Anion Binding by Bambus[6]uril Probed in the Gas Phase and in Solution

Large anion binding in water

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

Perchlorate Solid-Contact Ion-Selective Electrode Based on Dodecabenzylbambus[6]uril

Dodecabenzylbambus[6]uril (Bn12BU[6]) is an anion receptor that binds the perchlorate ion the most tightly (stability constant ~1010 M−1) of all anions due to the excellent match between the ion size in relation to the receptor cavity. This new bambusuril compound was used as an ionophore in the ion-selective membrane (ISM) to develop ion selective electrodes (ISEs) for determination of perchlorate concentration utilizing the poly(3,4-ethylenedioxythiophene) (PEDOT) polymer film as a solid-contact material. Variation of the content of Bn12BU[6] and tridodecylmethylammonium chloride (TDMACl) in the plasticized poly(vinyl chloride)-based ISM was also tested. All the prepared solid-contact ISEs and their analytical performance were characterized by potentiometry, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The ISEs showed rapid response and a sub-Nernstian slope (~57 mV/decade) during potentiometric measurements in perchlorate solutions in the concentration range from 10−1 to 10−6 M simultaneously with their high stability and sufficient selectivity to other common inorganic anions like bromide, chloride, nitrate and sulphate. The function of the ISE was further verified by analysis of real water samples (lake, sea, and mineral water), which gave accurate and precise results.

Halide Size-Selective Binding by Cucurbit[5]uril-Alkali Cation Complexes in the Gas Phase

We report data that suggest complexes with alkali cations capping the portals of cucurbit[5]uril (CB[5]) bind halide anions size-selectively as observed in the gas phase: Cl^- binds inside the CB[5] cavity, Br^- is observed both inside and outside, and I^- binds weakly outside. This is reflected in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: all detected Cl^- complexes dissociate at higher energies, and Br^- complexes exhibit unusual bimodal dissociation behavior, with part of the ion population dissociating at very low energies and the remainder dissociating at significantly higher energies comparable to those observed for Cl^-. Decoherence cross sections measured in SF₆ using cross-sectional areas by Fourier transform ion cyclotron resonance techniques for [CB[5] + M₂X]⁺ (M = Na, X = Cl or Br) are comparable to or less than that of [CB[5] + Na]⁺ over a wide energy range, suggesting that Cl^- or Br^- in these complexes are bound inside the CB[5] cavity. In contrast, [CB[5] + K₂Br]⁺ has a cross section measured about 20% larger than that of [CB[5] + Na]⁺, suggesting external binding that may correspond with the weakly bound component seen in SORI. While I^- complexes with alkali cation caps were not observed, alkaline earth iodides with CB[5] yielded complexes with cross sections 5-10% larger than that of [CB[5] + Na]⁺, suggesting externally bound iodide. Geometry optimization at the M06-2X/6-31+G* level of ab initio theory suggests that internal anion binding is energetically favored by approximately 50-200 kJ mol^-1 over external binding; thus, the externally bound complexes observed experimentally must be due to large energetic barriers hindering the passing of large anions through the CB[5] portal, preventing access to the interior. Calculation of the barriers to anion egress using MMFF//M06-2X/6-31+G* theory supports this idea and suggests that the size-selective binding we observe is due to anion size-dependent differences in the barriers.

Synthesis of urease inhibitory 2, 4-bis (4-cyanobenzyl)glycoluril using sandmeyer reaction and density functional theory investigation

AIMS

The aim of present research was to synthesize glycoluril derivative 2,4-Bis(4-cyanobenzyl)glycoluril through convergent scheme.

BACKGROUND

For this purpose Sandmeyer reaction procedure was employed for the synthesis of said compound. The structure of the pure compound was confirmed by using different spectroscopic techniques such as 1H-NMR, 13C-NMR and (HR-MS) Mass spectrometry.

OBJECTIVE

Convergent synthesis of 2,4-BIS (4-CYANOBENZYL)GLYCOLURIL USING SANDMEYER REACTION and urease inhibition study.

METHODS

The structure of the pure compound was confirmed by using different spectroscopic techniques such as 1H-NMR, 13C-NMR and (HR-MS) Mass spectrometry. The electronic properties of newly synthesized compound and thiourea were determined by using density functional theory.

