The use of the isokinetic relationship and molecular mechanics to investigate molecular interactions in inclusion complexes of cyclodextrins

Spectroscopic studies for the inclusion complexation of ketoprofen enantiomers with β-cyclodextrin

Inclusion complexes of R-ketoprofen and S-ketoprofen enantiomers with β-cyclodextrin (β-CD) in aqueous solution were studied using various spectroscopic techniques such as Raman, FTIR, UV and fluorescence. The different relative intensities and characteristic band shifts of the two enantiomers from Raman spectra suggests different interaction when complexed with β-CD. Raman experiments revealed a noticeable diminishing of the CC vibration and ring deformation, which indicate the embedding of ketoprofen inside the β-CD cavity. It's revealed that distinct differences between R- and S-ketoprofen in the presence of β-CD at neutral pH. The stoichiometry ratio and binding constant of the inclusion complexes were calculated using Benesi-Hildebrand plot. Both enantiomers showed stoichiometry ratio of 1:1 inclusion complex with β-CD. The binding constant of R-ketoprofen (4088 M^-1) is higher than S-ketoprofen (2547 M^-1). These values indicated that β-CD formed inclusion complexes more preferentially with R-ketoprofen than S-ketoprofen. Results demonstrated that β-CD can be used as a promising chiral selector for ketoprofen enantiomers.

Chemistry and engineering of cyclodextrins for molecular imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides bearing a basket-shaped topology with an "inner-outer" amphiphilic character. The abundance of hydroxyl groups enables CDs to be functionalized with multiple targeting ligands and imaging elements. The imaging time, and the payload of different imaging elements, can be tuned by taking advantage of the commercial availability of CDs with different sizes of the cavity. This review aims to offer an outlook of the chemistry and engineering of CDs for the development of molecular probes. Complexation thermodynamics of CDs, and the corresponding implications for probe design, are also presented with examples demonstrating the structural and physiochemical roles played by CDs in the full ambit of molecular imaging. We hope that this review not only offers a synopsis of the current development of CD-based molecular probes, but can also facilitate translation of the incremental advancements from the laboratory to real biomedical applications by illuminating opportunities and challenges for future research.

Thermodynamic investigation of the interaction between cyclodextrins and preservatives - Application and verification in a mathematical model to determine the needed preservative surplus in aqueous cyclodextrin formulations

Preservatives are inactivated when added to conserve aqueous cyclodextrin (CD) formulations due to complex formation between CDs and the preservative. To maintain the desired conservation effect the preservative needs to be added in apparent surplus to account for this inactivation. The purpose of the present work was to establish a mathematical model, which defines this surplus based upon knowledge of stability constants and the minimal concentration of preservation to inhibit bacterial growth. The stability constants of benzoic acid, methyl- and propyl-paraben with different frequently used βCDs were determined by isothermal titration calorimetry. Based upon this knowledge mathematical models were constructed to account for the equilibrium systems and to calculate the required concentration of the preservations, which was evaluated experimentally based upon the USP/Ph. Eur./JP monograph. The mathematical calculations were able to predict the needed concentration of preservation in the presence of CDs; it clearly demonstrated the usefulness of including all underlying chemical equilibria in a mathematical model, such that the formulation design can be based on quantitative arguments.

Stabilities of the Divalent Metal Ion Complexes of a Short-Chain Polyphosphate Anion and Its Imino Derivative

Abstract

The stability constants of ML-type complexes of the two linear triphosphate ligand anion analogues triphosphate (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{10}^{5 - } $$\end{document}P3O105-) and diimidotriphosphate (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{ 8} ( {\text{NH}})_{2}^{5 - } $$\end{document}P3O8(NH)25-) were investigated thermodynamically using potentiometric titrations according to Schwarzenbach’s procedure. The stability constants of the ML-type complexes of different divalent metal ions with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{ 8} ( {\text{NH}})_{2}^{5 - } $$\end{document}P3O8(NH)25- are larger than those of the corresponding complexes with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{10}^{5 - } $$\end{document}P3O105- because of the greater basicity of the imino group. The order of the stability constants for the ML-type complexes follows the Irving–Williams order, indicating that only non-bridging oxygen atoms are coordinated directly to the different metal ions in both ligands, and that the imino groups cannot participate in coordination to the metal ions. In the complexation reactions of the Ca²⁺, Sr²⁺, Ba²⁺–\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{10}^{5 - } $$\end{document}P3O105- and Cu²⁺, Zn²⁺, Ni²⁺–\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{ 8} ( {\text{NH}})_{2}^{5 - } $$\end{document}P3O8(NH)25- systems, each metal ion forms an enthalpically stable complex, and there was no suggestion of a conspicuous entropic effect based on the chelate effect. Monodentate complexes that are strongly coordinated with the ligands were therefore formed, whereas entropically stable bidentate complexes were formed in the complexation reactions of the Cu²⁺, Zn²⁺, Ni²⁺–\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{10}^{5 - } $$\end{document}P3O105- and Ca²⁺, Ba²⁺, Sr²⁺–\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{P}}_{ 3} {\text{O}}_{ 8} ( {\text{NH}})_{2}^{5 - } $$\end{document}P3O8(NH)25- systems. According to the HSAB concept, hard metal cations such as Ca²⁺, Ba²⁺ and Sr²⁺ should bind to the harder oxygen atoms rather than the softer nitrogen atoms of the imidopolyphosphate anions, preventing direct coordination to the imino nitrogen atom.

