Isothermal Titration Calorimetry and Theoretical Studies on Host-guest Interaction of Ibuprofen with <Emphasis Type="Bold">α</Emphasis>-, <Emphasis Type="Bold">β</Emphasis>- and <Emphasis Type="Bold">γ</Emphasis>-Cyclodextrin

Thermodynamic parameters for formation of the inclusion complexes of α-, β- and γ-cyclodextrin (α-, β- and γ-CD) with ibuprofen (BF) in Tris-HCl buffer solutions (pH=7.0) have been determined by isothermal titration calorimetry (ITC) with nanowatt sensitivity, and the inclusion structures have been investigated by using ¹H-NMR spectra at 298.15 K. A theoretical study on the inclusion processes between BF and CDs has been performed with the B3LYP/6-31G*//PM3 method in order to investigate the formation mechanism of the inclusion complexes. An analysis of the thermodynamic data indicates that the stoichiometries of α-, β- and γ-CD with BF are all 1:1 and formation of the inclusion complexes α-CD⋅BF and β-CD⋅BF are driven by enthalpy and entropy, whereas formation of γ-CD⋅BF is an entropy driven process. The ¹H-NMR spectra provide clear evidence for the inclusion phenomenon, and show that the isobutyl group and aromatic ring of the guest molecule are trapped inside the cavity of the CDs. Theoretical calculations suggest that the complex formed by the BF molecule entering into the cavity of the CD molecule from the wide side is more stable than that from the narrow side.     

Application of combined thermoanalytical techniques in the investigation of cyclodextrin inclusion complexes

Citronellol and citronellyl acetate have been entrapped with α-, β- and γ-cyclodextrin (CD). Evolved gas detection and TG-MS coupling was applied to prove the actual inclusion complex formation between monoterpens and CDs. The terpene content was determined by UV-VIS specrophotometry and RP-HPLC and the effect of storage time on the terpene content was also investigated. The α- and γ-cyclodextrin inclusion complexes showed higher thermal stabilities vs. dynamic heating compared to the β-CD complexes. On the contray, the retention of guest using β-cyclodextrin even after 10 years of storage was much more pronounced. Experimental data other than 1:1 complex compositions are assumed. Molecular modeling experiments also suggested multiple complex compositions.     

Study on the Association Phenomenon of Cyclodextrin to Porphyrin J-aggregates by NMR Spectroscopy

The nuclear magnetic resonance (NMR) spectroscopy demonstrated that the inclusion complexes of meso-tetrakis- (p-sulfonatophenyl) porphyrin (TPPS) with β-, Hydroxypropyl-β- and Methyl-β-cyclodextrin (β-, HP-β- and Me-β-CD) are formed, which resulted in the dissociation of TPPS J-aggregates efficiently under certain acidity. There are no significant differences in binding affinities and basic complexation mechanisms between TPPS and β-cyclodextrin (β-CD) or hydroxypropyl-β-cyclodextrin (HP-β-CD), i.e. porphyrin is included through the wide side of the cavity of β-CD or HP-β-CD. Alternatively, porphyrin is included through the narrow side of the Me-β-CD cavity.     

A Simulation on the Complexation of Cyclodextrins with Phospholipid Headgroups

The inclusion complexes of α-, β- and γ-cyclodextrin (CD) with three isolated phospholipid (PI – phosphatidylinositol; PS – phosphatidylserine; and PE – phosphatidylethanolamine) headgroups were studied using a flexible docking algorithm FDOCK based on molecular mechanics (CFF91 force filed). In the three phospholipid headgroups, PI headgroup exhibits the strongest affinity for CD, and the affinity of PS headgroup is greater than that of PE headgroup. By investigating the energy distribution and the complex structure in the inclusion procedure, it can be found that the van der Waals force is the main driving force responsible for the complexation. For the α-CD complex of PI headgroup, more than one inclusion complex should coexist due to the steric hindrance, which is reasonably consistent with the experimental results. Furthermore, analyses of the complex of PS and PE headgroup with α-CD also show that two or three possible complexes may appear in the inclusion process, and the complex structure with full inclusion is of the lowest energy and should be the most stable structure in the mixture. For β-␣and γ-CD, the energies of the most stable complexes structures for the three phospholipids headgroups were also discussed.     

