Effects of solvent, pH and beta-cyclodextrin on the photophysical properties of 4-hydroxy-3,5-dimethoxybenzaldehyde: intramolecular charge transfer associated with hydrogen bonding effect

The photophysical properties of 4-hydroxy-3,5-dimethoxybenzaldehyde (HDMB) in various solvents, pH and in aqueous beta-cyclodextrin (CD) have been investigated. In non-polar solvents, HDMB gives only one emission maxima; whereas, in polar solvents it shows a dual luminescence. The increase in Stokes shift with increase in polarity is much more for longer wavelength (LW) than for a shorter wavelength (SW) band. This behaviour indicates the formation of an intramolecular charge transfer (ICT) state through relaxation from the normal excited state. Especially in water, the ICT emission is further red shifted to 430 nm with the normal emission band at 330 nm and the relative fluorescence intensities between 330 nm and 430 nm emission bands are affected by the excitation wavelength. However, this excitation wavelength dependence is not large in aqueous beta-CD solutions. These results suggest that the ICT state in polar solvents/water is stabilized through exciplex formation by the hydrogen-bonding interaction between the carbonyl group and polar solvents/water. The ground and excited state pK(a) values for the neutral-monoanion equilibrium have been measured and discussed. HDMB forms a 1:1 inclusion complex with beta-CD. A mechanism is proposed to explain the inclusion process.     

Dual fluorescence of N-phenylanthranilic acid: Effect of solvents, pH and beta-cyclodextrin

Spectral characteristics of N-phenylanthranilic acid (NPAA) have been studied in different solvents, pH and beta-cyclodextrin (beta-CD) and compared with anthranilic acid (2-aminobenzoic acid, 2ABA). In all solvents a dual fluorescence is observed in NPAA, whereas 2ABA gives single emission. Combining the results observed in the absorption, fluorescence emission and fluorescence excitation spectra, it is found that strong intramolecular hydrogen bonding (IHB) interactions present in NPAA molecule. The inclusion complex of NPAA with beta-CD is analysed by UV-vis, fluorimetry, FT-IR, (1)H NMR, scanning electron microscope and AM 1 method. The above spectral studies show that NPAA forms a 1:1 inclusion complex with beta-CD and COOH group present in the beta-CD cavity. A mechanism is proposed to explain the inclusion process.     

Host-guest interaction of L-tyrosine with beta-cyclodextrin

The inclusion complexes of beta-cyclodextrin (beta-CD) with l-tyrosine (l-TYN) were investigated by using spectrophotometers. The absorption and fluorescence enhancement occurs with beta-CD and l-TYN forms 1:1 inclusion complex. The unusual blue shift of hydroxyl ion in the beta-CD medium confirms OH groups present in the interior part of the beta-CD cavity and -COOH group present in the upper part of the beta-CD cavity. A mechanism is proposed to explain inclusion process. The inclusion interaction was examined and the thermodynamic parameters of inclusion process DeltaG, DeltaH and DeltaS were determined. The results indicated that the inclusion process was an exergonic and spontaneous process. Stable solid inclusion complexes were established and characterized by FT-IR, scanning electron microscope (SEM) methods.     

Effect of beta-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate

The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous beta-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of beta-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of beta-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the beta-CD cavity with an association constant of 1915 M(-1) using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of beta-CD.     

Orientational dynamics of a charge transfer complex in cyclodextrin cavity as receptor

This paper reports the structure and dynamics of a twisted intermolecular charge transfer molecule 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide (o-DASPMI) included inside alpha-, beta- and gamma-cyclodextrin, investigated by using steady state and time-resolved emission spectroscopy and also theoretical modeling. A nice 1 : 1 inclusion complex with beta-CD in the excited state could be found with the dimethylamino group of the molecule sticking out as revealed from steady state and time-resolved emission. The inclusion complex has a longer decay time compared to that in neat water. Time-resolved anisotropy decay has been used to study the rotational dynamics of the molecule inside cyclodextrin cavity. The average angular structure of the inclusion complex as found from semiempirical PM3 calculations corroborates excellently the experimental results of angular orientation in beta-CD. The minimum energy of the complex is found to be nearly 5 A in the length of the molecule with the dimethylamino part sticking out in the bulk water. Hydrogen bonding at the rim hinders the inclusion complex of o-DASPMI in gamma-CD and instead it produces association at the rim. Hydrogen bond breaker urea breaks the bonding of o-DASPMI with the rim of gamma-CD and the formation of inclusion complex with gamma-CD ensues.     

