Guest‐responsive excimer emission in an α‐helix peptide bearing γ‐cyclodextrin and two naphthalene units

An α‐helix peptide (17 amino acids) bearing γ‐cyclodextrin (γ‐CD) and two naphthyl units (γ‐N₂17) was designed and prepared as a new type of chemosensor. The α‐helix peptide with γ‐CD sandwiched between two naphthyl moieties exhibits excimer emission by inserting the two naphthalene moieties into the γ‐CD cavity from the opposite sides in the side chain of the peptide. The two reference peptides, which have one naphthalene moiety and one γ‐CD unit, exhibit only monomer fluorescence and have larger binding constants for the examined guests than γ‐N₂17.     

NMR spectroscopic characterization of inclusion complexes comprising cyclodextrins and gallated catechins in aqueous solution: cavity size dependency

The structure of inclusion complexes of γ‐cyclodextrin (γ‐CD), (–)‐gallocatechin gallate (GCg), and (–)‐epigallocatechin gallate (EGCg) in D₂O was investigated using several NMR techniques. GCg formed a 1:1 inclusion complex with γ‐CD in which the A and C rings of GCg were inserted deep at the head of the A ring into the γ‐CD cavity from the wide secondary hydroxyl group side. In the 1:1 inclusion complex with GCg and γ‐CD, the GCg moiety maintained a conformation in which the B and B′ rings of GCg took both pseudoequatorial positions with respect to the C ring. The structure of the inclusion complex of GCg and γ‐CD obtained from NMR experiments supported well that determined from PM6 semiempirical SCF MO calculations. However, ¹H NMR experiments suggested that EGCg did not form any inclusion complex with γ‐CD in D₂O. The marked difference between GCg and EGCg in inclusion behavior toward γ‐CD may be explained in terms of the stabilization energy calculated with the PM6 method. Copyright © 2008 John Wiley & Sons, Ltd.     

Inclusion complex of β‐cyclodextrin with coffee chlorogenic acid: new insights from a combined crystallographic and theoretical study

This work reports the elusive structural evidence for the [4]pseudorotaxane of β‐cyclodextrin (β‐CD) with coffee chlorogenic acid (CGA), a conjugate of caffeic acid (CFA) and quinic acid (QNA). A single‐crystal X‐ray structure analysis of the inclusion complex β‐cyclodextrin–chlorogenic acid–water (2/2/17), 2C₄₂H₇₀O₃₅·2C₁₆H₁₈O₉·17H₂O, reveals that CGA threads through β‐CD and assembles via O—H…O hydrogen bonds and parallel‐displaced π–π interactions in the twofold symmetry‐related dimer yielding a [4]pseudorotaxane, which is crystallographically observed for the first time in CD inclusion complexes. The encapsulation of the aromatic ring and C=C—C(=O)O chain in the β‐CD dimeric cavity indicates that the CFA moiety plays a determinant role in complexation. This is in agreement with the DFT‐derived relative thermodynamic stabilities of the trimodal β‐CD–CGA inclusion complexes, that is, β‐CD complexed with different CGA components: C=C—C(=O)O chain > cyclohexane ring > aromatic ring. The complexation stability is further enhanced in the dimeric β‐CD–CGA complex, with the CFA moiety totally enclosed in the β‐CD dimeric cavity.     

Supramolecular aggregations through the inclusion complexation of cyclodextrins and polymers with bulky end groups

Mono‐polyhedral oligomeric sillsesquioxane‐end capped poly(ε‐caprolactone) (mPPCL) can form inclusion complexes (ICs) with α‐ and γ‐cyclodextrins (CDs) but not with β‐CD. These CD ICs have been characterized with X‐ray diffraction, solid‐state ¹³C cross‐polarization/magic‐angle‐spinning NMR spectroscopy, ¹H NMR spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy. The poly(ε‐caprolactone) (PCL) chain of mPPCL is included within the channel provided by the CDs to form a columnar, crystalline structure. The PCL/CD ratios determined by ¹H NMR spectroscopy for the ICs with α‐ or γ‐CDs are higher than the stoichiometries because of the steric hindrance of the bulky polyhedral oligomeric silsesquioxane chain end and result in a fraction of the ε‐caprolactone units free from complexation with the CDs. On the basis of these analyses, we propose some possible structures for these CD/mPPCL ICs. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 125–135, 2007     

Formation of a Polypseudorotaxane via Self‐Assembly of γ‐Cyclodextrin with Poly(N‐isopropylacrylamide)

A polypseudorotaxane (PPR) comprising γ‐cyclodextrin (γ‐CD) as host molecules and poly(N‐isopropylacrylamide) (PNIPAM) as a guest polymer is prepared via self‐assembly in aqueous solution. Due to the bulky pendant isopropylamide group, PNIPAM exhibits size‐selectivity toward self‐assembly with α‐, β‐, and γ‐CDs. It can fit into the cavity of γ‐CD to give rise to a PPR, but cannot pass through α‐CD and β‐CD under the same conditions. The ratio of the number of γ‐CD molecules to entrapped NIPAM repeat units is kept at 1:2.2 or 1:2.4, determined by ¹H NMR spectroscopy and TGA analysis, respectively, indicating that there are more than 2 but less than 3 NIPAM repeat units included by one γ‐CD molecule. This finding opens new avenues to PPR‐based supramolecular polymers to be used as solid, stimuli‐responsive materials.     

