steric constraints to complexation with carboxylate groups of larger molecules. Overall, diserinol isophthalamide is an effective complexation reagent that can be applied in future work to study retention of solution-phase structure, investigate intrinsic molecular properties, and examine solvation effects.

Urea- and Thiourea-Based Receptors for Anion Binding

ConspectusOver the past five decades, significant progress has been made in the field of anion recognition with a diverse variety of synthetic receptors because of the fundamental importance of anions in chemical, environmental, and biological processes. In particular, urea- and thiourea-based molecules offering directional binding sites are attractive receptors for anions due to their ability to bind anions employing primarily hydrogen-bonding interactions under neutral conditions and have gained a recent paramount attention in the area of supramolecular chemistry. The presence of two imine (-NH) groups on each urea/thiourea functionality in these receptors gives them potential for excellent binding of an anion, mimicking the natural binding process in living cells. The increased acidity offered by thiocarbonyl groups (C═S) in a thiourea-functionalized receptor could enhance its anion binding ability as compared to its analogous urea-based receptor containing a carbonyl (C═O) group. During the last several years, our group has been involved in exploring a wide variety of synthetic receptors, and we have studied them with anions experimentally and computationally. In this Account, we will highlight the overall summary of our group's efforts focusing on anion coordination chemistry of urea- and thiourea-based receptors with varying linkers (rigid and flexible), dimensions (dipodal and tripodal), and functionalities (bifunctional, trifunctional, and hexafunctional). Depending on the linkers and attached groups, bifunctional-based dipodal receptors can bind anions forming 1:1 or 1:2 complexes. A dipodal receptor with flexible aliphatic or rigid m-xylyl linkers forms a cleft to bind a single anionic species in the pocket. However, a dipodal receptor with p-xylyl linkers binds anions in both 1:1 and 1:2 binding modes. As compared to a dipodal receptor, a tripodal receptor provides a more organized cavity for an anion, forming predominantly a 1:1 complex, while the binding strength and selectivity are influenced by linking chains and terminal groups. A hexafunctional-based tripodal receptor bridged with o-phenylene groups provides two clefts that can host two small anions or one large anion. However, a hexafunctional receptor with p-phenylene groups as linkers binds two anions, one at an inner pocket and the other at an outer pocket. It was shown that the presence of suitable chromophores at the terminal groups makes the receptor useful for the naked-eye detection for certain anions (e.g., fluoride, acetate) in solution. The field of anion binding chemistry is rapidly growing, and this Account aims to provide fundamental aspects influencing the binding strength and selectivity of anionic species with abiotic receptors which might eventually be useful for the development of new devices for binding, sensing, and separating biologically and environmentally important anions.

Synthesis of new diphenyl urea-clubbed imine analogs and its Implications in diabetic management through in vitro and in silico approaches

Type II diabetes mellitus (T2DM) is a global health issue with high rate of prevalence. The inhibition of α-glucosidase enzyme has prime importance in the management of T2DM. This study was established to synthesize Schiff bases of 1,3-dipheny urea (3a-y) and to investigate their in vitro anti-diabetic capability via inhibiting α-glucosidase, a key player in the catabolism of carbohydrates. The structures of all compounds were confirmed through various techniques including, Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) and mass-spectrometry (MS) methods. Interestingly all these compounds displayed potent inhibition IC₅₀ values in range of 2.14-115 µM as compared to acarbose used as control. Additionally, all the compounds were docked at the active site of α-glucosidase to predict their mode of binding. The docking results indicates that Glu277 and Asn350 play important role in the stabilization of these compounds in the active site of enzyme. These molecules showed excellent predicted pharmacokinetics, physicochemical and drug-likeness profile. The anti-diabetic potential of these molecules signifies their medical importance and provide insights into prospective therapeutic options for the treatment of T2DM.

