Spectroscopic characterization of the binding and isomerization cycle of Brooker’s merocyanine with α-, β-, and γ-cyclodextrins

Complexes of Brooker’s merocyanine dye with α-, β- and γ-cyclodextrin (CD) have been characterized to determine the relative strength and thermodynamics of binding, as well as the effect of binding on the protolytic-photochemical isomerization cycle of the dye. It was found that the dye binds most tightly to β-CD, with a binding equilibrium constant of 430 M^?1, in agreement with previous results (Hamasaki et al. J. Incl. Phenom. Mol. Rec. Chem. 13, 349–359 (1992)), while α-CD and γ-CD complexes have a binding constant of approximately 110 M^?1 and 70 M^?1, respectively, determined using absorbance and fluorescence spectroscopy. The isomerization cycle for the dye in α- and γ-CD complexes was found to be the same as for the free dye. Complexation with β-CD, however, resulted in depressed trans-to-cis photoisomerization in acidic conditions followed by spontaneous cis-to-trans isomerization (with the addition of base). Thermodynamic results also indicated differences between α-CD (ΔS° = ?48 J K^?1) and β-CD (ΔS° =  +12 J K^?1) complexes. There was no temperature dependence observed for the γ-CD complexes. These results can be justified in terms of the location of the dye molecule within the cyclodextrin cavity for each of the complexes.     

非离子表面活性剂和β-环状糊精包结作用的微量热法研究

用Model-4200型恒温滴定量热系统,研究了非离子表面活性剂(TritonX-114,TritonX-100,TritonX-102,TritonX-405)和β-环状糊精的包结的热力学性质,结果表明:对所有体系,均形成1:1的包结物,ΔHⅲ和ΔSⅲ值均为负值,形成包结物的过程是焓有利而熵不利的过程,而且-ΔHⅲ和ΔSⅲ值随非离子表面活性剂环氧乙烷(EO)链长的增加而减少。另外,非离子表面活性剂环氧乙烷(EO)链越长,包结物稳定常数越小。     

Binding of cyclic carboxylates to octa-acid deep-cavity cavitand

As part of the fourth statistical assessment of modeling of proteins and ligands (sampl.eyesopen.com) prediction challenge, the strength of association of nine guests (1–9) binding to octa-acid host was determined by a combination of ¹H NMR and isothermal titration calorimetry. Association constants in sodium tetraborate buffered (pH 9.2) aqueous solution ranged from 5.39 × 10² M^?1 in the case of benzoate 1, up to 3.82 × 10⁵ M^?1 for trans-4-methylcyclohexanoate 7. Overall, the free energy difference between the free energies of complexation of these weakest and strongest binding guests was ΔΔG° = 3.88 kcal mol^?1. Based on a multitude of previous studies, the anticipated order of strength of binding was close to that which was actually obtained. However, the binding of guest 3 (4-ethylbenzoate) was considerably stronger than initially estimated.     

The detection of cyclodextrin self-association by titration with dyes

A simple procedure is proposed for the detection of association of cyclodextrins (CDs) and their derivatives in aqueous solutions by analyzing the curves of titration of these substances with dyes. Dissociation constants of cyclodextrin-dye complexes and concentrations of sites involved in the complexation are determined by the nonlinear regression analysis. The processes of CD association were characterized using deviations of the function of CD saturation with ligands from a simple scheme CD + ligand ? complex. Two association mechanisms were disclosed: cooperative association (for α-CD and dihydroxypropyl-β-CD) and step association (for β-CD, dimethyl-β-CD, hydroxypropyl-β-CD, and γ-CD). In the case of stepwise-associating compounds, association constants for head groups of cyclodextrins are found to vary from 3 × 10³ to 8 × 10³ M^?1. The titration data on CD association are confirmed by studying solutions of the above compounds using dynamic light scattering.     