Influence of substituent position and cavity size of the regioisomers of monocarboxymethyl‐α‐, β‐, and γ‐cyclodextrins on the apparent stability constants of their complexes with both enantiomers of Tröger's base

Enantiomers of Tröger's base were separated by capillary electrophoresis using 2^I‐O‐, 3^I‐O‐, and 6^I‐O‐carboxymethyl‐α‐, β‐, and γ‐cyclodextrin and native α‐, β‐, and γ‐cyclodextrin as chiral additives at 0–12 mmol/L for β‐cyclodextrin and its derivatives and 0–50 mmol/L for α‐ and γ‐cyclodextrins and their derivatives in a background electrolyte composed of sodium phosphate buffer at 20 mmol/L concentration and pH 2.5. Apparent stability constants of all cyclodextrin–Tröger's base complexes were calculated based on capillary electrophoresis data. The obtained results showed that the position of the carboxymethyl group as well as the cavity size of the individual cyclodextrin significantly influences the apparent stability constants of cyclodextrin–Tröger's base complexes.     

Poly(4‐acryloylmorpholine) oligomers carrying a β‐cyclodextrin residue at one terminus

A straightforward synthesis of amphiphilic β‐cyclodextrin‐poly(4‐acryloylmorpholine) (β‐CD‐PACM) polymers of controlled molecular weight, consisting of the radical polymerization of 4‐acryloylmorpholine in the presence of 6‐deoxy‐6‐mercapto‐β‐cyclodextrin (β‐CD‐SH) as chain‐transfer agent, has been established. These derivatives carry a single β‐cyclodextrin (β‐CD) moiety at one terminus and their average molecular weight is in the order of 10⁴. Thus, their β‐CD content is ～ 10% by weight. No evidence of un‐functionalized PACM was found in the final products. The chain‐transfer constant (C_T) of β‐CD‐SH was found to be 1.30 by independently determining the reaction constants of both chain‐transfer and propagation reactions. This ensures that the molecular weight, hence the β‐CD content of the polymers, does not significantly vary with conversion. These β‐CD‐PACM polymers are highly soluble in water as well as in several organic solvents such as chloroform and lower alcohols. They proved capable of solubilizing in water poorly soluble drugs such as 9‐[(2‐hydroxyethoxy)methyl]guanine (Acyclovir) and of gradually releasing them in aqueous systems. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1607–1617, 2008     

Reversible addition–fragmentation chain transfer polymerization of 2‐naphthyl acrylate with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate as a chain‐transfer agent

The reversible addition–fragmentation chain transfer (RAFT) polymerizations of 2‐naphthyl acrylate (2NA) initiated by 2,2′‐azobisisobutyronitrile were investigated with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate (CPDN) as a RAFT agent at various temperatures in a benzene solution. The results of the polymerizations showed that 2NA could be polymerized in a controlled way by RAFT polymerization with CPDN as a RAFT agent; the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with the monomer conversion. The polydispersities of the polymer were relatively low up to high conversions in all cases. The chain‐extension reactions of poly(2‐naphthyl acrylate) (P2NA) with methyl methacrylate and styrene successfully yielded poly(2‐naphthyl acrylate)‐b‐poly(methyl methacrylate) and poly(2‐naphthyl acrylate)‐b‐polystyrene block polymers, respectively, with narrow polydispersities. The P2NA obtained by RAFT polymerization had a strong ultraviolet absorption at 270 nm, and the molecular weights had no apparent effect on the ultraviolet absorption intensities; however, the fluorescence intensity of P2NA increased as the molecular weight increased and was higher than that of 2NA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2632–2642, 2005     

Nucleation effect of cyclodextrin inclusion compounds on the crystallization of polypropylene

The β‐cyclodextrin (β‐CD) and γ‐cyclodextrin (γ‐CD) inclusion compounds (ICs) with two different molecular weight isotactic polypropylene (iPP) were prepared. The ICs with high molecular weight iPP as guest molecule had lower inclusion rate. The crystallization behavior of iPP blended with the CDs and ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and light scattering. The iPP blended with the ICs was found to exhibit higher crystallization temperature (T_C), smaller spherulites, and faster crystallization rate than those of neat iPP. These results indicate that the ICs play a role of nucleating agent on the crystallization of iPP and induce the accelerated crystallization. Both β‐CD‐iPP ICs and γ‐CD‐iPP ICs with longer iPP molecular chains had better nucleation effect than the ICs with shorter iPP molecular chains. This suggested that the nucleation effect of these ICs was affected by the inclusion rate of ICs. The lower inclusion rate could result in better nucleation effect, due to the interaction of extended iPP molecules inside the CD cavity and iPP molecules in the matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 130–137, 2009     

