Dynamic study of interaction between beta-cyclodextrin and aspirin by the ultrasonic relaxation method

A single ultrasonic relaxational phenomenon was observed in aqueous solutions containing both beta-cyclodextrin (beta-CD) as host and nonionized or ionized acetylsalicylic acid (aspirin) as guest. The observed relaxation was responsible for a dynamic complexation reaction between beta-CD and aspirin molecules, concomitant with a volume change during the reaction. The kinetic and equilibrium constants for the complexation in the acid (nonionized) form of the aspirin system were derived from the guest concentration dependence of the relaxation frequency. The equilibrium constant for the carboxylate (ionized) form of aspirin was determined from the concentration dependence of a maximum absorption per wavelength, and the rate constants were calculated by using the determined equilibrium constant and the observed relaxation frequencies, which remained nearly almost constant over the concentration range studied. The results showed that the effect of charge on the aspirin molecule was reflected only in the dissociation process from the beta-CD cavity, while no remarkable change was seen in the association process whose rate was diffusion controlled. The results could be explained on the basis of the difference of the hydrophobic moieties in the two guests that were included in the host cavity. The results of the standard volume change for the complexation reaction were closely related to the number of expelled water molecules originally located in the beta-CD cavity and the volume of the aspirin molecule incorporated into the beta-CD cavity.     

Dynamic interaction between alkylammonium ions and beta-cyclodextrin by means of ultrasonic relaxation

Ultrasonic absorption measurements in the frequency range from 0.8 to 220 MHz were carried out in aqueous solutions of pentylammonium chloride (PEACL) and hexylammonium chloride (HEACL) with beta-cyclodextrin (beta-CD) at pH approximately 7.2 and 25 degrees C. A single relaxational absorption was attributed to a perturbation of a chemical relaxation associated with the formation of a complex between beta-CD and the alkylammonium chlorides. The rate and equilibrium constants for the complexation reaction were determined from the concentration dependence of the relaxation frequency. Increasing the chain length of the alkylammonium ion led to an increase in the stability of the complex and slowed the exit rate of the ion from the beta-CD cavity. The standard volume change of the reaction was obtained from a maximum absorption per wavelength and was attributed to water molecules being expelled from the cavity with concomitant alkylammonium ion insertion.     

Inclusion kinetics of a nucleotide into a cyclodextrin cavity by means of ultrasonic relaxation

To examine a dynamic interaction between nucleotide and cyclic oligosaccharide, ultrasonic absorption measurements were carried out in aqueous solution containing beta-cyclodextrin (beta-CD) and adenosine 5'-monophosphate (AMP) in the frequency range of 0.8-95 MHz. A relaxational absorption was observed in the solution, although it was not found in the individual solution of beta-CD or AMP. From the concentration dependences of AMP on the relaxation time and the maximum absorption per wavelength, the cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with a complex formation between beta-CD (host) and AMP (guest). The rate constants for the formation and breakup processes of the complex were determined. Also, a standard volume change of the reaction was obtained. From comparisons of the obtained rate and thermodynamic parameters with those for beta-CD and various guests, it has been concluded that the adenine moiety is included in the beta-CD cavity and that the hydrogen bonds may play a role in the complex formation.     

Inclusion complexation of novocaine by beta-cyclodextrin in aqueous solutions

The formation of inclusion complexes between beta-cyclodextrin (beta-CD) and the local anesthetic 2-(diethylamino)ethyl-p-amino-benzoate (novocaine) in aqueous solutions under different acidity conditions, using steady-state fluorescence or UV-vis spectroscopies, electrical conductivity, or the kinetic study of both the nitrosation reaction of the primary amine group in a mild acid medium and the hydrolysis of the ester function under an alkaline medium, has been studied. The inclusion complex formation between neutral or protonated novocaine and beta-CD of 1:1 stoichiometry was observed; however, the magnitude of the binding constants depends on the nature of both the guest and the host, and the higher-affinity guest-host was found under conditions when both the novocaine and the beta-CD were neutral molecules.     

Thermodynamic study on the effects of beta-cyclodextrin inclusion with berberine.