Synthesis and characterization of poly(ethylene glycol) based β-cyclodextrin polymers

A series of novel water soluble β-cyclodextrin (βCD) polymers has been synthesized from functionalized poly(ethylene glycol) (PEG). The chemical composition of the polymers has been characterized by ¹H NMR and the βCD content is found to be between 48 and 33% (w/w). The molecular weight has been determined by Size Exclusion Chromatography (SEC) and depends on the ratio between βCD and PEG, varying from 2.1 × 10⁴ to 8.6 × 10⁴ g mol^?1. The physico chemical properties have been characterized by differential scanning calorimetry (DSC), viscometry and isothermal titration calorimetry (ITC). ITC shows that the polymers have association constants comparable to βCD with different guest molecules, indicating a good accessibility of the CDs.     

Dipyrrolylquinoxaline-bridged hydrazones: a new class of chemosensors for copper(II)

Novel 2,3-bis(1H-pyrrol-2-yl)quinoxaline-functionalized hydrazones were prepared and characterized as new chemosensors for copper(II) ion. The binding properties of the compounds 4, 5, 6 and 7 for cations were examined by UV–vis, fluorescence spectroscopy, and linear sweep voltammetric experiments (LSV). The results indicate that a 1:1 stoichiometric complex is formed between compound 4 (or 5, 6, 7) and copper(II) ion, and the association constant is 1.3?×?10⁵ M^?1 for 4, 2.1?×?10⁶ M^?1 for 5, 4.1?×?10⁵ M^?1 for 6 and 8.0?×?10⁵ M^?1 for 7, respectively. The recognition mechanism between compound 4 (or 5, 6, 7) and metal ion was discussed based on their electrochemical properties, absorbance changes, and the fluorescence quenching effect when they interact with each other. Control experiments revealed that compound 4 (or 5, 6, 7) has a highly selective response to copper (II) ion.     

Alkyl Ammonium Ion Selectivity of Hexahomotrioxacalix[3]arene Triamide Derivative having the Intramolecular Hydrogen-Bonding Group

The lower rim functionalized hexahomotrioxacalix[3]arene triamide 4 with cone-conformation was synthesized from triol 1 by a stepwise reaction. The different extractability for alkali metal ions, transition metal ions, and alkyl ammonium ions from water into dichloromethane is discussed. Due to the strong intramolecular hydrogen bonding between the neighboring NH and CO groups in triamide 4, its affinity to metal cations was weakened. Triamide 4 shows a single selectivity to n-BuNH ₃ ⁺ . The anion complexation of triamide 4 was also studied by ¹H NMR titration experiments. Triamide 4 binds halides through the intermolecular hydrogen bonding among the NH hydrogens of amide in a 1:1 fashion in CDCl₃. The association constants calculated from these changes in chemical shifts of the amide protons are K _a = 223 M^?1 for Cl^? and K _a = 71.7 M^?1 for Br^?. Triamide 4 shows a preference for Cl^? complexation than Br^? complexation.     

Solubilization of cholesterol in aqueous solution by two β-cyclodextrin dimers and a negatively charged β-cyclodextrin derivative

Two βCD dimers (linked by succinic acid, 2, or ethylenediaminetetraacetic acid, EDTA, 3, bridges) and a negatively charged monomer derivative of βCD, 1, have been synthesized and their ability to solubilize cholesterol in aqueous solution was studied. The three compounds exhibit a great capacity in solubilizing cholesterol as, for instance, concentrations up to 6 mM of cholesterol were measured in the presence of 25 mM of 3. The phase-solubility diagrams of the two dimers exhibit A _L type profiles while the monomer 1 follows an A _P isotherm. The cholesterol/dimer complexes have 1:1 stoicheiometries while monomer 1 forms two complexes with molar ratios of 1:1 and 1:2 (cholesterol/1). The equilibrium constants are K _1:1 = (5.9 ± 0.3) × 10⁴ M^?1 and K _1:1 = (8.8 ± 0.2) × 10⁴ M^?1 for 2 and 3, respectively, and K _1:1 = 73 ± 19 M^?1 and K _1:2 = 204 ± 65 M^?1 for 1. The comparison of K _1:1(3) with the product K _1:1 × K _1:2 (1) reveals that a chelate effect in binding the cholesterol by 3 exists. The structure of the cholesterol/3 complex was studied by ROESY experiments and by molecular dynamics simulations.     

