Apparent Complexation Constants Research Articles

Lifespan extension in Caenorhabditis elegans by oxyresveratrol supplementation in hyper-branched cyclodextrin-based nanosponges.

In this work, the increase of the Caenorhabditis elegans (C. elegans) lifespan extension using hyper-branched cyclodextrin-based nanosponges (CD-NS) complexing oxyresveratrol (OXY), and the possible inhibition of C. elegans phosphodiesterase type 4 (PDE4) were evaluated. The titration displacement of fluorescein was used to calculate the apparent complexation constant (KF) between CD-NS and OXY. Moreover, PDE4 was expressed in E. coli, purified and refolded in presence of cyclodextrins (CDs) to study its possible inhibition as pharmacological target of OXY. The apparent activity was characterized and the inhibitory effect of OXY on PDE4 displayed a competitive in vitro inhibition corroborated in silico. A maximum increase of the in vivo life expectancy of about 9.6% of using OXY/CD-NS complexes in comparison with the control was obtained, in contrast to the 6.5% obtained with free OXY. No effect on lifespan or toxicity with CD-NS alone was found. These results as a whole represent new opportunities to use OXY and CD-NS in lifespan products.

Modeling-based optimization of the simultaneous enantiomeric separation of multicomponent mixtures of phenoxy acid herbicides using dual cyclodextrin systems by Capillary Electrophoresis

In this work, the Dubsky's model proposed for Capillary Electrophoresis (CE) enantioseparation systems with a mixture of chiral selectors was applied to the rapid optimization of the simultaneous enantiomeric separation of a multicomponent mixture of six phenoxy acid herbicides using a dual system of two cyclodextrins (CDs), (2-hydroxypropyl)-β-CD (HP-β-CD) and heptakis(2,3,6-tri-O-methyl)-β-CD (TM-β-CD). Simply by carrying out a small number of individual experiments separately with each CD, the Dubsky's model enabled to foresee the results that could be obtained for any possible combination of concentrations and relative proportion of both CDs in the mixture. Results obtained in this work demonstrated that the model was successful by improving the previous results experimentally obtained by the trial and error method for the simultaneous enantiomeric separation of the six phenoxy acid herbicides studied in this work. In fact, the separation was improved in terms of enantiomeric resolutions obtained (from 1.2 to 4.2 for concentrations of CDs of 4 mM HP-β-CD and 16 mM TM-β-CD) and by considerably reducing the time to optimize the separation conditions enabling to find, in a faster and efficient way, the most adequate proportion of both CDs and the concentration of each CD in the mixture to obtain baseline separation of the twelve enantiomers. Additionally, the apparent complexation constants between enantiomers and each CD were calculated. This is the first time that the above-mentioned model was applied to a multicomponent mixture of chiral compounds.

Thermodynamic study of the complexation of humic acid by calorimetry

Although the thermodynamic quantities (Gibbs free energy, reaction enthalpy, and entropy) of the complexation of humic acid are necessary for the discussion of the reaction thermodynamics, their accurate determination, especially concerning dissolved humic acid in deep groundwater, has not been carried out. In this study, a combination of potentiometry and calorimetry was used for the determination of the thermodynamic values of complexation of typical humic acid and groundwater humic acid, which was isolated from deep groundwater at Horonobe, Hokkaido, Japan, with copper (II) ions and uranyl (VI) ions. The apparent complexation constant of Horonobe humic acid was independent of the pH of the bulk solution, whereas that of typical humic acid was dependent on the pH. This observation indicates that the polyelectrolyte effect of Horonobe humic acid is negligible because of its relatively small molecular size. In addition, the effect of the heterogeneity of Horonobe humic acid was not significant. Moreover, the complexation enthalpy of Horonobe humic acid was consistent with that of homogeneous poly(acrylic acid), which means the complexation of Horonobe humic acid was not affected by the functional group heterogeneity. Consequently, the unique complexation mechanism of Horonobe humic acid was revealed based on the determined thermodynamic quantities. The migration of radionuclides in the deep underground environment must be affected by these characteristics; thus, the accurate determination of thermodynamic quantities of in situ humic substances is very helpful for the safety assessment of geological disposal, when disposal sites for radioactive waste are chosen.