Heterodimerization of dye-modified cyclodextrins with native cyclodextrins

The heterodimerization behavior of dye-modified beta-cyclodextrins (1-6) with native cyclodextrins (CDs) was investigated by means of absorption and induced circular dichroism spectroscopy in an aqueous solution. Three types of azo dye-modified beta-CDs (1-3) show different association behaviors, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl red-modified beta-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4' position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its beta-CD cavity. It also associates with the native alpha-CD by inserting the moiety of 1 into the alpha-CD cavity. The association constants for such heterodimerization are 198 M(-1) at pH 1.00 and 305 M(-1) at pH 6.59, which are larger than the association constant of 1 for beta-CD (43 M(-1) at pH 1.00). Methyl red-modified 2, which has the same dye moiety as that for 1 although its substituent position is different from that of 1, does not associate even with alpha-CD due to the stable self-intramolecular complex, in which the dye moiety is deeply included in its own cavity of beta-CD. Alizarin yellow-modified CD (3), which has an azo dye moiety different from that of 1 and 2, caused a slight spectral variation upon addition of alpha-CD, suggesting that the interaction between 3 and alpha-CD is weak. On the other hand, phenolphthalein-modified beta-CD (4), which forms an intermolecular association complex in its higher concentrations, binds with beta-CD with an association constant of 787 M(-1) at pH 10.80, where 4 exists as the dianion monomer in the absence of beta-CD. p-Nitorophenol-modified beta-CDs (5 and 6), each having p-nitorophenol moieties with a different connecting part with an amido and amidophenyl group, respectively, associated with alpha-CD with association constants of 66 and 16 M(-1) for 5 and 6, respectively. The phenyl unit in the connecting part of 6 may prevent the smooth binding with alpha-CD. All these results suggest that the dye-modified CDs, in which the dye part is not tightly included in its CD cavity, associate with the native CD to form heterodimer composed of two different CD units by inserting the dye moiety into the native CD unit. The resulting heterodimers have a cavity that can bind another appending moiety of host molecules. On this basis, more ordered molecular arrays or the supramolecular hereropolymers can be constructed.     

Molecular modeling (MM2 and PM3) and experimental (NMR and thermal analysis) studies on the inclusion complex of salbutamol and beta-cyclodextrin

The inclusion complex of salbutamol and beta-cyclodextrin (beta-CD) is studied by computational (MM2 and PM3) and experimental techniques. Molecular modeling calculations predict two different orientations of salbutamol in the beta-CD cavity in vacuo and in aqueous solution. In vacuo calculations show that the introduction of the aromatic ring of salbutamol is preferred to the introduction of the tert-butyl group into the beta-CD cavity. However, in aqueous solution both computational methods predict the introduction of the alkyl chain instead of the aromatic ring in the beta-CD cavity contrary to experimental results published previously. These quantitative predictions were experimentally confirmed here by studying the inclusion complex in solution by NMR. A 1:1 stoichiometry was found by (1)H NMR studies for this complex. A 2D ROESY (rotating-frame Overhauser enhancement spectroscopy) experiment shows that there are no cross-peaks between the aromatic protons of salbutamol and any of the protons of beta-CD. Cross-peaks for the protons of the tert-butyl group and protons inside the cavity of beta-CD demonstrate the full involvement of this group in the complexation process and confirm the orientation of the complex predicted by molecular modeling. The solid-state complex was prepared and its stoichiometry (beta-CD.C(13)H(21)NO(3).8H(2)O) and dissociation process studied by thermogravimetric analysis.     

