Site-Specific Interaction between 2-Dibenzofuran Carboxylate and β- and γ-Cyclodextrins Determined by Intermolecular NOE and Molecular Modeling

A Comprehensive Study of Gemfibrozil Complexation with β-Cyclodextrins in Aqueous Solution Using Different Analytical Techniques

Gemfibrozil (GEM) is a hypolipidemic agent, which is effective in reducing serum cholesterol and triglyceride levels. Complexation of GEM with native β-cyclodextrin (β-CD) and with the derivatives hydroxypropyl-β- and randomly methylated β-CD (HPβ-CD and Meβ-CD) was studied in aqueous solution of pH 2.8 and 7.0. The stability constants were determined by spectrofluorimetry, ¹H-NMR spectroscopy and solubility assays. Considering the well-known difficulties to obtain similar stability constants by different techniques, the agreement of the values obtained supports the reliability of the results presented. The advantages and drawbacks of each analytical technique for the study of inclusion complexation were discussed as well. In addition, the thermodynamic parameters of complexation, enthalpy (ΔH) and entropy (ΔS), were determined and related to the type of molecular interactions that take place between GEM and the different cyclodextrins. Finally, solid dispersions were prepared by co-evaporation, kneading, vacuum desiccation, and coprecipitation, and complexation was evaluated by X-ray diffraction.

Theoretical Study of the β-Cyclodextrin Inclusion Complex Formation of Eugenol in Water

The interaction between eugenol and β-cyclodextrin in the presence of water is studied by molecular mechanics and dynamics simulations. A force field model is used in molecular mechanics to determine the interaction energy and the complex configuration at the absolute minimum. The van der Waals term is the main contribution to the total energy, and so directly determines the configuration of the inclusion complex. The formation of inclusion complexes is simulated by molecular dynamics, in which their configurations are deduced from the position probability density that represents the preferred location and orientation of the guest in the simulation. When eugenol approaches from the rims of β-cyclodextrin, it tends to enter the cavity, remain inside for a short period and then exit from it. The guest tends to include the phenyl ring inside the cavity in the most probable configurations. Two inclusion complex configurations are proposed, each with the hydroxyl and methoxyl groups pointing towards one different rim of β-cyclodextrin. The initial guest orientation is the main factor determining these configurations. The model presented in this study reproduces the experimental findings on inclusion complex formation and proposes two possible complex configurations, one previously suggested by different authors.

Molecular Dynamics Study of the Factors Influencing the β-Cyclodextrin Inclusion Complex Formation of the Isomers of Linear Molecules

The influence of the size, composition, and atomic distribution of linear guests on β-cyclodextrin inclusion complex formation is clarified by means of a molecular dynamics simulation at constant temperature. The intermolecular energy is modelled by a Lennard-Jones potential, where the molecular composition is represented by various parameters and by a continuum description of the guest and cavity walls. It is concluded that the parameters related to the atomic size require minimum values for the confinement of linear molecules inside the cavity. The isomer with optimal affinity for β-cyclodextrin as predicted by the free energy presents an asymmetrical molecular structure, and the position probability density shows that the isomer tends to insert the portion with largest atoms into the cavity, although the preferential binding site of the guest is not always located in regions of the host with maximum discriminatory power.

Using inclusion complexes with cyclodextrins to explore the aggregation behavior of a ruthenium metallosurfactant

The aggregation behavior of a chiral metallosurfactant, bis(2,2'-bipyridine)(4,4'-ditridecyl-2,2'-bipyridine)ruthenium(II) dichloride (Ru2(4)C13), synthesized as a racemic mixture was characterized by small-angle neutron scattering, light scattering, NMR, and electronic spectroscopies. The analysis of the SANS data indicates that micelles are prolate ellipsoids over the range of concentrations studied, with a relatively low aggregation number, and the micellization takes place gradually with increasing concentration. The presence of cyclodextrins (β-CD and γ-CD) induces the breakup of the micelles and helps to establish that micellization occurs at a very slow exchange rate compared to the NMR time scale. The open structure of this metallosurfactant enables the formation of very stable complexes of 3:1 stoichiometry, in which one CD threads one of the hydrocarbon tails and two CDs the other, in close contact with the polar head. The complex formed with β-CD, more stable than the one formed with the wider γ-CD, is capable of resolving the Δ and Λ enantiomers at high CD/surfactant molar ratios. The chiral recognition is possible due to the very specific interactions taking place when the β-CD covers-via its secondary rim-part of the diimine moiety connected to the hydrophobic tails. A SANS model comprising a binary mixture of hard spheres (complex + micelles) was successfully used to study quantitatively the effect of the CDs on the aggregation of the surfactant.