CEval: All-in-one software for data processing and statistical evaluations in affinity capillary electrophoresis

We introduce CEval software (downloadable for free at echmet.natur.cuni.cz) that was developed for quicker and easier electrophoregram evaluation and further data processing in (affinity) capillary electrophoresis. This software allows for automatic peak detection and evaluation of common peak parameters, such as its migration time, area, width etc. Additionally, the software includes a nonlinear regression engine that performs peak fitting with the Haarhoff‐van der Linde (HVL) function, including automated initial guess of the HVL function parameters. HVL is a fundamental peak-shape function in electrophoresis, based on which the correct effective mobility of the analyte represented by the peak is evaluated. Effective mobilities of an analyte at various concentrations of a selector can be further stored and plotted in an affinity CE mode. Consequently, the mobility of the free analyte, μA, mobility of the analyte-selector complex, μAS, and the apparent complexation constant, K′, are first guessed automatically from the linearized data plots and subsequently estimated by the means of nonlinear regression. An option that allows two complexation dependencies to be fitted at once is especially convenient for enantioseparations. Statistical processing of these data is also included, which allowed us to: i) express the 95% confidence intervals for the μA, μAS and K′ least-squares estimates, ii) do hypothesis testing on the estimated parameters for the first time.We demonstrate the benefits of the CEval software by inspecting complexation of tryptophan methyl ester with two cyclodextrins, neutral heptakis(2,6-di-O-methyl)-β-CD and charged heptakis(6-O-sulfo)-β-CD.

Dynamic computer simulation of electrophoretic enantiomer migration order and separation in presence of a neutral cyclodextrin.

One-dimensional dynamic computer simulation was employed to investigate the separation and migration order change of ketoconazole enantiomers at low pH in presence of increasing amounts of (2-hydroxypropyl)-β-cyclodextrin (OHP-β-CD). The 1:1 interaction of ketoconazole with the neutral cyclodextrin was simulated under real experimental conditions and by varying input parameters for complex mobilities and complexation constants. Simulation results obtained with experimentally determined apparent ionic mobilities, complex mobilities, and complexation constants were found to compare well with the calculated separation selectivity and experimental data. Simulation data revealed that the migration order of the ketoconazole enantiomers at low (OHP-β-CD) concentrations (i.e. below migration order inversion) is essentially determined by the difference in complexation constants and at high (OHP-β-CD) concentrations (i.e. above migration order inversion) by the difference in complex mobilities. Furthermore, simulations with complex mobilities set to zero provided data that mimic migration order and separation with the chiral selector being immobilized. For the studied CEC configuration, no migration order inversion is predicted and separations are shown to be quicker and electrophoretic transport reduced in comparison to migration in free solution. The presented data illustrate that dynamic computer simulation is a valuable tool to study electrokinetic migration and separations of enantiomers in presence of a complexing agent.

Preparation and in vitro Evaluation of Inclusion Complexes of Nelfinavir with Chemically Modified ?-cyclodextrins

Nelfinavir is a poorly water-soluble antiretroviral drug with relatively low bioavailability. In the present study, the practically insoluble drug, nelfinavir (NFV) and its inclusion complexes with hydroxypropyl-?-cyclodextrin (HP-?-CD) were investigated to improve the aqueous solubility and the dissolution rate of the drug, thus enhancing its bioavailability. The phase solubility diagram with HP-?-CD was classified as AL-type at all temperatures investigated, indicating the formation of higher order complexes. The apparent complexation constants (K1:1) calculated from phase solubility diagram were 145.49, 188.45 and 255.54 M-1 at 25, 37 and 45 ± 0.5°C, respectively. Aqueous solubility and dissolution studies indicated that the dissolution rates were remarkably increased; this could be mainly attributed to the improved solubility and dissolution associated with inclusion complex between drug and HP- ? -CD. Absence of endothermic and characteristic diffraction peaks corresponding to NFV was observed for the inclusion complex in DSC and PXRD. FT-IR study indicated that the presence of intermolecular hydrogen bonds between NFV and HP-?-CD in inclusion complex, resulting in the formation of amorphous form. DOI: http://dx.doi.org/10.3329/dujps.v11i2.14558 Dhaka Univ. J. Pharm. Sci. 11(2): 107-116, 2012 (December)