A Cu+ /Thiourea Dendrimer Achieves Excellent Cytosolic Protein Delivery via Enhanced Cell Uptake and Endosome Escape

Intracellular protein delivery has attracted considerable attention in the development of protein-based therapeutics, however, the design of highly efficient materials for robust delivery of native proteins remains challenging. This study proposes a Cu⁺ -based coordination polymer for cytosolic protein delivery with high efficacy and robustness. The phenylthiourea grafted dendrimer is coordinated with cuprous ions to prepare the polymeric carrier, which efficiently bind cargo proteins via a combination of coordination, ionic and hydrophobic interactions. The incorporation of Cu⁺ ions in the polymer greatly improves its cellular uptake and endosomal escape. The cuprous-based coordination polymer successfully delivered a variety of structurally diverse proteins into various cell lines with reserved bioactivities. This study provides a new type of coordination polymers for cytosolic delivery of biomacromolecules.

Unravelling Structural Dynamics, Supramolecular Behavior, and Chiroptical Properties of Enantiomerically Pure Macrocyclic Tertiary Ureas and Thioureas

The introduction of urea or thiourea functionality to the macrocycle skeleton represents an alternative way to control conformational dynamics of chiral, polyamines of a figure-shaped periodical structure. Formally highly symmetrical, these macrocycles may adapt diverse conformations, depending on the nature of an amide linker and on a substitution pattern within the aromatic units. The type of heteroatom X in the N-C(═X)-N units present in each vertex of the macrocycle core constitutes the main factor determining the chiroptical properties. In contrast to the urea-containing derivatives, the electronic circular dichroism of thioureas is controlled by the chiral neighborhood closest to the chromophore. The dynamically induced exciton couplet is observed when the biphenyl chromophores are present in the macrocycle core. In the solid state, the seemingly disordered molecules may create ordered networks stabilized by intermolecular S···halogen, H···halogen, and S···H interactions. The presence of two bromine substituents in each aromatic unit in thiourea-derived trianglamine gives rise to a self-sorting phenomenon in the crystal. In solution, this particular macrocycle exists as a dynamic equimolar mixture of two conformational diastereoisomers, differing in the spatial (clockwise and counter clockwise) arrangement of the C-Br bonds. In the crystal lattice, macrocycles of a given handedness assemble into homohelical layers.

Protonophoric and mitochondrial uncoupling activity of aryl-carbamate substituted fatty acids

Aryl-urea substituted fatty acids are protonophores and mitochondrial uncouplers that utilise a urea-based synthetic anion transport moiety to carry out the protonophoric cycle. Herein we show that replacement of the urea group with carbamate, a functional group not previously reported to possess anion transport activity, produces analogues that retain the activity of their urea counterparts. Thus, the aryl-carbamate substituted fatty acids uncouple oxidative phosphorylation and inhibit ATP production by collapsing the mitochondrial proton gradient. Proton transport proceeds via self-assembly of the deprotonated aryl-carbamates into membrane permeable dimeric species, formed by intermolecular binding of the carboxylate group to the carbamate moiety. These results highlight the anion transport capacity of the carbamate functional group.

OSCAR: an extensive repository of chemically and functionally diverse organocatalysts

The automated construction of datasets has become increasingly relevant in computational chemistry. While transition-metal catalysis has greatly benefitted from bottom-up or top-down strategies for the curation of organometallic complexes libraries, the field of organocatalysis is mostly dominated by case-by-case studies, with a lack of transferable data-driven tools that facilitate both the exploration of a wider range of catalyst space and the optimization of reaction properties. For these reasons, we introduce OSCAR, a repository of 4000 experimentally derived organocatalysts along with their corresponding building blocks and combinatorially enriched structures. We outline the fragment-based approach used for database generation and showcase the chemical diversity, in terms of functions and molecular properties, covered in OSCAR. The structures and corresponding stereoelectronic properties are publicly available (https://archive.materialscloud.org/record/2022.106) and constitute the starting point to build generative and predictive models for organocatalyst performance.