The fluorescence enhancement of berberine (Berb) as a result of complex with beta-cyclodextrin (beta-CD) is investigated. The association constants of alpha-CD and beta-CD with Berb are 60 and 137 M(-1) at 20 degrees C in pH 7.20 aqueous solution. Effects of temperature on the forming inclusion complexes of beta-CD with Berb have been examined through using fluorescence titration. Enthalpy and entropy values calculated from fluorescence data are -33.7 kJ mol(-1) and 74.3 J x mol(-1) K(-1) respectively. It was found that the dielectric constant of beta-CD cavity is about 24 in a rough analogy with absolute alcohol. These results suggest that the extrusion of 'high energy water' molecules from the cavity of beta-CD and hydrophobic interaction upon the inclusion complex formation are the main forces of the inclusion reaction. Effect of pH on the association of beta-CD with Berb was also studied. Mechanism of the inclusion of beta-CD with Berb is further studied by absorption and NMR measurements. Results show that beta-CD forms a 1:1 inclusion complex with Berb.     

A spectroscopic study of the inclusion of azulene by beta- and gamma-cyclodextrins

The inclusion of azulene (AZ) inside the cavities of beta-cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) was studied using absorption, fluorescence and induced-circular dichroism spectroscopy. The inclusion of AZ into the cavity of beta-CD has a stoichiometry of 1:1, whereas that of AZ/gamma-CD complex is 1:2. The equilibrium constants for the formation of the two complexes were calculated to be 780+/-150 M(-1) for AZ:beta-CD and (4.5+/-0.86)x10(5) M(-2) for AZ:(gamma-CD)(2). The latter is due to a stepwise equilibrium mechanism in which a 1:1 complex is formed with a binding constant of 775 M(-1), followed by the formation of a 1:2 complex with a binding constant of 580 M(-1). The difference between the two binding constant values is slight, indicating an almost equal contribution from each of the gamma-CD molecules to the overall binding in AZ:(gamma-CD)(2). From the induced-circular dichroism spectra, the inclusion of AZ was found to be axial in AZ:beta-CD and nearly axial in AZ:(gamma-CD)(2).     

Modeling competitive guest binding to beta-cyclodextrin molecular printboards

Anchoring of functionalized guest molecules to self-assembled monolayers (SAMs) is key to the development of molecular printboards for nanopatterning. One very promising system involves guest binding to immobilized beta-cyclodextrin (beta-CD) hosts, with guest:host recognition facilitated by a hydrophobic interaction between uncharged anchor groups on the guest molecule and beta-CD hosts self-assembled at gold surfaces. We use molecular dynamics free energy (MDFE) simulations to describe the specificity of guest:beta-CD association. We find good agreement with experimental thermodynamic measurements for binding enthalpy differences between three commonly used phenyl guests: benzene, toluene, and t-butylbenzene. van der Waals interaction with the inside of the host cavity accounts for almost all of the net stabilization of the larger phenyl guests in beta-CD. Partial and full methylation of the secondary rim of beta-CD decreases host rigidity and significantly impairs binding of both phenyl and larger adamantane guest molecules. The beta-CD cavity is also very intolerant of guest charging, penalizing the oxidized state of ferrocene by at least 7 kcal/mol. beta-CD hence expresses moderate specificity toward uncharged organic guest molecules by van der Waals recognition, with a much higher specificity calculated for electrostatic recognition of organometallic guests.     

Study of inclusion complex formation between tropaeolin OO and beta-cyclodextrin by spectrophotometry and Infrared spectroscopy

The mechanism of the inclusion of tropaeolin OO (TPOO) and beta-cyclodextrin (beta-CD) has been studied by spectrophotometry. The inclusion depth of the guest molecule in the host molecule was demonstrated by infrared spectrometry. Effect of the pH, concentrations of beta-CD, solvents and ionic strength on the inclusion of TPOO and beta-CD were examined. The result showed that TPOO reacts with beta-CD to form a 1:1 host-guest complex with an apparent formation constant of 1.50 x 10(3) l mol(-1). The thermodynamic parameters of inclusion reaction, DeltaG degrees , DeltaH degrees and DeltaS degrees were obtained.     