Self-assembly behavior of inclusion complex formed by β-cyclodextrin with α-aminopyridine

An inclusion complex (1) has been prepared by β-cyclodextrin with α-aminopyridine. The result of X-ray crystallographic analyses showed that the α-aminopyridine molecules in the β-cyclodextrin cavities possess two opposite orientations, i.e. the amine group of α-aminopyridine pointing to the primary side (1a, occupancy: 41.2%) or the secondary side (1b, occupancy: 58.8%) of β-cyclodextrin, forming two scalelike supramolecular aggregations. The studies of 2D NMR and circular dichroism spectra indicated that the α-aminopyridine molecule is deeply embedded in the β-cyclodextrin cavity to form host-guest inclusion complex, showing a circular dichroism spectrum induced by the chiral cavity of cyclodextrin. The results obtained are helpful for understanding the molecular recognition and aggregation mechanism between the host and guest.     

Cyclodextrins Binding to Paeonol and Two of Its Isomers in Aqueous Solution. Isothermal Titration Calorimetry and 1H NMR Investigations of Molecular Recognition

Interactions between CDs with three substituted phenols, paeonol (Pae), acetovanillone (Ace) and 2-hydroxyl-5-methoxy-acetophone (Hma), which are isomers, have been determined by isothermal titration calorimetry (ITC) and ¹H NMR in aqueous solution at 298.2 K. Both the binding thermodynamics and ¹H NMR spectra show that the interaction between α-cyclodextrin (α-CD) molecule and each guest molecule is extremely weak. The thermodynamic parameters indicate that the binding processes of β-cyclodextrin (β-CD) with the isomers are mainly entropy driven and that β-CD binds with Pae or Ace in 1:1 stoichiometry, whereas with Hma binds in 1:1 and 2:1 stoichiometries. The thermodynamic parameters also suggest that γ-cyclodextrin (γ-CD) binds each isomer in the same 1:1 stoichiometry. The binding processes of Pae and Hma with γ-CD are enthalpy driven whereas Ace with γ-CD is predominantly driven by entropy. The ¹H NMR spectra reveal that the three isomers were trapped into the torus cavity of the β-CD molecule from the narrow side during the binding process. Pae penetrates into the γ-CD cavity from the primary rim of the macrocycle whereas Ace does so from the secondary rim, but Hma appears not interact with the internal cavity of γ-CD at all.     

Study on inclusion complexes of meso-tetrakis(2-thienyl)porphyrin and Cu-meso-tetrakis(2-thienyl)porphyrin with cyclodextrins by spectroscopy method

In phosphate buffer solution of pH5.4, the interaction of meso-tetrakis(2-thienyl)porphyrin(H₂TTP) and Cu-meso-tetrakis(2-thienyl)porphyrin(Cu-TTP) with α-cyclodextrin(α-CD), β-CD, γ-CD, heptakis(2,3,6-tri-O-methyl)-β-CD(TM-β-CD) has been studied by means of UV-vis, fluorescence and ¹HNMR spectroscopy, respectively. The H₂TTP and Cu-TTP can form 1:2 inclusion complexes with TM-β-CD and 1:1 inclusion complexes with the other three cyclodextrins. In this paper, the inclusion constants (K) of H₂TTP and Cu-TTP for the formation of the inclusion complexes have been estimated from the changes of absorbance and fluorescence intensity in phosphate buffer solution. The inclusive capabilities of different kinds of cyclodextrins are compared. The result shows that the inclusion ability of α-CD with H₂TTP and Cu-TTP is the strongest among the three native CDs. The inclusion ability of modified β-CD with H₂TTP and Cu-TTP is stronger, compared to the native β-CD, which indicates that the capacity matching plays a crucial role in the inclusion procedure except for the hydrophobic effect. In addition ¹HNMR spectra supports the inclusion conformation of the TM-β-CD-Cu-TTP inclusion complex, indicating the interaction mechanism of inclusion processes.     