Syntheses and Structural Characterization of Divalent Metal Complexes (Co, Ni, Pd and Zn) of Sterically Hindered Thiourea Ligand and A Theoretical Insight of their Interaction with SARS-CoV-2 Enzyme

Reacting two equivalents of sterically hindered 1,3-bis(2,6-diethylphenyl)thiourea ligand (L) with CoCl₂, NiBr₂, PdX₂ (X = Cl; Br) and ZnI₂ in acetonitrile afforded the corresponding bulky thiourea ligand stabilized four coordinated monomeric [L₂CoCl₂] (1), [L₂NiBr₂] (2), [L₂PdX₂] (3a: X = Cl; 3b: X = Br) and [L₂ZnI₂] (4.2CH₃CN) complexes. Compound 1, 2 and 4.2CH₃CN are tetrahedral whereas Pd complexes (3a and 3b) are square planar. In solution, palladium complexes are dominated by cis-isomers. Structural characterization shows inter- and intramolecular hydrogen bonding. Hirshfeld surface and fingerprint plots indicated significant intermolecular interactions in the crystal network. Molecular docking analysis revealed relatively higher SARS-CoV-2 enzyme interacting abilities of the synthesized complexes compared to the free ligand. All compounds have been characterized by elemental analyses, NMR spectroscopy and single-crystal X-ray diffraction.

Structure-Activity Relationship and Mechanistic Studies of Bisaryl Urea Anticancer Agents Indicate Mitochondrial Uncoupling by a Fatty Acid-Activated Mechanism

Targeting the cancer cell mitochondrion is a promising approach for developing novel anticancer agents. The experimental anticancer agent N,N'-bis(3,5-dichlorophenyl)urea (SR4) induces apoptotic cell death in several cancer cell lines by uncoupling mitochondrial oxidative phosphorylation (OxPhos) using a protein-free mechanism. However, the precise mechanism by which SR4 depolarizes mitochondria is unclear because SR4 lacks an acidic functional group typically found in protein-independent uncouplers. Recently, it was shown that structurally related thioureas can facilitate proton transport across lipid bilayers by a fatty acid-activated mechanism, in which the fatty acid acts as the site of protonation/deprotonation and the thiourea acts as an anion transporter that shuttles deprotonated fatty acids across the phospholipid bilayer to enable proton leak. In this paper, we show that SR4-mediated proton transport is enhanced by the presence of free fatty acids in the lipid bilayer, indicating that SR4 uncouples mitochondria through the fatty acid-activated mechanism. This mechanistic insight was used to develop a library of substituted bisaryl ureas for structure-activity relationship studies and subsequent cell testing. It was found that lipophilic electron-withdrawing groups on bisaryl ureas enhanced electrogenic proton transport via the fatty acid-activated mechanism and had the capacity to depolarize mitochondria and reduce the viability of MDA-MB-231 breast cancer cells. The most active compound in the series reduced cell viability with greater potency than SR4 and was more effective at inhibiting adenosine triphosphate production.

Dynamic Covalent Self-Assembly of Chloride- and Ion-Pair-Templated Cryptates

While supramolecular hosts capable of binding and transporting anions and ion pairs are now widely available, self-assembled architectures are still rare, even though they offer an inherent mechanism for the release of the guest ion(s). In this work, we report the dynamic covalent self-assembly of tripodal, urea-based anion cryptates that are held together by two orthoester bridgeheads. These hosts exhibit affinity for anions such as Cl- , Br- or I- in the moderate range that is typically advantageous for applications in membrane transport. In unprecedented experiments, we were able to dissociate the Cs⋅Cl ion pair by simultaneously assembling suitably sized orthoester hosts around the Cs+ and the Cl- ion.

Mononuclear Tricoordinate Copper(I) and Silver(I) Halide Complexes of a Sterically Bulky Thiourea Ligand and a Computational Insight of Their Interaction with Human Insulin