Complexation of adamantyl compounds by beta-cyclodextrin and monoaminoderivatives

Since the beta-cyclodextrin cavity is not a smooth cone but has constrictions in the neighborhoods of the H3 and H5 atoms, the hypothesis that bulky hydrophobic guests can form two isomeric inclusion complexes (one of them, c(p), is formed by the entrance of the guest by the primary side of the cavity, and the other one, c(s), results from the entrance by the secondary side) is checked. Thus, the inclusion processes of two 1-substituted adamantyl derivatives (rimantidine and adamantylmethanol) with beta-cyclodextrin and its two monoamino derivatives at positions 6 (6-NH2beta-CD) and 3 (3-NH2beta-CD) were studied. From rotating-frame Overhauser enhancement spectroscopy experiments, it was deduced that both guests form c(s) complexes with beta-CD and 6-NH2beta-CD but c(p) complexes with 3-NH2beta-CD. In all cases, the hydrophilic group attached to the adamantyl residue protrudes toward the bulk solvent outside the cyclodextrin cavity. The thermodynamic parameters (free energy, equilibrium constant, enthalpy, and entropy) associated with the inclusion phenomena were measured by isothermal titration calorimetry experiments. From these results, the difference in the free energy for the formation of the two complexes, c(s) and c(p), for the same host/guest system has been estimated as being 11.5 +/- 0.8 kJ mol(-1). This large difference explains why under normal experimental conditions only one of the two complexes (c(s)) is detected. It is also concluded that a hyperboloid of revolution can be a better schematic picture to represent the actual geometry of the cyclodextrin cavities than the usual smooth cone or trapezium.     

Spectral properties and inclusion of 3-(4'-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one in organized media of micellar solutions, beta-cyclodextrin and viscous medium

On the line of a previous work on the spectral properties of some of heteroaryl chalcone, the absorption and fluorescence emission spectral properties of 3-(4'-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP), have been investigated in organized media of aqueous micellar and beta-cyclodextrin (beta-CD) solutions. While the absorption spectra are less sensitive to the nature of the added surfactant or beta-CD, the characteristics of the intramolecular charge transfer (ICT) fluorescence are highly sensitive to the properties of the medium. The ICT maximum is strongly blue-shifted with a great enhancement in the fluorescence quantum yield on adding micellar or beta-CD. This indicates the solubilization of DMAFP in the micellar core and formation of an inclusion complex with beta-CD. The critical micelle concentrations (CMC) as well as the polarity of the micellar core of SDS, CTAB and TX-100 have been determined. The CMC values are in good agreement with the reported values while the polarity is lower indicating that DMAFP molecules are incorporated in the micellar core not at the micellar interface. The inclusion constants of binding of DMAFP in micellar or beta-CD have been also determined. The thermodynamic parameters of formation of DMAFP:CD inclusion complex have been calculated from the temperature dependence of the fluorescence spectra of the formed complex. The highly negative value of formation entropy (DeltaS=-98.0Jmol(-1)K(-1)) reflects the high restrictions imposed on the movement of both the host and included guest molecules which is consistent with the increase of the fluorescence yield and blue shift of the fluorescence maximum.     

Organic anion recognition of naphthalenesulfonates by steroid-modified beta-cyclodextrins: enhanced molecular binding ability and molecular selectivity

Two beta-cyclodextrin (beta-CD) derivatives bearing steroid groups (1 and 2) were synthesized by the condensation of mono(6-aminoethylamino-6-deoxy)-beta-CD with cholic acid and deoxycholic acid, respectively, and their original conformations and binding behavior to the organic anion of naphthalenesulfonate derivatives were investigated by using 1H NMR spectroscopy and spectrofluorometric titration in combination with computational methods. The 2D NMR experiments reveal that the steroid groups attached to the beta-CD rim could be deeply embedded in the beta-CD cavity to form the intramolecular (for 1) or intermolecular (for 2) inclusion complexes in aqueous solution. Upon complexation with naphthalenesulfonate derivatives, modified beta-CDs display two obviously different binding modes, that is, the competitive inclusion mode and the induced-fit inclusion mode, which is consistent with the results of molecular modeling study. The two modes and the strict size/shape fitting relationship between the hosts and guests reasonably explain the different binding behaviors and molecular selectivity of host beta-CDs 1 and 2 toward the naphthalenesulfonate guests. Therefore, the cholic acid- or deoxycholic acid-modified beta-CDs could effectively recognize the size/shape of guest molecules as compared with the parent beta-CD, giving good molecular selectivity up to 24.9 for the disodium 2,6-naphthalenedisulfonate/disodium 1,5-naphthalenedisulfonate pair by the host 1.     