RESULTS

Furthermore compound was evaluated against urease enzyme and was found to be potent inhibitors with IC50 value of 11.5 ± 1.50 µM when compared with standard inhibitor thiourea (IC50 = 21.0 ± 1.90 µM). Compound may serve as lead compound for the synthesis of new cyano based bambusuril in future with enhanced biological properties.

CONCLUSION

We have synthesized a new glycoluril derivative 2,4-Bis(4-cyanobenzyl)glycoluril by the sandmeyer reaction. It has obtained in the form of light yellowish powder in good yield (96%). Glycoluril based macrocycles have been used in various fields. Starting from the 2,4-Bis(4-nitrobenzyl)glycoluril (already reported compound) which has undergone reduction (CH3OH,Pt/C) , diazotization (NaNO2/HCl), cyanation (CuCl/KCN) respectively in order to synthesize the desired new glycoluril derivative. The obtained product will be used as a building block for the synthesis of the cyano based bambusuril marcocycle in future. The yield of the obtained product has been monitored by using different amount of cyanating reagent but the best results shown by the use of 4 mmol of CuCl/KCN. KCN with CuCl assisted the conversion of diazo group into cyano group with enhanced yield when used in excess amount. It act as a catalyst. Solubility characteristic of 2,4-Bis(4-cyanobenzyl)glycoluril has determined also in different organic solvents. 1H NMR technique proved to be very helpful for the structure determination of our desired product. Benzylic protons give signals at 7.5 ppm and 7.8 ppm respectively. The downfield peaks confirm about the presence of CN group near the benzylic protons. Methine protons show signal at 5.2 ppm which ensures about the basic skeleton of glycoluril. Ureidyl protons also confirm the synthesis of the heterocyclic 2,4-Bis(4-cyanobenzyl)glycoluril compound. The negative and positive electrostatic potential sites, molecular descriptors, and charge density distribution of frontier molecular orbitals are revealing that 4a with promising sites for electrophilic and nucleophilic attacks would result to enhance the urease inhbition which is in good agreement with the experimental data.

Aza-Bambusurils En Route to Anion Transporters

Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza-bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio-bambusurils in a one-pot reaction, which converts a single anion receptor into a potential anion channel. Solid-state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Å is reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition-state energy for double-anion movement through the channel suggests that although these host-guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.

Acyclic cucurbit[n]uril-type molecular containers: influence of glycoluril oligomer length on their function as solubilizing agents

We present the synthesis of a series of six new glycoluril derived molecular clips and acyclic CB[n]-type molecular containers (1–3) that all feature SO3(−) solubilizing groups but differ in the number of glycoluril rings between the two terminal dialkoxyaromatic sidewalls. We report the X-ray crystal structure of 3b which shows that its dialkoxynaphthalene sidewalls actively define a hydrophobic cavity with high potential to engage in π–π interactions with insoluble aromatic guests. Compounds 1–3 possess very good solubility characteristics (≥38 mM) and undergo only very weak self-association (Ks < 92 M(−1)) in water. The weak self-association is attributed to unfavorable SO3(−)···SO3(−) electrostatic interactions in the putative dimers 12–42. Accordingly, we created phase solubility diagrams to study their ability to act as solubilizing agents for four water insoluble drugs (PBS-1086, camptothecin, β-estradiol, and ziprasidone). We find that the containers 3a and 3b which feature three glycoluril rings between the terminal dialkoxy-o-xylylene and dialkoxynaphthalene sidewalls are less efficient solubilizing agents than 4a and 4b because of their smaller hydrophobic cavities. Containers 1 and 2 behave as molecular clip type receptors and therefore possess the ability to bind to and thereby solubilize aromatic drugs like camptothecin, ziprasidone, and PBS-1086.