Reaction of two equivalents of the bulky 1,3-bis(2,6-diethylphenyl)thiourea ligand (L) with MX (being M = Cu+, Ag+; and X = Cl-, Br-, I-) in acetonitrile afforded neutral complexes of the type [MXL2] [CuClL2].2CH3CN (1a); [CuBrL2].2CH3CN (1b); [CuIL2] (1c): [AgClL2] (2a); [AgBrL2] (2b) and [AgIL2] (2c). The two aromatic groups in free ligand were found to be trans with respect to the thiourea unit, which was a reason to link the ligand molecules via intermolecular hydrogen bonding. Intramolecular hydrogen bonding was observed in all metal complexes. The copper complexes 1a and 1b are acetonitrile solvated and show not only intra- but also intermolecular hydrogen bonding between the coordinated thiourea and the solvated acetonitrile molecules. Silver complexes reported here are the first examples of structurally characterized tricoordinated thiourea-stabilized monomeric silver(I) halides. Molecular docking studies were carried out to analyze the binding modes of the metal complexes inside the active site of the human insulin (HI) protein. Analysis of the docked conformations revealed that the electrostatic and aromatic interactions of the protein N-terminal residues (i.e., Phe and His) may assist in anchoring and stabilizing the metal complexes inside the active site. According to the results of docking studies, the silver complexes exhibited the strongest inhibitory capability against the HI protein, which possesses a deactivating group, directly bonded to silver. All compounds were fully characterized by elemental analysis, NMR spectroscopy, and molecular structures of the ligand, and five out of six metal complexes were also confirmed by single-crystal X-ray diffraction.

Three modes of interactions between anions and phenolic macrocycles: a comparative study

Macrocyclic polyphenolic compounds such as resorcin[4]arenes can be considered as multidentate anion receptors. In the current work, we combine new experimental data and reports from the previous literature (solution data and deposited crystal structures from the CCDC) to systematically analyze binding motifs between resorcin[4]arene derivatives and anions, determine the role of supporting interactions from CH donors, ion pairing and estimate their relative strength. We have found that in medium polarity solvents (THF) anion binding is a main driving force for the formation of complexes between resorcinarenes and Alk₄NX salts. Three binding modes have been detected using ¹H NMR and DOSY, depending on the type of additional interactions. Mode I was observed for upper-rim unsubstituted resorcinarenes, which use OH groups and aromatic CH from the upper rim as hydrogen bond donors to form multidentate and multivalent binding sites at the upper rim. Mode II was observed for upper-rim halogenated resorcinarenes (tetrabromo- and tetraiodo-derivatives), which use OH groups and aliphatic CH atoms from the bridges to support the chelation of anions between aromatic units. This binding mode is also multidentate and multivalent, but weaker and more anion-selective than mode I (works effectively for chlorides but not for bromides). For O-substituted derivatives, mode III is observed, with anions bound in a nest formed by aromatic CH atoms in the lower rim (multidentate but monovalent binding). The relative strength of these three binding modes, their solvent-dependence, and emergence in the crystal structures (CCDC) have been evaluated.

Hydrogen Bonding Phase-Transfer Catalysis with Alkali Metal Fluorides and Beyond

Phase-transfer catalysis (PTC) is one of the most powerful catalytic manifolds for asymmetric synthesis. Chiral cationic or anionic PTC strategies have enabled a variety of transformations, yet studies on the use of insoluble inorganic salts as nucleophiles for the synthesis of enantioenriched molecules have remained elusive. A long-standing challenge is the development of methods for asymmetric carbon-fluorine bond formation from readily available and cost-effective alkali metal fluorides. In this Perspective, we describe how H-bond donors can provide a solution through fluoride binding. We use examples, primarily from our own research, to discuss how hydrogen bonding interactions impact fluoride reactivity and the role of H-bond donors as phase-transfer catalysts to bring solid-phase alkali metal fluorides in solution. These studies led to hydrogen bonding phase-transfer catalysis (HB-PTC), a new concept in PTC, originally crafted for alkali metal fluorides but offering opportunities beyond enantioselective fluorination. Looking ahead, the unlimited options that one can consider to diversify the H-bond donor, the inorganic salt, and the electrophile, herald a new era in phase-transfer catalysis. Whether abundant inorganic salts of lattice energy significantly higher than those studied to date could be considered as nucleophiles, e.g., CaF₂, remains an open question, with solutions that may be found through synergistic PTC catalysis or beyond PTC.