Aggregation and Host–Guest Interactions in Dansyl-Substituted Poly(acrylate)s in the Presence of β-Cyclodextrin and a β-Cyclodextrin Dimer in Aqueous Solution: A UV–Vis, Fluorescence, 1H NMR, and Rheological Study

A UV–vis, steady-state, and time-resolved fluorescence 2D 1H NOESY NMR spectroscopic and rheological study of new poly(acrylate)s 3% randomly substituted with either N-(2-aminoethyl)-, N-(6-aminohexyl)-, or N-(12-aminododecyl)-5-dansyl-sulfonamide to give the substituted polymers PAADSen, PAADShn, and PAADSddn, respectively, is reported. Their dansyl substituent aggregation and complexation by β-cyclodextrin, βCD, and its covalently linked dimer N,N′-bis(6A-deoxy-6A-β-cyclodextrin)urea, 66βCD2ur, is also reported. The βCD complexation of the dansyl substituents is characterized by apparent complexation constants, K = 89, 105, and 55 dm3 mol–1 for PAADSen, PAADShn, and PAADSddn, respectively, and the analogous complexations by 66βCD2ur are characterized by K = 3.04 × 103, 3.42 × 104, and 2.42 × 105 dm3 mol–1 at pH 7.0 in aqueous 0.10 mol dm–3 NaCl at 298.2 K. Under the same conditions the dansyl substituent shows three fluorescence decay constants assigned to the aggregated (τ1 = 2.2, 2.5, and 3.2 ns), sin...

PREPARATION AND CHARACTERIZATION OF RESVERATROL/HYDROXYPROPYL‐β‐CYCLODEXTRIN INCLUSION COMPLEX USING SUPERCRITICAL ANTISOLVENT TECHNOLOGY

ABSTRACTThe inclusion complex of resveratrol (RV) and hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) prepared by supercritical antisolvent (SAS) process was investigated to improve its aqueous solubility. The phase solubility diagram with HP‐β‐CD was classified as AL type, and the calculated apparent complexation constant (KC) was 5,988 M−1 at 298K. Inclusion complexes were analyzed by differential scanning calorimetry (DSC), powder X‐ray diffractometry (PXRD), Fourier‐transform infrared spectroscopy (FT‐IR) and scanning electron microscopy (SEM). No endothermic and characteristic diffraction peaks corresponding to RV were observed for the inclusion complex in DSC and PXRD. FT‐IR study demonstrated the presence of intermolecular hydrogen bonds between RV and HP‐β‐CD in inclusion complex, resulting in the formation of amorphous form. Furthermore, the SAS‐processed RV/ HP‐β‐CD powders showed a significant enhancement in the solubility of RV (up to 25.13 mg/mL) in an aqueous medium. The percent dissolution of RV increased considerably and about 100% of RV was dissolved within 5–10 min.PRACTICAL APPLICATIONSIn this study, the supercritical antisolvent (SAS) process was effectively utilized to prepare solid‐state inclusion complexes of resveratrol (RV) and hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD). The SAS‐processed RV/HP‐β‐CD powders showed a significant enhancement in the solubility of RV (up to 25.13 mg/mL) in an aqueous medium. The percent dissolution of RV increased considerably and about 100% of RV was dissolved within 5–10 min. This work thoroughly overcomes the high hydrophobicity and sensitivity of resveratrol to air and light, so that obviously enlarges the application in pharmaceutical, chemical, food and cosmetic industries.