An investigation of the complexation of a TTF derivative with α-, β- and γ-cyclodextrins in aqueous media

This Letter, describes the complexation of α-, β- and γ-cyclodextrins (1-3) with TTF derivative 4 in water. In particular, we show using ¹H, ¹³C NMR, UV-vis spectroscopy and isothermal titration calorimetry that β-cyclodextrin 2 forms an effective complex with 4. Complex 2·4 can be conveniently disassembled upon the addition of 1-adamantanol.     

Characterization of β-cyclodextrin inclusion complex with procaine hydrochloride by 1H NMR and ITC

The inclusion of local anesthetic drug procaine hydrochloride by β-cyclodextrin was investigated by 1D and 2D proton NMR spectroscopy and isothermal titration calorimetry (ITC) at 298 K. The stoichiometry of the complex was determinate by the method of continuous variation, using the chemical induced shift of both host and guest protons. The association constant K, of the obtained complex was calculated and found to be 293.17 M^?1. Rotating frame NOE spectroscopy, was used to ascertain the solution geometry of the host–guest complex. The result reveals that the procaine molecule penetrates into the β-cyclodextrin cavity with the aromatic ring. The energetics of complexation process is investigated by ITC technique. The analysis indicates that the complexation of procaine by β-CD is an exothermic process and show that both enthalpy and entropy contribute to the binding process. The obtained value for the association constant is in good agreement with that obtained from NMR.     

The Micellization and Clouding of Nonionic Surfactant,Poly(Ethylene Glycol) t-Octylphenyl Ether (Triton X-100): Effect of Halide Ions of (Sodium Salt) Electrolytes

In this investigation, the micellization and the clouding phenomena of a nonionic surfactant, poly(ethylene glycol) t-octylphenyl ether (Triton X-100) in the absence and presence of halide ions (sodium salt) electrolytes has been reported. The critical micelle concentration (CMC) of Triton X-100 (in the absence and presence of electrolytes) was measured by surface tension measurements. A decreasing trend of CMC was found with increasing the temperature as well as the concentration of electrolyte. The effectiveness of the halide ions was found in the order: F^? > Cl^? > Br^? > I^?. The surface properties of Triton X-100 were evaluated. The thermodynamic parameters of the micellar systems of Triton X-100 were evaluated and from these thermodynamics data, it was found that in the presence of electrolyte the stability of the micellar system is more. The cloud points (CPs) of Triton X-100 were also measured in the absence and presence of halide ions of electrolytes. With the addition of halide ions of sodium salt (electrolyte), a decrease in CP values was observed and the order was found to be: F^? > Cl^? > Br^? > I^?.     

Molecular association of benzene with a new cyclophane receptor

Host/guest interactions in the cyclophane-2/benzene system have been investigated by absorption and fluorescence spectroscopy in dichloromethane. The cyclophane serves as a host and the benzene as a guest. Absorption and fluorescence titration experiments are carried out by holding either the concentration of the host or guest constant while varying the concentration of the other component. When the concentration of benzene is kept constant, an isostilbic point at 288 nm is observed in the fluorescence spectral data, suggesting that only two absorbing species are present in equilibrium. Keeping the concentration of cyclophane-2 constant while increasing the concentration of benzene results in a hyposchromic shift of the emission peaks in the range 275–360 nm. The shift is attributed to interaction of the cyclophane with benzene. The average association constant of cyclophane-2 with benzene, K _a = 425 ± 54 M^?1, obtains from fitting the absorption and the fluorescence spectral data to the Bourson et al. equation using non-linear regression analysis.     

Inclusion complex of α-cyclodextrin and the extended viologen dication: a model of an insulated molecular wire

An extended viologen dication 1, containing one viologen subunit, was used as a model for the inclusion complex formation between cyclodextrin (CD) molecules and molecular wires comprising several subunits. UV–Vis and fluorescence spectroscopic measurements confirmed the formation of two types of the inclusion complexes 1:1 and 2:1 between αCD and 1 in the aqueous solution containing 20% of ethanol. The complex formation constants were obtained from the fluorescence spectral changes: K _a  = 25 ± 3 mM^?1 for [αCD–1] complex and K _a  = 0.21 ± 0.07 mM^?2 for [(αCD)₂–1] complex, respectively. Cyclodextrins βCD and γCD do not form the inclusion complexes with 1 in these aqueous solutions. The time-dependent differential capacitance measurements confirmed the adsorption of 1 in the form of a complex at the electrode/electrolyte interface. These studies were conducted with the aim to find the most suitable CD cavity that would separate individual molecular wires from each other on the electrode/electrolyte interface.     