Graphical Abstract

Electronic supplementary material

The online version of this article (doi:10.1007/s10953-013-0099-2) contains supplementary material, which is available to authorized users.

Biomimetic Interactions of Proteins with Functionalized Nanoparticles: A Thermodynamic Study

Gold nanoparticles (NPs) functionalized with L-amino acid-terminated monolayers provide an effective platform for the recognition of protein surfaces. Isothermal titration calorimetry (ITC) was used to quantify the binding thermodynamics of these functional NPs with alpha-chymotrypsin (ChT), histone, and cytochrome c (CytC). The enthalpy and entropy changes for the complex formation depend upon the nanoparticle structure and the surface characteristics of the proteins, e.g., distributions of charged and hydrophobic residues on the surface. Enthalpy-entropy compensation studies on these NP-protein systems indicate an excellent linear correlation between DeltaH and TDeltaS with a slope (alpha) of 1.07 and an intercept (TDeltaS0) of 35.2 kJ mol(-1). This behavior is closer to those of native protein-protein systems (alpha = 0.92 and TDeltaS0 = 41.1 kJ mol(-1)) than other protein-ligand and synthetic host-guest systems.

Thermodynamics of rhodium hydride reactions with CO, aldehydes, and olefins in water: organo-rhodium porphyrin bond dissociation free energies

Tetra(p-sulfonato-phenyl) porphyrin rhodium hydride ([(TSPP)Rh-D(D2O)](-4)) (1) reacts in water (D2O) with carbon monoxide, aldehydes, and olefins to produce metallo formyl, alpha-hydroxyalkyl, and alkyl complexes, respectively. The hydride complex (1) functions as a weak acid in D2O and partially dissociates into a rhodium(I) complex ([(TSPP)Rh(I)(D2O)](-5)) and a proton (D+). Fast substrate reactions of 1 in D2O compared to reactions of rhodium porphyrin hydride ((por)Rh-H) in benzene are ascribed to aqueous media promoting formation of ions and supporting ionic reaction pathways. The regioselectivity for addition of 1 to olefins is predominantly anti-Markovnikov in acidic D2O and exclusively anti-Markovnikov in basic D2O. The range of accessible equilibrium thermodynamic measurements for rhodium hydride substrate reactions is substantially increased in water compared to that in organic media through exploiting the hydrogen ion dependence for the equilibrium distribution of species in aqueous media. Thermodynamic measurements are reported for reactions of a rhodium porphyrin hydride in water with each of the substrates, including CO, H2CO, CH3CHO, CH2=CH2, and sets of aldehydes and olefins. Reactions of rhodium porphyrin hydrides with CO and aldehydes have nearly equal free-energy changes in water and benzene, but alkene reactions that form hydrophobic alkyl groups are substantially less favorable in water than in benzene. Bond dissociation free energies in water are derived from thermodynamic results for (TSPP)Rh-organo complexes in aqueous solution for Rh-CDO, Rh-CH(R)OD, and Rh-CH2CH(D)R units and are compared with related values determined in benzene.

Effect of PVP K-25 on the formation of the naproxen:beta-ciclodextrin complex

The aim of this study was to investigate the effects of the presence of the water-soluble polymer polyvinylpyrrolidone K-25 (MW=24000g/mol) on the complexation of the AINE naproxen, in its sodium salt form, with the beta-cyclodextrin. The data revealed that the polyvinylpyrrolidone K-25 interacts with the drug as well as with the drug:beta-cyclodextrin inclusion complex. The polymer shows more affinity for the inclusion complex, K=(6.67+/-0.292) x 10(-5)M(-1) than for the free drug, (2.08+/-0.208) x 10(-5)M(-1). The presence of different proportions of polymer, in a range 0-1% (w/w) of polyvinylpyrrolidone, does not increase the ability of drug-cyclodextrin complexation but important changes in the driving force of complex formation were detected, depending on the percentage of polyvinylpyrrolidone K-25 present. At low polymer concentrations, the complexation process is driven entropically, while at higher PVP proportions it is enthalpically favored. In the ternary system, polyvinylpyrrolidone K-25 partially or totally coats the drug:beta-cyclodextrin inclusion complex interacting with the beta-cyclodextrin (through hydrogen bonds), and with the naproxen.