Development of a reversed phase high performance liquid chromatography method based on the use of cyclodextrins as mobile phase additives to determine pterostilbene in blueberries

In this work, a reversed phase high performance liquid chromatography (RP-HPLC) method was developed for the determination of pterostilbene in food samples. The novel method is based on the addition of cyclodextrins (CDs) to the mobile phase where the complexation of pterostilbene by CDs is carried out. In order to select the most suitable conditions for the RP-HPLC method, the effect of several physico-chemical parameters on the complexation of pterostilbene by CDs was studied. Our results show that the addition of 12 mM HP-β-CD to a 50:50 (v/v) methanol:water mobile phase at 25 °C and pH 7.0 significantly improves the main analytical parameters. In addition, it was seen that pterostilbene forms a 1:1 complex with HP-β-CD, showing an apparent complexation constant of 251 ± 13 M −1. Finally, in order to study the validity of the proposed method, blueberries were analyzed and the concentration of pterostilbene has been determined.

Simplified Modelling of the Complexation of Humic Substance for Equilibrium Calculations

For the prediction of chemical species distribution in systems containing humic substances (HS), a simplified complexation model was proposed in our previous study. In the model, the logarithm of the apparent complexation constant is described using the equation log Kc app = log Kc + ac log α bc log I − mc log [M] consisting of four HS specific parameters, log Kc , ac , bc , and mc , which are the minimized number of parameters to reproduce the complexation as a function of α (degree of dissociation of functional groups of HS), I (ionic strength of bulk solution), and [M] (free metal ion concentration). In this study, the applicability of this model was demonstrated by applying the equation to the complexation data of Ca(II) obtained by potentiometric titration in awide range of pcH (= −log [H+]), Ca(II) concentration, and ionic strength, as well as to the complexation data of Al(III), Ca(II), Cu(II), Pb(II), Am(III), Cd(II), Cm(III), Co(II), Dy(III), Eu(III), and U(VI) reported in the literature. The proposed equation fairly well reproduced a variety of complexation data with the minimum specific parameters, indicating the suitability of the proposed model for the purpose of equilibrium calculation of metal ion species in the system containing HS as a soluble ligand.

Reciprocal principle of molecular recognition in supramolecular chromatography—highly selective analytical separation of cyclodextrin congeners on a silica-bonded [60]fullerene stationary phase

The supramolecular pair fullerene/cyclodextrin represents a new example for the concept of reciprocal molecular recognition in liquid chromatography. Through inverting their roles as selectand and selector, small-to-large-ring cyclodextrins were chromatographed on silica-bonded [60]fullerene, whereas [60]- and [70]fullerenes were previously separated on a silica-bonded β-cyclodextrin. The recognition of cyclodextrins of intermediate ring size on silica-bonded [60]fullerene is highly selective. Thus, CD8, CD9 and CD10 are separated both from lower (CD6 and CD7) and higher (CD11–CD25) congeners with a remarkable and unprecedented retention window. By using the retention-increment method, employing a reactor column (with selector) and a reference column (without selector), apparent relative complexation constants Krel of CD6–CD12 and [60]fullerene were determined by supramolecular liquid chromatography. The historical development of the reciprocal principle of molecular recognition in chromatography is reported in the introduction.

Simplified Modelling of the Complexation of Humic Substance for Equilibrium Calculations

For the prediction of chemical species distribution in systems containing humic substances (HS), a simplified complexation model was proposed in our previous study. In the model, the logarithm of the apparent complexation constant is described using the equation log Kc app = log Kc + ac log α bc log I − mc log [M] consisting of four HS specific parameters, log Kc , ac , bc , and mc , which are the minimized number of parameters to reproduce the complexation as a function of α (degree of dissociation of functional groups of HS), I (ionic strength of bulk solution), and [M] (free metal ion concentration). In this study, the applicability of this model was demonstrated by applying the equation to the complexation data of Ca(II) obtained by potentiometric titration in awide range of pcH (= −log [H+]), Ca(II) concentration, and ionic strength, as well as to the complexation data of Al(III), Ca(II), Cu(II), Pb(II), Am(III), Cd(II), Cm(III), Co(II), Dy(III), Eu(III), and U(VI) reported in the literature. The pro...