An artificial molecular chaperone: poly-pseudo-rotaxane with an extensible axle

Poly-pseudo-rotaxanes CDs contains as a subset 1 (CDs; cyclodextrins, 1; poly(delta-valerolactone) having single beta-CD at the end of the polymer chain) initiate polymerization of delta-valerolactone (delta-VL) in the solid state when CDs (alpha-CD, beta-CD, and 2,6-di-O-methyl-beta-CD) are threaded onto the polymer chain. 1 without threaded CDs did not show any polymerization ability for delta-VL. An adamantane molecule (Ad) inhibited the polymerization ability of CDs contains as a subset 1 for delta-VL, indicating that beta-CD at the end of CDs contains as a subset 1 could not bind delta-VL because the beta-CD cavity was occupied by Ad. It should be noted that the insertion reaction and the polymerization took place inside the beta-CD cavity at the end of CDs contains as a subset 1 and that the formation of poly-pseudo-rotaxane is necessary for the initiation of delta-VL. The structures of beta-CD contains as a subset 1 and 1 were characterized by powder X-ray diffraction measurements and solid-state NMR spectroscopies. The polymer chain of beta-CD contains as a subset 1 was found to elongate in the solid state, whereas the polymer chain of 1 formed a random coil conformation. 1 was deactivated for the polymerization by blocking the active cavity of beta-CD with the polymer chain. CDs threaded onto 1 are immune to the initiation of delta-VL directly but have an essential role to fold the polymer chain in a proper way as an artificial chaperone.     

An anthracene-appended beta-cyclodextrin-based dyad: study of self-assembly and photoinduced electron-transfer processes

The self-assembly of a beta-cyclodextrin (beta-CD)-based supramolecular dyad is reported, in which the donor anthracene moiety is covalently linked to the smaller rim of the beta-CD and the acceptor pyromellitic diimide (PMDI) is encapsulated within the beta-CD cavity. Encapsulation of the PMDI into the beta-CD cavity was studied by a variety of techniques, which suggested that PMDI is encapsulated so as to position the aromatic part at the centre of the cavity with the 2-propyl end at the narrower rim among the overhanging primary OH groups and the N-ethylpyridinium end situated at the wider rim exposed to water. Photoinduced electron transfer (PET) in the system was studied by fluorescence quenching and laser flash photolysis techniques. At [PMDI]<10(-4) M, the equilibrium is in favour of the free molecules, and under these conditions fluorescence quenching is negligible and diffusion-mediated electron transfer involving the triplet excited state of anthracene predominates. At higher concentrations of PMDI, the equilibrium is largely in favour of the supramolecular dyad and intra-ensemble PET processes predominate. The experimentally determined electron-transfer rate constant agrees very well with that calculated by using the Marcus equation. It was observed that a fraction of the ion pairs survived for more than 200 micros.     

Spectroscopic study on binding behaviors of different structural nonionic surfactants to cyclodextrins

The binding of polyethylene glycol (10) n-octylphenyl ether (OPE) and polyethylene glycol (10) tert-octylphenyl ether (Triton X-100, TX) to beta-cyclodextrin (beta-CD) and heptakis(2,3- beta-dimethyl)- beta-CD (DM- beta-CD) was described in detail by surface tension, steady-state fluorescence of OPE and TX, and phosphorescence of 1-bromonaphthalene (BN) probe. Surface tension and fluorescence measurements show that beta-CD entraps the hydrophobic moieties of OPE and TX to form inclusion complexes with the stoichiometry of 1:1. Unlike the n-octyl group of OPE, however, the tert-octyl group of TX fails to be encapsulated into the cavity of DM- beta-CD because of the steric hindrance of methyl groups at the rim of the cavity. The inclusion of the phenyl group of OPE and TX was demonstrated by dynamic quenching effect of iodide ion on fluorescence of OPE and TX in the presence of beta-CD. Static fluorescence quenching of OPE and TX by BN, phosphorescence of BN, and energy transfer between TX and BN provide additional evidence for the inclusion of their phenyl groups into the CD cavity. Analyses of molecular size suggest that the longer tert-octyl group of OPE is situated in curled manner in the cavity and the tert-octyl group of TX undergo a slight distortion for fit of beta-CD. Further introduction of the third guest component into the CD cavity occupied by OPE and TX will force the flexible octyl groups of OPE and TX to deform to a greater extent.     