Solvent and guest isotope effects on complexation thermodynamics of alpha-, beta-, and 6-amino-6-deoxy-beta-cyclodextrins

The stability constant (K), standard free energy (DeltaG degrees ), enthalpy (DeltaH degrees ), and entropy changes (TDeltaS degrees ) for the complexation of native alpha- and beta-cyclodextrins (CDs) and 6-amino-6-deoxy-beta-CD with more than 30 neutral, positively, and negatively charged guests, including seven fully or partially deuterated guests, have been determined in phosphate buffer solutions (pH/pD 6.9) of hydrogen oxide (H(2)O) or deuterium oxide (D(2)O) at 298.15 K by titration microcalorimetry. Upon complexation with these native and modified CDs, both nondeuterated and deuterated guests examined consistently exhibited higher affinities (by 5-20%) in D(2)O than in H(2)O. The quantitative affinity enhancement in D(2)O versus H(2)O directly correlates with the size and strength of the hydration shell around the charged/hydrophilic group of the guest. For that reason, negatively/positively charged guests, possessing a relatively large and strong hydration shell, afford smaller K(H2O)/K(D2O) ratios than those for neutral guests with a smaller and weaker hydration shell. Deuterated guests showed lower affinities (by 5-15%) than the relevant nondeuterated guests in both H(2)O and D(2)O, which is most likely ascribed to the lower ability of the C-D bond to produce induced dipoles and thus the reduced intracavity van der Waals interactions. The excellent enthalpy-entropy correlation obtained can be taken as evidence for the very limited conformational changes upon transfer of CD complexes from H(2)O to D(2)O.

Mechanistic implications of the equality of compensation temperatures in chromatography

A common interpretation of the observation that two processes exhibit similar compensation temperatures in an enthalpy-entropy plot is that the two processes occur via the same "mechanism". We show that this interpretation is not rigorously allowed. In fact, the only thing that can be concluded from the observation of identical compensation temperatures is that the relative contributions of enthalpy and entropy to the overall free energy are the same in the two processes. Since it is possible that two processes occur via different mechanisms that, by chance, result in the same relative blends of enthalpy and entropy, the observation of identical compensation temperatures cannot be used as evidence for mechanistic identity. If two processes exhibit different compensation temperatures, however, it can logically be concluded that the two processes are mechanistically distinct.

Inclusion of phthalate esters by a self-assembled monolayer of thiolated cyclodextrin on a gold electrode

Complexation of phthalic acid esters (PAEs) by a self-assembled monolayer (SAM) of thiolated alpha-cyclodextrin (6-(2-mercaptoethylamino)-6-deoxy-alpha-cyclodextrin, MEA-alpha-CD) on a gold electrode was examined by a cyclic voltammetry using hydroquinone (HQ) as a probe. From the inhibitory effect of the phthalate esters on the inclusion of HQ by the surface-confined cyclodextrin (CD), the association constants (Kasn) of the esters with the immobilized CD were estimated. For comparison, the association of PAEs with free alpha-CD was examined spectrophotometrically using methyl orange as a probe. It was concluded that, in both free and surface-confined CD systems, the Kasn value increased with an increase in the length of aliphatic alcohols conjugated to phthalic acid. Furthermore, the Kasn values for PAEs in the SAM system were much larger than those in a free CD system. This could be intuitively ascribed to the steric factor for the PAEs to come out from the cavity of surface-confined CD, whose rim was in contact with the PAEs. Thermodynamic parameters indicated that the inclusion of PAEs in the SAM system was entropy-driven, which is different from the free CD system where the inclusion was favored by both enthalpy and entropy. This is partly due to the difficulties in cancellation of strain energy by the inclusion into the cavity of the densely fixed CD (97% of the calculated maximum) and partly due to the reduction of hydrogen bonding between the PAEs and the surface-confined CD. Desolvation of the PAEs and CD by the friction at the penetration into the cavity of CD, which was rigidly fixed to the electrode, might also contribute to the positive entropy change. These factors might emphasize the apolar factor of binding to be characterized by a favorable entropy change in the immobilized CD system.