Molecular recognition in cyclodextrin complexes of amino acid derivatives. 2. A new perturbation: the room-temperature crystallographic structure determination for the N-acetyl-p-methoxy-L-phenylalanine methyl ester/beta-cyclodextrin complex

Cyclodextrins (CDs) are cyclic oligosaccharides that encapsulate various small organic molecules, forming inclusion complexes. Because CD complexes are held together purely by noncovalent interactions, they function as excellent models for the study of chiral and molecular recognition mechanisms. Recently, room-temperature crystallographic studies of both the 2:2 N-acetyl-L-phenylalanine methyl ester/beta-CD and 2:2 N-acetyl-L-phenylalanine amide/beta-CD complexes were reported. The effect of changes in carboxyl backbone functional group on molecular recognition by the host CD molecule was examined for the nearly isomorphous supramolecular complexes. A new perturbation of the system is now examined, specifically perturbation of the aromatic side chain. We report a room-temperature crystal structure determination for the 2:2 N-acetyl-p-methoxy-L-phenylalanine methyl ester/beta-CD inclusion complex. The complex crystallizes isomorphously with the two previously reported examples in space group P1; the asymmetric unit consists of a hydrated head-to-head host dimer with two included guest molecules. The crystal packing provides both a nonconstraining extended hydrophobic pocket and an adjacent hydrophilic region, where hydrogen-bonding interactions can potentially occur with primary hydroxyl groups of neighboring CD molecules and waters of hydration. The rigid host molecules show no sign of conformational disorder, and water of hydration molecules exhibit the same type of disorder observed for the other two complexes, with a few significant differences in locations of water molecules in the hydrophilic region near guest molecules. There is evidence for modest disorder in the guest region of an electron density map. In comparing this system with the two previously reported complexes of phenylalanine derivatives, it is found that the packing of the guest molecules inside the torus of the CD changes upon substitution of a methoxy group at the para position of the aromatic phenyl ring. Backbone hydrogen-bonding interactions for the guest molecules with the CD primary hydroxyls and waters also change. This structure determination is a new and revealing addition to a small but growing database of amino acid and peptidomimetic interactions with carbohydrates.     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

Molecular electrostatic potentials and hydrogen bonding in alpha-, beta-, and gamma-cyclodextrins

Cyclodextrins (CDs) are cyclic oligomers of glucose having the toroid of sugars elaborating a central cavity of varying size depending on the number of glucoses. The central hydrophobic cavity of CD shows a binding affinity toward different guest molecules, which include small substituted benzenes to long chain surfactant molecules leading to a variety of inclusion complexes when the size and shape complementarity of host and guest are compatible. Further, interaction of guest molecules with the outer surface of alpha-CD has also been observed. Primarily it is the electrostatic interactions that essentially constitute a driving force for the formation of inclusion complexes. To gain insights for these interactions, the electronic structure and the molecular electrostatic potentials in alpha-, beta-, and gamma-CDs are derived using the hybrid density functional theory employing the three-parameter exchange correlation functional due to Becke, Lee, Yang, and Parr (B3LYP). The present work demonstrates how the topography of the molecular electrostatic potential (MESP) provides a measure of the cavity dimensions and understanding of the hydrogen-bonded interactions involving primary and secondary hydroxyl groups. In alpha-CD, hydrogen-bonded interactions between primary -OH groups engender a "cone-like" structure, while in beta- or gamma-CD the interactions from the primary -OH with ether oxygen in glucose ring facilitates a "barrel-like" structure. Further, the strength of hydrogen-bonded interactions of primary -OH groups follows the rank order alpha-CD > beta-CD > gamma-CD, while the secondary hydrogen-bonded interactions exhibit a reverse trend. Thus weak hydrogen-bonded interactions prevalent in gamma-CD manifest in shallow MESP minima near hydroxyl oxygens compared to those in alpha- or beta-CD. Furthermore, electrostatic potential topography reveals that the guest molecule tends to penetrate inside the cavity forming the inclusion complex in beta- or gamma-CD.     