Host–guest interactions between niobocene dichloride and α-, β-, and γ-cyclodextrins: preparation and characterization

The possible inclusion complexes of Cp₂NbCl₂ into α-, β-, and γ-CD hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed by a combination of thermogravimetric analysis, FTIR, PXRD, and ¹³C CP-MAS NMR spectroscopies. The solid-state results demonstrated that α-cyclodextrin does not form inclusion complexes with Cp₂NbCl₂ whereas β- and γ-cyclodextrins do form such complexes. PXRD, NMR, and thermal analysis showed that the organometallic molecules of Cp₂NbCl₂OH are included in the cavities of β- and γ-cyclodextrins, possibly adopting a symmetrical conformation in the complex, with each glucose unit in a similar environment. In solution, ¹H NMR experiments suggest that niobocene has a shallow penetration on the β-CD leading to upfield shift on H-3 signal with a minor perturbation on the H-5 proton while for γ-CD, both H-3 and H-5 are shifted upfield substantially. This suggests that niobocene penetrates deeper into the γ-CD cavity than in the β-CD cavity, as a result of the cavity size. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Inclusion Complex Formation Between Modified Cyclodextrins and Riboflvin and Alloxazine in Aqueous Solution

Inclusion complex formation of hydroxypropylated α-, β- and γ-cyclodextrins with riboflavin (vitamin B₂) and alloxazine was studied by spectroscopic and solubility methods. Alloxazine, which is a structural analog of riboflavin, was considered in order to evaluate the role of ribityl and methyl substituents in complexation. Thermodynamic parameters for 1:1 complex formation were obtained and analyzed in terms of influence of the reagent structure on the binding process. It was shown that the cavity of hydroxypropyl-β-cyclodextrin is more appropriate for formation of stable complexes. The complexes are enthalpy stabilized, due to prevalence of van der Waals interactions and possible hydrogen bonding. The partial insertion of riboflavin into the cyclodextrin cavity was revealed by ¹H NMR and computer modeling. The ribityl side chain, which prevents deep inclusion, is located nearby the wider rim of the cyclodextrin molecule and can undergo destruction. Penetration of the alloxazine molecule into the macrocyclic cavity is deeper and accompanied by formation of more stable inclusion complexes. Hydroxypropyl-β-cyclodextrin was found to be the more efficient solubilizing agent for riboflavin and alloxazine, whereas a stabilization action of cyclodextrins towards riboflavin was not observed.     

A Study of b-Cyclodextrin Inclusion Complexes with Progesterone and Hydrocortisone Using Rotating Frame Overhauser Spectroscopy

Summary.  Inclusion complexes of β-cyclodextrin with two steroid derivatives, progesterone (pregn-4-ene-3,20-dione) and hydrocortisone (11,17,21-trihydroxy-pregn-4-ene-3,20-dione), were studied in the liquid state by NMR spectroscopy. The complex formation process was monitored by intermolecular dipolar interactions between ¹H signals in the hydrophobic β-cyclodextrin cavity (H-3 and H-5 of the α-glucose units) and the steroid moiety in ROESY spectra. The data revealed that progesterone is fully immersed in the β-cyclodextrin cavity; however, complete inclusion of the hydrocortisone molecule was prevented by the polar hydroxyl groups on its surface. Received April 26, 2001. Accepted (revised) May 18, 2001     

The Effect of Cyclodextrins on the Luminol-Hydrogen Peroxide Chemiluminescence

The action of different molar ratios of α, β, γ-cyclodextrin upon the chemiluminescence of the luminol-H₂O₂ in alkaline buffer Tris-HCl, pH=8.5 has been evidenced. It was found out that α, β, γ- cyclodextrin have an antioxidant capacity, probably due to the free radicals (that are generate in the system) encapsulation in the their cavity. This behaviour depends on α, β, γ-cyclodextrin molar ratio; α-cyclodextrin and γ-cyclodextrin protects more efficiently against free radicals than β-cyclodextrin. These findings could be very important regarding the oxidative stress process.     

Inclusion complexes of fusidic acid and three structurally related compounds with cyclodextrins

The inclusion complexes between fusidate, 3-keto fusidate, 11-keto fusidate and 11-deoxy fusidate and α-, β-, and γ-cyclodextrin (CD) were studied using capillary electrophoresis. By monitoring the changes in mobility of the negatively charged compounds in the presence of varying amount of CD the stability constants of the complexes formed could be obtained. In the case of α- and β-CD the obtained results could be modelled to a simple model assuming 1:1 stoichiometry, revealing, not surprisingly, that β-CD formed a stronger complex compared to α-CD. A model assuming 1:2 (fusidate:CD) stoichiometry could be fitted to the data obtained with γ-CD. The results showed that the different fusidanes formed very strong 1:1 complexes with γ-CD as well as a quite weak 1:2 complex. 3-keto-, 11-keto- and 11-deoxy-fusidate formed stronger complexes compared to fusidate, probably due to an decrease in hydrophilicity caused by the reduced number of hydroxyl groups. The complex between γ-CD and fusidate was studied by use of 2D-NMR spectroscopy. The results showed that most of the hydrogen atoms of fusidate show interactions with the hydrogen atoms in the cavity of γ-CD. The interaction pattern suggests that fusidate may be fully embedded in the cavity of γ-CD. No interactions between fusidate and the hydrogen atoms situated at the outside of the CD were found.     