Estimation of Binding Constants of Cd(II), Ni(II) and Zn(II) with Polyethyleneimine (PEI) by Polymer Enhanced Ultrafiltration (PEUF) Technique

Continuous and batch processes of polymer enhanced ultrafiltration for the estimation of binding constants of divalent cadmium, nickel, and zinc ions have been elaborated. Polyethyleneimine was used as a complexation agent. Effect of pH and ionic strength on the binding ability of target metal ions to polyethyleneimine were estimated by continuous mode PEUF system. A mathematical model was developed to estimate apparent complexation constants of metal ions with PEI. The development of this model, which is in good agreement with experimental data, enables to compare the data obtained in both continuous and batch system and correlate the effect of pH and ionic strength with apparent binding constants.

A new example of reversal of the order of migration of enantiomers, as a function of cyclodextrin concentration and pH, by cyclodextrin-modified capillary zone electrophoresis: enantioseparation of 6,6′-dibromo-1,1′-binaphthyl-2,2′-diol

Enantioseparation of 6,6'-dibromo-1,1'-binaphthyl-2,2'-diol (DBBD) by cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was studied using the three native alpha, beta, and gamma cyclodextrins, the three hydroxypropylated cyclodextrins (2-hydroxypropyl-alpha, beta, and gamma), heptakis-2,6-di-O-methyl-beta-CD (DM-beta-CD), and heptakis-2,3,6-tri-O-methyl-beta-cyclodextrin (TM-beta-CD). First, the acidity constants of DBBD were determined using capillary electrophoresis, before performing enantioseparation. The influence of the concentrations of the studied cyclodextrins on the enantioseparation was explored and the experimental optimal concentrations were determined and compared to the theoretical optimal concentrations. Moreover, the apparent complexation constants between each studied cyclodextrin and the two DBBD enantiomers were evaluated using a non-linear curve fitting method and three linear plotting methods (x-reciprocal, y-reciprocal and double reciprocal). For TM-beta-CD, the order of migration of the enantiomers of DBBD reversed as a function of TM-beta-CD concentration. The influence of the nature of methylated cyclodextrin derivatives (methyl-beta-CD (M-beta-CD) and DM-beta-CD) was then studied. Inversion of the order of migration of the enantiomers of DBBD was observed for DM-beta-CD, whereas the S enantiomer of DBBD always migrated first for M-beta-CD.

Enantioseparation of binaphthol and its monoderivatives by cyclodextrin-modified capillary zone electrophoresis: A mathematical approach

A simple model for the separation of atropisomers of binaphthol and its monoderivatives by means of cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was used to describe the migration behavior of poly charged enantiomers in a chiral separation system. This mathematical approach allowed for the determination of the optimal cyclodextrin concentrations for the enantioseparation of binaphthols by taking into account the influence of the formed complex mobilities. Moreover, using this theoretical approach, the reversal of the enantiomers’ migration order as a function of cyclodextrin concentration was predicated. The apparent complexation constants between the cyclodextrins and the binaphthol and its monoderivatives could be calculated using a non-linear curve fitting method and three linear plotting methods ( x-reciprocal, y-reciprocal and double reciprocal). Good agreements between the theoretical and experimental cyclodextrin concentrations were obtained.

Kinetic Study of the Activation of Banana Juice Enzymatic Browning by the Addition of Maltosyl-β-cyclodextrin

In recent years, the use of cyclodextrins (CDs) as antibrowning agents in fruit juices has received growning attention. However, there has been no detailed study of the behavior of these molecules as substances, which can lead to the darkening of foods. In this paper, when the color of fresh banana juice was evaluated in the presence of different CDs, the evolution of several color parameters was the opposite of that observed in other fruit juices. Moreover, a kinetic model based on the complexation by CDs of the natural browning inhibitors present in banana is developed for the first time to clarify the enzymatic browning activation of banana juice. Finally, the apparent complexation constant between the natural polyphenoloxidase inhibitors present in banana juice and maltosyl-beta-CD was calculated (Kci = 27.026 +/- 0.212 mM (-1)).