Spectral study on the inclusion complex of cryptophane-E and CHCl3

Cryptophane-E was synthesized from vanillin by a three-step method, and its absorption and fluorescence spectroscopic properties were determined. Two absorption bands at about 245–260 and 280–290 nm were observed for cryptophane-E and the fluorescence emission maxima were at 320–330 nm depending on the solvent used. The interaction of cryptophane-E with CHCl₃ was studied in detail by absorption and fluorescence spectroscopies. The results showed that cryptophane-E and CHCl₃ can easily form a stable 1:1 host–guest inclusion complex. Their binding constant (K) was determined by Benesi–Hildebrand equation and the nonlinear least squares fit method. The binding constant is largest in ethyl acetate, followed by dioxane and with acetonitrile as the smallest. In addition, the effect of guest volume on the host–guest inclusion complex was investigated. Guest molecules including CH₂Cl₂ and CCl₄ were unable to form inclusion complex with cryptophane-E because of sizes mismatching with the host cavity.     

Ester hydrolysis and enol nitrosation reactions of ethyl cyclohexanone-2-carboxylate inhibited by beta-cyclodextrin

Both the ester hydrolysis and the nitrosation reactions of the enol tautomer of ethyl cyclohexanone-2-carboxylate (ECHC) are investigated in the absence and presence of beta-cyclodextrin (beta-CD). The ester hydrolysis reaction is studied in dilute H2O and D2O solutions of hydrochloric acid and in aqueous buffered solutions of carboxylic acids (acetic acid and its chloro derivatives). The pseudo-first-order rate constant increases with both the [H+] and the total buffer concentration, indicating that the hydrolysis is subject to acid and general base catalysis. Substantial solvent isotope effects in the normal direction (kH/kD > 1) for the acid-catalyzed hydrolysis was observed. Addition of beta-CD strongly slows the hydrolysis reaction. The variation of the observed rate constant (k(o)) with [beta-CD] exhibits saturation behavior, consistent with 1:1 binding between the enol of ECHC and beta-CD. The binding is quite strong, and bound ECHC-enol is unreactive. The nitrosation reaction of ECHC in aqueous acid medium, using sodium nitrite in great excess over the concentration of ECHC, yields perfect first-order kinetics, indicating that the slow step is the nitrosation of the enol tautomer. This finding suggests that a great percentage of the total ECHC concentration must exist in the enol form. The nitrosation reaction is of first order in [nitrite] and is catalyzed by the presence of Cl-, Br-, or SCN- ions, which indicates that the attack of the nitrosating agent is the slow step. The nitrosation reaction is also strongly inhibited by the presence of beta-CD because of the formation of unreactive inclusion complexes between the host, beta-CD, and the guest, the enol of ECHC. In alkaline medium, the formation of the enolate ion is observed, which absorbs at higher wavelengths (lambda(max) = 256 nm in acid medium shifts to lambda(max) = 288 nm in alkaline medium). This anion also undergoes ester hydrolysis spontaneously, but shows neither specific basic catalysis nor appreciable effect by the presence of beta-CD. From kinetic and spectroscopic measurements the pKa of the enol of ECHC has been determined as 12.35.     

Caging effects on the ground and excited states of 2,2'-bipyridine-3,3'-diol embedded in cyclodextrins

The 2,2'-bipyridine-3,3'-diol (BP(OH)(2)) molecule shows unique spectroscopic features in water that may position it as a new biological probe. In an attempt to mimic biological environments, we explore in this paper the caging effects of cyclodextrins on the steady state spectra of BP(OH)(2). The caging effects of gamma-, beta-, and 2,6-di-O-methyl-beta-cyclodextrins (CDs) on the ground and excited state properties of BP(OH)(2) in aqueous solutions are investigated by steady state absorption and fluorescence spectroscopy, and by ab initio calculations. The stoichiometry of the three complexes was found to be 1:1 and the binding constants were estimated from the absorption and fluorescence spectra. In the case of gamma-CD, the large cavity size supports only small binding, whereas such binding increases in the cases of the smaller cavity sizes of beta-CD and 2,6-di-O-methyl-beta-CD. Maximum binding was measured in the case of 2,6-di-O-methyl-beta-CD due to the increased hydrophobicity of the host cavity. The unique absorption features of BP(OH)(2) in water show a dramatic decrease in intensity due to caging effects. The decrease in intensity correlates very well with the extent of binding and hydrophobicity of the host molecules. Similar results were also obtained from the fluorescence spectra. The calculated structure of the BP(OH)(2):beta-CD complex predicts that the inclusion of BP(OH)(2) is nearly axial and centered inside the beta-CD cavity. The BP(OH)(2) molecule maintains its dienol moiety in the complex with no possible hydrogen bonding with the host interior H-atoms. The results are discussed in light of the possible use of BP(OH)(2) as a water sensor in biological systems.     