Computational and ion mobility MS study of (all-S)-cyclohexylhemicucurbit[6]uril structure and complexes

A computational study of (all-S)-cyclohexylhemicucurbit[6]uril and its complexes with anions (Cl(-), Br(-), I(-) and HCOO(-)), the proton (H(+)) and non-dissociated acid (HCl, HBr, HI and HCOOH) guests was performed. The geometries of guest-host complexes were optimized via density functional theory using the BP86 functional, SV(P) basis set and Stuttgart pseudopotentials for iodide. Binding affinities and their trends were evaluated at the BP86/TZVPD level of theory. In addition, the quantum theory of atoms in molecules was used to gain insight into guest-host interactions. A computational study in the gas phase and ion-mobility mass-spectrometry analysis revealed that the studied macrocycle formed inclusion complexes with anions. Protonation of the macrocycle is preferred at the nitrogen atom pointing inside of the cavity. In the studied conditions, non-dissociated acids formed complexes at the oxygen atom pointing outside of the macrocycle.

Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle

A p-xylyl-based macrocycle L has been synthesized and its binding properties with halides have been investigated by (1)H NMR titrations, single crystal X-ray diffraction analysis, and density functional theory (DFT) calculations. As investigated by (1)H NMR titrations, the ligand preferentially binds a halide in a 1:2 binding mode, with the association constants (in log K2) of 2.82, 2.70, 2.28, and 2.20 for fluoride, chloride, bromide, and iodide, respectively. The overall binding trend was found to be in the order of fluoride > chloride > bromide > iodide, reflecting that the binding strength correlates with the relative basicity and size of the respective halide. Crystallographic studies indicate that the ligand forms 1:2 complexes with chloride, bromide and iodide. In the chloride complex, the ligand is hexaprotonated and each chloride is held via three NH···Cl(-) bonds. The ligand is tetraprotonated for the other complexes, where each halide is H-bonded to two secondary ammonium NH(+) groups via NH···X(-) bonds. The results of DFT calculations performed on [H6L](6+) at M062x/6-311G (d,p) level in both gas and solvent phases, suggest that the ligand binds halides with the binding energy in the order of F(-) > Cl(-) > Br(-) > I(-), supporting the experimental data obtained from (1)H NMR studies. Results from DFT calculations further indicate that a 1:2 binding is energetically more favorable than a 1:1 binding of the ligand.

Cucurbiturils: from synthesis to high-affinity binding and catalysis

Major developments in the synthesis of cucurbiturils and applications related to their high-affinity binding and catalysis have recently taken place.

Real-time analysis of multiple anion mixtures in aqueous media using a single receptor

Bambusuril macrocycles allow qualitative and quantitative sensing of anion mixtures in water using ¹H NMR spectroscopy.

A bambusuril macrocycle that binds anions in water with high affinity and selectivity

Synthetic receptors that function in water are important for the qualitative and quantitative detection of anions, which may act as pollutants in the environment or play important roles in biological processes. Neutral receptors are particularly appealing because they are often more selective than positively charged receptors; however, their affinity towards anions in pure water is only in range of 1-10(3)  L mol(-1) . The anion-templated synthesis of a water-soluble bambusuril derivative is shown to be an outstanding receptor for various inorganic anions in pure water, with association constants of up to 10(7)  L mol(-1) . Furthermore, the macrocycle discriminates between anions with unprecedented selectivity (up to 500 000-fold). We anticipate that the combination of remarkable affinity and selectivity of this macrocycle will enable the efficient detection and isolation of diverse anions in aqueous solutions, which is not possible with current supramolecular systems.

Water-mediated inclusion of benzoates and tosylates inside the bambusuril macrocycle

A supramolecular complex between benzoates and a bambusuril crystallizes out immediately after mixing in chloroform but only in the presence of residual water molecules. In this complex each of the two portals of the macrocycle is occupied by one benzoate. Carboxylate groups are connected through hydrogen bonding interactions with one molecule of water positioned between them in the center of the bambusuril cavity. Similar water assisted host-guest behavior was also observed when tosylates instead of benzoates were used.