Inclusion compounds of cystostatic active (C5H5)2VCl2 and (CH3C5H4)2VCl2 with α-, β- and γ-cyclodextrines: Synthesis, EPR study and microbiological behavior toward Escherichia coli

The inclusion of vanadocene dichloride (VDC) and 1,1′-dimethyl vanadocene dichloride (MeVDC) into cyclodextrines (α-CD, β-CD and γ-CD) was studied by EPR spectroscopy. It was found that VDC and MeVDC with β-CD and γ-CD form true inclusion compounds, but with α-CD, VDC and MeVDC gave only fine dispersion mixtures. The inclusion was validated by anisotropic EPR spectra of solid samples. In addition, the antimicrobial was validated by anisotropic EPR spectra of solid samples. In addition, the antimicrobial behavior (against E. coli) of each of the complexes was determined. It was established that not only did VDC and MeVDC cause elongation of E. coli, but also the new vanadocene inclusion complexes were effective in this regard.     

Interactions of tetrakis(4-N-methylpyridyl)porphyrin with cyclodextrins and disodium phthalate in aqueous solution

In pH 7.3 buffers, the interactions of a cationic porphyrin, tetrakis(4-N-methylpyridyl)porphyrin (TMPyP), with cyclodextrins (CDs) and disodium phthalate (DSP) have been examined by means of absorption, fluorescence, and induced circular dichroism spectroscopy. α-CD, β-CD, and γ-CD form a 1:1 inclusion complex with a TMPyP monomer, which dimerizes in solution without CD. TMPyP also forms a 1:1 organic cation–organic anion complex with DSP. The 1:1 TMPyP–DSP complex forms a ternary CD–TMPyP–DSP inclusion complex with α-, β-, and γ-CD, in which a DSP molecule is not incorporated into the CD cavity. From the fluorescence intensity change, the␣equilibrium constants have been evaluated for the formation of the inclusion complexes and the organic cation–anion complexes.     

Study on the Inclusion Compounds of Eugenol with α-, β-, γ- and Heptakis (2,6-di-O-methyl)-β-cyclodextrins

Several host–guest inclusion compounds of eugenol as a guest molecule and cyclodextrins (α-,β-,γ-CD) and heptakis (2,6-di-O-methyl)-β-cyclodextrin (DMβ-CD) as hosts were investigated in the solid state and in aqueous solution. The one-to-one solid inclusion compounds of eugenol and β-CD or γ-CD were prepared, but those of eugenol with α- or DMβ-CD were not obtained under the same condition. However, the UV-visible absorption spectroscopy data indicated that the liquid guest could form a 1:1 inclusion compound with all four hosts respectively in aqueous solution. The two solid inclusion compounds were characterized by powder X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric analysis (TG), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR). The association constants (K), calculated from the modified Benesi–Hidebrand equation, of eugenol with α-, β-, γ- and DMβ-CD is 4.95 × 10⁴, 3.96 × 10⁵, 1.47 × 10⁵ and 9.33 × 10⁴ mol⁻¹ dm³, respectively.     

The Interactions of Titanocene Dihalides with α-, β- and γ-cyclodextrin Host Molecules

The inclusion of titanocene dihalides (X = F, Cl) into -, - and -cyclodextrin hosts was studied by NMR spectroscopy, thermal analysis and mass spectrometry. It was found that -cyclodextrin does not form inclusion complexes with titanocene halides whereas - and -cyclodextrin do form such complexes. According to the changes in NMR spectra we propose that there is a shallow penetration of a guest molecule of titanocene dihalide into the cavity in the case of -cyclodextrin, but deeper penetration in the case of -cyclodextrin. The stability of the latter inclusion complexes was studied by NMR shift titration.     