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.     

Study of the complexation of fisetin with cyclodextrins

In this work, the interaction between fisetin (3,3',4',7-tetrahydroxyflavone) (Fis) and cyclodextrins (CDs) (alpha and beta) was studied through UV-vis absorption, steady-state fluorescence, induced circular dichroism, and (1)H NMR experiments with dependence on temperature and pH. Some experimental data were compared with quantum-mechanics studies based on the SAM1 (AMPAC) semiempirical model, as well as with the B3LYP and MPW1PW91 functional models from density functional theory using the 6-311G and 3-21G basis sets. The spectroscopic measurements show that Fis does not form stable complexes with alpha-CD. On the other hand, at pH 4.0 and 6.5, the complex Fis-beta-CD is formed in a Fis:beta-CD 1:1 stoichiometry and an equilibrium constant (K) of 900 +/- 100 M(-1). In basic medium (pH 11.5), K decreases to 240 +/- 90 M(-1) because Fis deprotonation leads to its better solubilization in water. Molecular modeling points out that Fis is not totally inserted into the inner cavity of beta-CD. The formation of the inclusion complex renders an environment that enhances intramolecular excited state proton transfer. The inclusion complex is formed preferentially via entry of the Fis phenyl group into beta-CD.     

Changing mechanisms in the beta-cyclodextrin-mediated hydrolysis of phenyl esters of perfluoroalkanoic acids

The rate of hydrolysis of esters CF(3)(CF(2))(n)COOPh (1 (n = 1), 2 (n = 2), and 3 (n = 6)) was measured at pH 6.00 and at pH higher than 9.00 in the presence of beta-cyclodextrin (beta-CD). For compounds 1 and 2 the reaction rate decreases as the beta-CD concentration increases, and they show saturation effects at all pH. It is suggested that the substrate forms an inclusion complex with cyclodextrin. Analysis of the rate data allows calculation of the association equilibrium constant, K(CD), the rate constant for the reaction of the included compound, k(c), and K(TS) which is the hypothetical association equilibrium constant for the transition state of the cyclodextrin-mediated reaction. The dependence of log K(CD) and log K(TS) with the number of atoms in the chain is different. We suggest that the reactions of 1 and 2 take place with the perfluorinated alkyl chain included in the cavity, whereas the transition state for the reaction of phenyl trifluoroacetate involves a complex with the aryl ring inside the cavity. At low pH the inhibition comes from the protection of the carbonyl group toward nucleophilic attack by water. In basic pH the reaction of HO(-) as an external nucleophile is also inhibited. The cyclodextrin-mediated reaction involves the ionized OH group at the rim of the cyclodextrin cavity with poor efficiency due to an unfavorable orientation of the substrate in the complex. On the other hand, the reaction of compound 3 is strongly accelerated by cyclodextrin because the association of the substrate with cyclodextrin competes with the monomer-aggregate equilibrium and at high enough cyclodextrin concentration the main species present in solution is the complex between 3 and cyclodextrin.     