Study of inclusion complex between 2,6-dinitrobenzoic acid and β-cyclodextrin by 1H NMR, 2D 1H NMR (ROESY), FT-IR, XRD, SEM and photophysical methods

The formation of host-guest inclusion complex of 2,6-dinitrobenzoic acid (2,6-DNB) with nano-hydrophobic cavity of β-cyclodextrin (β-CD) in solution phase has been studied by UV-visible spectroscopy and electrochemical analysis (cyclic voltammetry, CV). The effect of acid-base concentrations of 2,6-DNB has been studied in presence and absence of β-CD to determination for the ground state acidity constant (pKa). The binding constant of inclusion complex at 303 K was calculated using Benesi-Hildebrand plot and thermodynamic parameter (ΔG) was also calculated. The solid inclusion complex formation between β-CD and 2,6-DNB was confirmed by 1H NMR, 2D 1H NMR (ROESY), FT-IR, XRD and SEM analysis. A schematic representation of this inclusion process was proposed by molecular docking studies using patch dock server.

Spectral and electrochemical study of host-guest inclusion complex between 2,4-dinitrophenol and β-cyclodextrin

The formation of host-guest inclusion complex of 2,4-dinitrophenol (2,4-DNP) with nano-hydrophobic cavity of β-cyclodextrin (β-CD) in solution phase was studied by UV-visible spectrophotometer and electrochemical method (cyclic voltammetry, CV). The prototropic behaviors of 2,4-DNP with and without β-CD and the ground state acidity constant (pK(a)) of host-guest inclusion complex (2,4-DNP-β-CD) were studied. The binding constant of inclusion complex at 303K was calculated using Benesi-Hildebrand plot and thermodynamic parameter (ΔG) was also calculated. The solid inclusion complex formation between β-CD and 2,4-DNP was confirmed by (1)H NMR, FT-IR, XRD and SEM analysis. A schematic representation of this inclusion process is proposed by molecular docking studies using PatchDock server.

Influence of amino acid sequence on the interaction of short peptides containing 3-[2-(9-anthryl)benzoxazol-5-yl]-alanine with β-cyclodextrin

The influence of peptide sequence and Leu chirality in linear and cyclic peptides containing 3-[2-(9-anthryl)benzoxazol-5-yl]alanine on interaction with β-cyclodextrin were studied using fluorescence and NMR spectroscopy. The analysis of enthalpy-entropy compensation effect (α=1.05±0.02 and TΔS00=15.1±0.5 kJ mol−1) indicates that the entropic contribution connected with the solvent reorganization is the major factor governing the peptides-β-cyclodextrin complexation. Moreover, spatial orientation of guest-host molecule depends more than association constant on Leu residue configuration. However, the cyclization of the peptide chain substantially decrease the association constant with β-CD. An analysis of 2D NMR spectra reveals that inclusion complex is formed by penetration of cyclodextrin cavity from wider and narrow rims by anthryl group in the case of Box(Ant)-SPKL or anthryl and Leu residues for Box(Ant)-SPK(D)L analogue.

Inclusion Complexes between β-Cyclodextrin and a Gemini Surfactant in Aqueous Solution: An NMR Study

(1)H NMR spectra, diffusion-ordered NMR (DOSY), and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments for aqueous solutions at 298 K containing the gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO(1)-12), in the absence and presence of beta-cyclodextrin (beta-CD) were used to characterize the surfactant and to determine the effects of the complexation in the micellization. For the binary system, the critical micelle concentration (cmc), the aggregation number, the stepwise micellization constant, and the size of the monomer have been obtained by studying the dependence of the chemical shifts and the self-diffusion coefficients with the concentration of surfactant. For the ternary system, the analysis of the (1)H NMR spectra and the self-diffusion coefficients reveal the formation of complexes of 1:1 and 2:1 stoichiometry (beta-CD:gemini), with a calculated stability constant for the second binding step higher than that of the first. The values of the hydrodynamic radii of the complexes were obtained from the calculated diffusion coefficients. The presence of beta-CD modifies the cmc in an extension that indicates mainly the formation of a 2:1 complex. The analysis of the chemical shifts of the surfactant indicates the nonparticipation of the complexes into the micelles. ROE enhancements depend substantially on the amount of the macrocycle added and therefore on the stoichiometry; at low concentrations of beta-CD, one of the hydrocarbon chains binds favorably with the cavity whereas the other interacts with the outer face. By contrast, at higher concentrations of beta-CD, the two hydrocarbon tails are included in two different macrocycles.

Influence of the separation of the charged groups and aromatic ring on interaction of tyrosine and phenylalanine analogues and derivatives with β-cyclodextrin

Interactions of tyrosine and phenylalanine analogues with beta-cyclodextrin have been examined in terms of structural features of the ligand such as the separation of the charged amino group and aromatic ring, the presence of additional functional group attached to the amino or phenyl ring, and the presence of a charge on amino or carboxyl group, and steric effects using steady-state and time-resolved fluorescence spectroscopy and microcalorimetry. The studied aromatic amino acids possess low binding constant to beta-cyclodextrin, diversified with respect to the presence or absence of a substituent in para position of the phenyl ring. However, calculated, based on the global analysis of the fluorescence intensity decays, binding constants do not allow to estimate unequivocally the influence of the distance between the charged groups and phenol/phenyl ring on the inclusion complex stability because of their low diversification.