The use of electrospray mass spectrometry to determine speciation in a dynamic combinatorial library for anion recognition

The composition of a dynamic mixture of similar 2,2'-bipyridine complexes of iron(II) bearing either an amide (5-benzylamido-2,2'-bipyridine and 5-(2-methoxyethane)amido-2,2'-bipyridine) or an ester (2,2'-bipyridine-5-carboxylic acid benzylester and 2,2'-bipyridine-5-carboxylic acid 2-methoxyethane ester) side chain have been evaluated by electrospray mass spectroscopy in acetonitrile. The time taken for the complexes to come to equilibrium appears to be dependent on the counteranion, with chloride causing a rapid redistribution of two preformed heteroleptic complexes (of the order of 1 hour), whereas the time it takes in the presence of tetrafluoroborate salts is in excess of 24 h. Similarly the final distribution of products is dependent on the anion present, with the presence of chloride, and to a lesser extent bromide, preferring three amide-functionalized ligands, and a slight preference for an appended benzyl over a methoxyethyl group. Furthermore, for the first time, this study shows that the distribution of a dynamic library of metal complexes monitored by ESI-MS can adapt following the introduction of a different anion, in this case tetrabutylammonium chloride to give the most favoured heteroleptic complex despite the increasing ionic strength of the solution.

Electrospray ionization mass spectrometric investigations of the complexation behavior of macrocyclic thiacrown ethers with bivalent transitional metals (Cu, Co, Ni and Zn)

RATIONALE

Heavy metals are both a problem for the environment and an important resource for industry. Their selective extraction by means of organic ligands therefore is an attractive topic. The coordination of three thiacrown ethers to late 3d-metal ions was investigated by a combination of electrospray ionization mass spectrometry (ESI-MS) and electron paramagnetic resonance (EPR).

METHODS

The mass spectrometric experiments were carried out in an ion trap mass spectrometer with an ESI source. Absolute binding constants were estimated by comparison with data for 18-crown-6/Na(+). EPR spectroscopy was used as a complementary method for investigating the Cu(I) /Cu(II) redox couple.

RESULTS

The study found that thiacrown ethers preferentially bind traces of copper even at an excess of other metal ions (Co(II), Ni(II), and Zn(II)). The absolute association constants of the Cu(I) complexes were about 10(8) M(-1), and about two orders of magnitude lower for the other 3d-metal cations. The EPR spectra demonstrated that the reduction from Cu(II) to Cu(I) upon formation of the [(thiacrown)Cu](+) species takes place in solution.

CONCLUSIONS

ESI-MS demonstrated that the three thiacrown ligands examined had high binding constants as well as good selectivities for copper(I) at low concentrations, and in the presence of other metal ions. By a combination of ESI-MS and EPR spectrometry it was shown that the reduction from Cu(II) to Cu(I) occurred in solution.

Applications of electrospray ionization mass spectrometry in mechanistic studies and catalysis research

Mechanistic studies form the basis for a better understanding of chemical processes, helping researchers develop more sustainable reactions by increasing the yields of the desired products, reducing waste production, and lowering the consumption of resources and energy overall. Conventional methods for the investigation of reaction mechanisms in solution include kinetic studies, isotope labeling, trapping of reactive intermediates, and advanced spectroscopic techniques. Within the past decade, electrospray ionization mass spectrometry (ESI-MS) has provided an additional tool for mechanistic studies because researchers can directly probe liquid samples by mass spectrometry under gentle conditions. Specifically, ESI-MS allows researchers to identify the molecular entities present in solution over the course of a chemical transformation. ESI-MS is particularly useful for investigations of organic reactions or metal catalysis that involve ionic intermediates. Accordingly, researchers are increasingly using ESI-MS in mechanistic studies and catalyst development. However, a further understanding of the ESI process and how it can facilitate mechanistic studies has not accompanied this increased use of the technique. Therefore, at least in part the ESI-MS method not only has offered great promise for the elucidation of reaction mechanisms but also became a black box with the occasional risk of misinterpretation. In this Account, we summarize applications of ESI-MS for synthetic and mechanistic research. Recently researchers have established direct linkages between gas-phase data obtained via ESI-MS and processes occurring in solution, and these results reveal qualitative and quantitative correlations between ESI-MS measurements and solution properties. In this context, time dependences, concentration series, and counterion effects can serve as criteria that allow researchers assess if the gas-phase measurements correlate with the situation in the solution. Furthermore, we report developments that bridge the gap between gas-phase and solution-phase studies. We also describe predictions derived from ESI-MS that have been verified with solution-phase chemistry experiments.