Encapsulation of sodium nimesulide and precursors in beta-cyclodextrin

Crystalline 1:1 inclusion complexes with beta-cyclodextrin (beta-CD) and the sodium salt of nimesulide (4-nitro-2-phenoxymethanesulfonanilide), and the sodium salt of the derivative 2-phenoxymethanesulfonanilide, have been prepared by co-precipitation from aqueous solution. The presence of true inclusion complexes was supported by elemental analysis, thermogravimetry and powder X-ray diffraction. FTIR and 13C CP MAS NMR spectroscopy confirmed that no chemical modification of the guests occurred upon formation of inclusion complexes. The reaction of the precursors 2-phenoxynitrobenzene and 2-phenoxyaniline with beta-CD was also studied and crystalline inclusion complexes with a 2:1 (host-to-guest) stoichiometry were isolated. The interaction of the different guest species with beta-CD host molecules was studied theoretically by carrying out ab initio calculations. Favourable inclusion geometries were obtained for the four guests mentioned above. On the other hand, it was found that the inclusion of the neutral guests nimesulide and 2-phenoxymethanesulfonanilide was considerably less favourable. This is in agreement with the experimentally observed difficulty in isolating true inclusion complexes containing these guests and beta-CD. The calculated lower stability is attributed to the different steric hindrance arising from the different conformational preferences of neutral and anionic forms.     

Study of the interaction between progesterone and beta-cyclodextrin by electrochemical techniques and steered molecular dynamics

The interaction of progesterone with beta-cyclodextrin (beta-CD) was studied by differential pulse polarography. The aim of the present work was to study the effect of beta-CD on the electrochemical behavior of progesterone in aqueous solution and also to analyze the molecular interactions involved in formation of the inclusion complex. The complex with stoichiometry of 1:1 was thermodynamically characterized. In addition, steered molecular dynamics (SMD) was used to investigate the energetic properties of formation of the inclusion complex along four different pathways (reaction coordinates), considering two possible orientations. From multiple trajectories along these pathways, the potentials of mean force for formation of the beta-CD progesterone inclusion complex were calculated. The energy analysis was in good agreement with the experimental results. In the beta-CD progesterone inclusion complex, a large portion of the steroid skeleton is included in the beta-CD cavity. The lowest energy was found when the D-ring of the guest molecule is located near the secondary hydroxyls of the beta-CD cavity. In the most probable orientation, one intermolecular hydrogen bond is formed between the O of the C-20 keto group of the progesterone and a secondary hydroxyl of the beta-CD.     

Inclusion of the Niflumic Acid Anion in β-cyclodextrin: A Solution NMR and X-ray Structural Investigation

We report parallel solution and solid state studies of the inclusion of the anionic form of the non-steroidal anti-inflammatory drug niflumic acid (2-[[3-(trifluoromethyl)phenyl]-amino]-3-pyridinecarboxylic acid) in the host g -cyclodextrin ( g -CD). 1 H NMR data for the interaction between host and guest in aqueous solution recorded at 300 MHz indicated a strong preference for insertion of the trifluoromethylphenyl residue, rather than the pyridinecarboxylate moiety, in the host cavity. A 1:1 complex stoichiometry was determined by the continuous variation method utilising chemical shifts of both host and guest protons. Analysis of the data using a new flexible program developed for this purpose yielded an overall association constant K of 336 M m 1 at 298 K. The NMR data indicate a dynamic equilibrium between complexed and uncomplexed species but do not distinguish guest entry from the primary and secondary sides of the host. Reaction between the Cs + salt of niflumic acid and g -CD yielded the crystalline complex ( g -CD) 2 ·(Cs + niflumate m ) 4 ·22H 2 O whose single crystal X-ray structure was determined. A novel inclusion mode for this host, namely entry of guest trifluoromethylphenyl residues from both the primary and secondary sides, was revealed by the X-ray analysis.