Host–Guest Interaction Study of Resveratrol With Natural and Modified Cyclodextrins

The aim of this work is to increase the stability and water solubility of resveratrol by complexation with different cyclodextrins. Furthermore, physical–chemical properties of each inclusion compound were investigated. Complexes of resveratrol with cyclodextrins both native (α, β, γ) and modified (2-hydroxypropyl-β-cyclodextrin, dimethyl-β-cyclodextrin) were obtained by using the suspension method. An inclusion complex with β-cyclodextrin was also prepared by using the microwave. Solid state characterization of the products was carried out using Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), X-ray diffraction (DRX); solution studies were performed by UV–Vis spectrophotometry and ¹H-NMR spectroscopy. Phase solubility profiles with all cyclodextrins employed were classified as A_N type, indicating the formation of 1:1 stoichiometric inclusion complexes. Stability constants (K _c) from the phase solubility diagrams were calculated. Stability studies in the solid state and in solution were performed; the photodegradation by UV–Vis spectrophotometry was monitored. The isomerization rate trans to cis, in ethanol solution, decreased with inclusion. The dissolution studies revealed that resveratrol dissolution rate was improved by the formation of inclusion complexes.     

Binary Systems of Nifedipine And Various Cyclodextrins in The Solid State. Thermal,FTIR, XRD studies

Nifedipine complexes with β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), 2-hydroxypropyl-β-cyclodextrin (2HP-β-CD), randomly methylated-β-cyclodextrin (RM-β-CD) and heptakis(2,6-O-dimethyl)-β-cyclodextrin (DM-β-CD) have been prepared by both kneading and heating methods and their behaviour studied by differential scanning calorimetry (DSC), diffuse reflectance mid-infrared spectroscopy (FTIR) and X-ray diffractometry (XRD). DSC revealed the nifedipine melting endotherm with onset at approximately 171°C for the kneaded mixtures with β-CD, γ-CD and 2HP-β-CD, thus confirming the presence of nifedipine in the crystalline state, while some decrease in crystallinity was observed in the DM-β-CD kneaded mixture. With RM-β-CD, however, broadening and shifting of the nifedipine endotherm and reduction in its intensity suggested that the kneading could have produced an amorphous inclusion complex. These differing extents of interaction of nifedipine with the cyclodextrins were confirmed by FTIR and XRD studies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermodynamic Study of the Inclusion Interaction between Gemini Surfactants and Cyclodextrins by Isothermal Titration Microcalorimetry

The inclusion complexes of a series of bis-quarternary ammonium surfactants, (C _n N)₂Cl₂ (where n = 12, 14, 16) and sodium bis(2-ethylhexyl) sulfosuccinae (AOT), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) in aqueous solutions were investigated by using isothermal titration calorimetry (ITC) at 298.15 K. The stability constants, stoichiometry, and formation enthalpies, entropies and Gibbs energies for the complexes in aqueous solutions have been derived from the calorimetric data. The values of the binding constant, K ^∘ _i , are very large, which indicates that these complexes are quite stable in their aqueous solutions. The enthalpy changes (ΔH ^∘) for all of the inclusion processes are negative, showing that the complex process is enthalpy driven. The entropy effect (TΔS ^∘) is negative, so the inclusion process is entropically unfavorable. The large negative Gibbs energy changes indicate that formation of host-guest inclusion complexes is generally a spontaneous process. The thermodynamic parameters are discussed in the light of the different structures of the host and guest molecules.     

New Aspects of Cyclodextrin Chemistry Induced by Outside Type Complex Formation; Asymmetric Reduction of Indol-3-Pyruvic Acid with NaBH4 in Aqueous Solution

Asymmetric reduction of indol-3-pyruvic acid (IPA) with NaBH₄ in aqueous solution in the presence of various cyclodextrins (α-, β-, γ-, mono-6-amino-6-deoxy-β- and di-6^ABamino-6^AB-deoxy-β-cyclodextrin) was investigated. From the NMR and circular dichroism spectral studies, the conformation of the CyD–substrate complexes is suggested; the part of carboxylic group stay in the cavity of α-CyD, whole of IPA in β-CyD, two molecules in a γ-CyD cavity, and IPA(s) is/are on the rim of the cavity of mono-6-amino-6-deoxy-β- and di-6^ABamino-6^AB-deoxy-β-CyD (AβCyD, DAβCyD) with electrostatic interaction between amino group and carboxylic group. This conformational difference provides in the difference in the optical selectivity of reduction.