Assembly behavior of inclusion complexes of beta-cyclodextrin with 4-hydroxyazobenzene and 4-aminoazobenzene

To further reveal the factors governing the supramolecular assembly of beta-cyclodextrin (beta-CD) inclusion complexes, two aggregates (1 and 2) were prepared from the inclusion complexes of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene, respectively, and their binding behavior were investigated by means of X-ray analysis, UV-vis, NMR, and circular dichroism spectra in both solution and the solid state. The obtained results indicated that the beta-CD/4-hydroxyazobenzene complex 1 could form head-to-head dimers (triclinic system, space group P1) in the solid state, which were further self-assembled to a linear supramolecular architecture by the intra- and interdimer hydrogen bond interactions as well as the intradimer pi-pi interactions. However, when the included guest 4-hydroxyazobenzene was switched to a 4-aminoazobenzene, the resultant beta-CD/4-aminoazobenzene complex 2 (monoclinic system, space group P2(1)) could be self-assembled to a wave-type supramolecular aggregate under similar conditions. Furthermore, the combination of crystallographic and spectral investigations jointly revealed the inclusion complexation geometry of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene in both solution and the solid state, which demonstrated that the disparity of substituents in the azobenzenes played an important role in the inclusion complexation and molecular assembly, affecting not only the structural features of aggregates but also the binding abilities of azobenzenes with beta-CD.     

Diastereomeric difference of inclusion modes between (-)-epicatechin gallate, (-)-epigallocatechin gallate and (+)-gallocatechin gallate, with beta-cyclodextrin in aqueous solvent

Inclusion complexes of (-)-epicatechin gallate (ECg) as well as (+)-gallocatechin gallate (GCg) and beta-cyclodextrin (beta-CD) in an aqueous solution were investigated using several NMR techniques and a computational method. ECg and EGCg formed a 1:1 complex with beta-CD, in which the A ring and a portion of the C ring were included from the wide secondary hydroxyl group side of the beta-CD cavity, and the B and B' rings were left outside the cavity. GCg formed a 1:2 complex with beta-CD, in which the A and B rings of GCg were included by two molecules of beta-CD. The difference between the two modes of inclusion of the 1:1 complex of ECg, EGCg.beta-CD and the 1:2 complex of GCg.beta-CD might have resulted from the size of the space between the B and B' rings in aqueous solution. As a result of nuclear Overhauser effect (NOE) experiments, GCg was considered to have a large enough space between the B and B' rings to include the B ring in the beta-CD cavity; on the other hand, ECg and EGCg have no such large space.     

Mechanistic study of opposite migration order of dimethindene enantiomers in capillary electrophoresis in the presence of native beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin

The possible mechanisms of the opposite affinity pattern of the enantiomers of dimethindene [(R,S)-N,N-dimethyl-3[1(2-pyridyl)ethyl]indene-2-ethylamine] (DIM) towards native beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl-)-beta-CD (TM-beta-CD) were studied using capillary electrophoresis (CE), NMR spectrometry, electrospray ionization mass spectrometry (ESI-MS) and X-ray crystallography. NMR spectrometry allowed to estimate the stoichiometry of the complex and to determine the binding constants. As found using ESI-MS, together with more abundant 1:1 complex, a complex with 1:2 stoichiometry may also be present in a rather small amount in a solution of DIM and beta-CD. One-dimensional ROESY experiments indicated that the geometry of the complexes of DIM with native beta-CD depends on the ratio of the components in the solution. In the 1:1 solution of DIM and beta-CD the complex may be formed by inclusion of the indene moiety of DIM into the cavity of beta-CD on the primary side and into the cavity of TM-beta-CD into the secondary side. The most likely structural reason for lower affinity of the enantiomers of DIM towards the cavity of TM-beta-CD compared to native beta-CD could be elucidated. The indene moiety does not enter the cavity of TM-beta-CD as deeply as the cavity of beta-CD. This may be the most likely explanation of significantly higher affinity constants of DIM enantiomers towards the latter CD compared to the former one. The marked difference between the structure of the complexes may also be responsible for the opposite affinity pattern of the DIM enantiomers towards beta-CD and TM-beta-CD.     