Formation of an Inclusion Complex of a New Transition Metal Ligand in β-Cyclodextrin

The inclusion complex of a new transition metal ligand, 2,4,9-trithia-tricyclo[3.3.1.1^3,7]decane-7-carboxylic acid (2,4,9-trithia-adamantane-7-carboxylic acid, TPCOOH) in β-cyclodextrin was studied by ¹H NMR, 2D NOESY NMR spectroscopy, host-induced CD spectroscopy, and tandem mass spectrometry. ¹H NMR, MS–MS and NOESY data show that the TPCOOH guest forms a 1:1 inclusion complex with the host β-cyclodextrin. The NOESY experiments also show that TPCOOH is oriented in the complex with the thioketal end preferentially located at the larger opening of β-cyclodextrin. The orientation of the guest in the host molecule is also confirmed by the induced CD of the ligand, which shows a positive Cotton effect. An association constant of 660±20?M^?1 was determined by ¹H NMR titration for the complex at room temperature in D₂O.     

Adamantane-bearing benzylamines and benzylamides: novel building blocks for supramolecular systems with finely tuned binding properties towards β-cyclodextrin

Novel building blocks for the synthesis of supramolecular components based on adamantane-bearing benzylamines were prepared. The binding properties of these amines and the corresponding acetamides towards β-cyclodextrin (β-CD) were studied using mass spectrometry, NMR spectrometry, isothermal titration calorimetry and semi-empirical calculations. It was found that all of the examined guests predominantly formed 1:1 inclusion complexes in an enthalpy-driven manner with association constants of the order of 10²–10³ M^? 1. Stronger binding to the β-CD cavity was observed for guests with a longer spacer between the adamantane and benzene moieties and/or a 1,4-disubstituted benzene ring.     

Synthesis of carbohydrate-functionalized thiophene-capped cyclopenta[c]thiophene for concanavalin A recognition

Synthetic route to two different carbohydrate-functionalized oligothiophenes comprising central cyclopenta[c]thiophene (CPT) moiety was demonstrated. Turbidimetric analysis revealed enhanced interaction for divalent α-mannose-functionalized terthiophene 7 with Con A. The extent of binding interaction for 7-Con A complex formation was determined from isothermal titration calorimetry (ITC) measurements and the binding constant found (K_ITC) 2.4(6) × 10⁵ M^?1.     

Measuring Positive Cooperativity Using the Direct ESI-MS Assay. Cholera Toxin B Subunit Homopentamer Binding to GM1 Pentasaccharide

Direct electrospray ionization mass spectrometry (ESI-MS) assay was used to investigate the stepwise binding of the GM1 pentasaccharide β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp (GM1os) to the cholera toxin B subunit homopentamer (CTB₅) and to establish conclusively whether GM1os binding is cooperative. Apparent association constants were measured for the stepwise addition of one to five GM1os to CTB₅ at pH 6.9 and 22 °C. The intrinsic association constant, which was established from the apparent association constant for the addition of a single GM1os to CTB₅, was found to be (3.2 ± 0.2) × 10⁶ M^–1. This is in reasonable agreement with the reported value of (6.4 ± 0.3) × 10⁶ M^–1, which was measured at pH 7.4 and 25 °C using isothermal titration calorimetry (ITC). Analysis of the apparent association constants provides direct and unambiguous evidence that GM1os binding exhibits small positive cooperativity. Binding was found to be sensitive to the number of ligand-bound nearest neighbor subunits, with the affinities enhanced by a factor of 1.7 and 2.9 when binding occurs next to one or two ligand-bound subunits, respectively. These findings, which provide quantitative support for the binding model proposed by Homans and coworkers [14], highlight the unique strengths of the direct ESI-MS assay for measuring cooperative ligand binding.