Influence of buffer substances and urea on the β‐cyclodextrin‐mediated chiral separation of dipeptides in CE

The influence of buffering substances and urea on the beta-CD-mediated chiral separations of the dipeptides Ala-Phe and Ala-Tyr was studied in the pH range of 2.5-3.8. Only minor effects of the buffer substances on the chiral separation selectivity alpha were observed at a beta-CD concentration of 15 mg/mL. In contrast, the selectivity improved at pH 2.5 but decreased at pH 3.8 upon the addition of 2 M urea. Complexation by beta-CD resulted in a shift of the pK(a) values toward higher values which was more pronounced for the DD-enantiomers of both dipeptides than for the LL-enantiomers. Addition of urea further increased the pK(a) shift. The consequence of this pK(a) shift is an increase of the fraction of the protonated, positively charged form of the peptides which explained the improved chiral separation at pH 2.5 and the reduced selectivity at pH 3.8. A pK(a) shift by the addition of urea was also observed for N-tert-butyloxycarbonyl phenylalanine (BOC-Phe) as a model compound that is strongly complexed by beta-CD. This effect was not stereospecific. Addition of urea resulted in a decrease of the apparent complexation constants between beta-CD and the BOC-Phe enantiomers to the same extent but this did not affect the separation selectivity alpha. For chiral separations that display strong pH dependence such as peptide enantioseparations close to the pK(a) values of the compounds, urea may not solely be regarded as a solubility enhancer for beta-CD but may also influence the separation.

Use of Cyclodextrins as Secondary Antioxidants to Improve the Color of Fresh Pear Juice

In this paper, the color of fresh pear juice was evaluated for the first time in the presence of some natural and modified cyclodextrins (CDs) and the effect of these compounds as browning inhibitors was determined using the color space CIELAB system. Furthermore, because contradictory claims have been published concerning the mechanism by which enzymatic browning is inhibited by CDs, a kinetic model in the presence of CDs is proposed and the corresponding apparent complexation constants between the mixtures of diphenols present in pear juice and alpha-, beta-, and maltosyl-beta-CD have been calculated. Moreover, pear juice color was studied in the presence of different polyphenoloxidase inhibitors. Finally, we show that maltosyl-beta-CD enhances the ability of ascorbic acid to prevent enzymatic browning because of the protection that maltosyl-beta-CD offers against ascorbic acid oxidation. In this respect, maltosyl-beta-CD seems to act as a "secondary antioxidant", reducing pear juice browning and enhancing the naturally occurring antioxidant capacity of pear juice.

Use of Natural and Modified Cyclodextrins as Inhibiting Agents of Peach Juice Enzymatic Browning

Although cyclodextrins (CDs) have been successfully used as antibrowning agents in different fruit juices, no research has studied the effect of these compounds on enzymatic browning in peach juice. In this paper, the color of fresh peach juice was evaluated in the presence of two types of natural (alpha-CD and beta-CD) and a modified (maltosyl-beta-CD) CD, and the effectiveness of these compounds as browning inhibitors was determined using the color space CIELAB system. Moreover, to clarify the mechanism by which CDs inhibit peach juice enzymatic browning, the process was kinetically modeled in the absence and presence of CDs using a colorimetric method; the apparent complexation constants between the mixtures of diphenols present in peach juice and some types of CD were calculated. The results show that the highest affinity constant was presented by alpha-CD (Kc = 18.31 mM-1) followed by maltosyl-beta-CD (Kc = 11.17 mM-1), whereas beta-CD was incapable of inhibiting peach juice enzymatic browning. Cyclodextrin; browning; peach; juice; color; polyphenol oxidase.

Amphiphilic Pyranines: Chromophore Wrapped in a Hydrophobic Alkyl Chain and Micelle Formation Induced by Complexation with Methylviologen

Abstract Amphiphilic pyranines POCn (n = 8, 12, and 16) were synthesized, and their aggregation behaviors were investigated in water. 1H NMR studies implied that POC16 existed in the conformation of the pyranine chromophore wrapped with a hexadecyl methylene chain, which showed no indication of aggregation at the concentration up to 10−4 M. However, the formation of micelles was readily induced by the addition of methylviologen (MV2+) to a POC16 solution, which was responsible for an apparent large complexation constant of ca. 106 M−1.