Spectral and photophysical studies of inclusion complexes of 2-amino-4,6-dimethyl pyrimidine with beta-cyclodextrin

The interaction of 2-amino-4,6-dimethyl pyrimidine (ADMP) with beta-cyclodextrin (beta-CD) has been studied by means of UV absorption, steady state and time resolved fluorescence techniques. Spectral characteristics, bandwidths and photophysical parameters indicating that ADMP experience two different environments in aqueous solutions: bulk water and 1:1 (ADMP:beta-CD) inclusion complexation. The size restriction of the upper rim of beta-CD partially include ADMP and prevent the possibility of formation of 1:2 complex. The effective polarity of the cyclodextrin cavity experienced by the induced ADMP is equivalent with the polarity of an 80:20 methanol-water mixture.     

Comparison of binding of tetraphenylborate and tetraphenylphosphonium ions to cyclodextrins studied by capillary electrophoresis

Binding constants for tetraphenylborate and tetraphenylphosphonium ions (Ph4B- and Ph4P+) to cyclodextrins (CDs) to give 1:1 host-guest complexes have been measured using capillary electrophoresis. Mobilities of the ions as a function of gamma-CD concentration give binding constants, K, of 1.08 x 10(5) M-1 for Ph4B- and 0.6 x 10(1) M-1 for Ph4P+. This dramatic difference of four orders of magnitude in binding constants is not seen with beta-CD (K = 7.7 x 10(1) M-1 for Ph4B- and 3.7 x 10(1) M-1 for Ph4P+) or dimethyl (DM)-beta-CD (K = 46 x 10(1) M-1 for Ph4B-1 and 7.7 x 10(1) M-1 for Ph4P+). The crystal and hydrodynamic radii of the ions, the latter calculated from their absolute mobilities, indicate that Ph4B- is smaller than the gamma-CD cavity, whereas Ph4P+ is approximately the gamma-CD cavity size. Results suggest that Ph4B- fits exactly into a gamma-CD cavity, with hydrophobic contacts involving several of the phenyl rings, whereas Ph4P+ is too large to enable these multiple contacts to be made. When only a single phenyl ring can fit into the CD cavity, binding strengths are in the order DM-beta-CD > beta-CD > gamma-CD and Ph4B- > Ph4P+. Measurement of electrophoretic mobilities of the complexes shows that their hydrodynamic radii are in the order gamma-CD < beta-CD approximately DM-beta-CD for Ph4B- and gamma-CD > beta-CD approximately DM-beta-CD for Ph4P+.     

Inter- and intramolecular excited state proton transfer in 2-hydroxy-9H-carzole-1-carboxylic acid

Absorption, fluorescence and fluorescence excitation spectroscopy and single photon counting time dependence spectrofluorimetry have been used to study the inter- and intramolecular excited state proton transfer (ESIPT) reactions in 2-hydroxy-9H-carbazole-1-carboxylic acid (2-HCA). Except in cyclohexane and water (pH 5) dual fluorescence is observed in rest of the solvents used. Normal Stokes shifted band seems to originate from 2-HCA-1-c and tautomer emission band from the tautomer formed by ESIPT in 2-HCA-1-c followed by structural reorganization. Both these emission band systems originate from the same ground state species. AM1 and CNDO/S-CI calculations have been carried out to establish the identity of the species. Different prototropic equilibria have been determined and discussed.     

Structure and intramolecular flexibility of beta-cyclodextrin complex with (-)-epigallocatechin gallate in aqueous solvent

The probable structure of the inclusion complex of beta-cyclodextrin (beta-CD) and (-)-epigallocatechin gallate (EGCg) in D2O was investigated using several NMR techniques. EGCg formed a 1:1 complex with beta-CD, in which the A ring and a portion of the C ring of EGCg were included at the head of the phenolic hydroxyl group attached to C7 of EGCg in the beta-CD cavity from the wide secondary hydroxyl group side. In the 1:1 complex with beta-CD, EGCg maintained the conformation in which the B and B' rings of EGCg took pseudoequatorial and pseudoaxial positions with respect to the C ring, respectively. The structure of the inclusion complexes of beta-CD and EGCg obtained from NMR experiments supported those determined from AM1 semiempirical SCF MO calculations well.