Enhancing the lateral-flow immunoassay for viral detection using an aqueous two-phase micellar system

Availability of a rapid, accurate, and reliable point-of-care (POC) device for detection of infectious agents and pandemic pathogens, such as swine-origin influenza A (H1N1) virus, is crucial for effective patient management and outbreak prevention. Due to its ease of use, rapid processing, and minimal power and laboratory equipment requirements, the lateral-flow (immuno)assay (LFA) has gained much attention in recent years as a possible solution. However, since the sensitivity of LFA has been shown to be inferior to that of the gold standards of pathogen detection, namely cell culture and real-time PCR, LFA remains an ineffective POC assay for preventing pandemic outbreaks. A practical solution for increasing the sensitivity of LFA is to concentrate the target agent in a solution prior to the detection step. In this study, an aqueous two-phase micellar system comprised of the nonionic surfactant Triton X-114 was investigated for concentrating a model virus, namely bacteriophage M13 (M13), prior to LFA. The volume ratio of the two coexisting micellar phases was manipulated to concentrate M13 in the top, micelle-poor phase. The concentration step effectively improved the M13 detection limit of the assay by tenfold from 5?×?10⁸ plaque forming units (pfu)/mL to 5?×?10⁷ pfu/mL. In the future, the volume ratio can be further manipulated to yield a greater concentration of a target virus and further decrease the detection limits of the LFA.

Tensammetry with accumulation on the hanging mercury drop electrode : Part 3. The behaviour of triton X-100 in mixtures with PEG-9000

The behaviour of Triton X-100, which can be present in monomeric or associated form, and its mixtures with PEG-9000, which does not undergo association, is described. The tensammetric curve of Triton X-100 alone shows one or two peaks at negative potentials, depending on the concentration of Triton X-100, i.e., on the presence of associated forms. For <2 mg l^?1, there is one broad peak, related to monomers of Triton X-100. The calibration plot for this peak is sigmoidal but its rising section (0.05–0.20 mg l^?1) is approximately linear. The calibration curve of the second, much narrower, peak related to associated forms of Triton X-100, grows parabolically with increasing concentration of Triton X-100. The behaviour of a mixture of PEG-9000 with a larger amount of Triton X-100 is similar to the behaviour of a model mixture of components with sufficiently different properties (e.g., PEG 1500/PEG 9000). The peak for PEG-9000, the stronger surfactant, is relatively less affected by a large amount of Triton X-100. Even this effect can be decreased by using a suitable preconcentration potential (?1.45 V vs. SCE) so that PEG-9000 can be determined in the presence of a 1000-fold amount of Triton X-100. Both peaks of Triton X-100 are greatly decreased by the presence of PEG-9000 and the broad peak can be completely suppressed. Triton X-100 cannot be determined accurately in the presence of unknown amounts of PEG-9000.     

Investigation of the inclusion of the herbicide cycluron in native cyclodextrins by X-ray diffraction,nuclear magnetic resonance spectroscopy and isothermal titration calorimetry

The formation of inclusion complexes between the native cyclodextrins (CDs) and the urea herbicide cycluron has been investigated both in solution and in the solid state. Single-crystal X-ray structures of both the uncomplexed guest and the β-CD·cycluron complex were determined while powder X-ray diffraction was used to confirm complexation between γ-CD and cycluron in the solid state. Solution-state complexation between the herbicide and α-, β- and γ-CD was established using ¹H NMR spectroscopy and isothermal titration calorimetry (ITC). From the ¹H NMR spectroscopic studies 1:1 complex stoichiometry was indicated in all cases and association constant values (K) were determined as 228, 3254 and 155 for the complexes α-CD·cycluron, β-CD·cycluron and γ-CD·cycluron, respectively. Assigning a 1:1 host–guest ratio, the ITC technique produced K values of the same order as those determined using the spectroscopic method. The thermodynamic parameters ΔH, ΔS and ΔG obtained using ITC provide insights into the driving forces involved during complex formation.