The effect of γ‐cyclodextrin addition in the self‐assembly behavior of pyrene labeled poly(acrylic) acid with different chain sizes

The interaction between poly(acrylic acid) polymers (PAA) of low‐ (2000 g/mol) and high‐ (450,000 g/mol) molecular weight (M_w) hydrophobically modified with pyrene (PAAMePy) and β‐ and γ‐cyclodextrins (β‐CD, γ‐CD) was investigated with fluorescent techniques. The interaction with β‐CD promotes little variation in the spectral and photophysical behavior of the polymer, whereas significant changes are observed upon addition of γ‐CD. The degree of inclusion (between the pyrene groups of the polymer and the cyclodextrins) is followed through the observation of the changes in the absorption, excitation (collected in the monomer and excimer emission regions) and emission (I_E/I_M ratio) spectra and from time‐resolved data. Within the studied range of γ‐CD concentration, the fluorescence decays of the long chain (high M_w) PAAMePy polymers were found tri‐exponential in the monomer and excimer emission regions in agreement with previous studies. In the case of the low M_w PAAMePy polymers, tri‐exponential decays were observed at the monomer and excimer emission wavelengths. However, when a γ‐CD concentration of 0.01 and 0.03 M is reached for, respectively, the low‐ and high‐labeled pyrene short chain (low M_w) polymers, the fluorescence decays in the excimer region become biexponential (two excimers) with no rising component, thus showing that all pyrene groups are encapsulated (and preassociated) into the γ‐CD cavity. In the case of the high M_w polymers, the addition of γ‐CD has been found to change the level of polymer interaction from pure intramolecular (water in the absence of cyclodextrin) to a coexistence of intra‐ with intermolecular interactions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1402–1415, 2008     

Photophysical Properties of 7‐(diethylamino)Coumarin‐3‐carboxylic Acid in the Nanocage of Cyclodextrins and in Different Solvents and Solvent Mixtures

The photophysical properties of 7‐(diethylamino) coumarin‐3‐carboxylic acid (7‐DCCA) were studied in cyclodextrins (α, β, γ,‐CDs), different neat solvents and solvent mixtures by using steady state absorption, emission and time‐resolved fluorescence spectroscopy. We have observed that with gradual increase in concentration of β‐CD the fluorescence quantum yield and lifetime decreased in a regular pattern whereas with gradual increase in concentration of γ‐CD the fluorescence quantum yield and lifetime gradually increased. With addition of urea, the fluorescence quantum yield and lifetime of 7‐DCCA in CDs increased. Binding constant calculation shows that 7‐DDCA forms 1:1 complex with β‐CD and with γ‐CD it forms 1:1 and 1:2 (guest:host) inclusion complex. We proposed that the dye molecule formed capping complex with β‐CD by means of hydrogen bonding and after addition of urea the hydrogen bonding network broke down and part of dye molecule entered inside the cavity of β‐CD. The photophysics of 7‐DCCA was studied in dioxane‐water mixture and ethylene glycol‐acetonitrile mixture to know the effect of polarity and viscosity of the media. The photophysics of 7‐DCCA was also studied in different neat solvents. It was found that the photophysics of 7‐DCCA depended on the structural feature of the solvents and solvent mixtures.     

Heat‐Induced Supramolecular Organogels Composed of α‐Cyclodextrin and “Jellyfish‐Like” β‐Cyclodextrin–Poly(ε‐caprolactone)

In general, the complexation and gelation behavior between biocompatible poly(ε‐caprolactone) (PCL) derivatives and α‐cyclodextrin (α‐CD) is extensively studied in water, but not in organic solvents. In this article, the complexation and gelation behavior between α‐CD and multi‐arm polymer β‐cyclodextrin‐PCL (β‐CD‐PCL) with a unique “jellyfish‐like” structure are thoroughly investigated in organic solvent N,N‐dimethylformamide and a new heat‐induced organogel is obtained. However, PCL linear polymers cannot form organogels under the same condition. The complexation is characterized by rheological measurements, DSC, XRD, and SEM. The SEM images reveal that the complexes between β‐CD‐PCL and α‐CD present a novel topological helix porous structure which is distinctly different from the lamellar structure formed by PCL linear polymers and α‐CD, suggesting the unique “jellyfish‐like” structure of β‐CD‐PCL is crucial for the formation of the organogels. This research may provide insight into constructing new supramolecular organogels and potential for designing new functional biomaterials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1598–1606     

Nuclear magnetic resonance to study the interactions acting in the enantiomeric separation of homocysteine by capillary electrophoresis with a dual system of γ‐cyclodextrin and the chiral ionic liquid EtCholNTf2

The enantiomeric separation of 9‐fluorenylmethoxycarbonyl chloride (FMOC)‐homocysteine (Hcy) by CE was investigated using γ‐CD and the chiral ionic liquid (R)‐(1‐hydroxybutan‐2‐yl)(trimethyl)azanium‐bis(trifluoromethanesulfon)imidate (also called (R)‐N,N,N‐trimethyl‐2‐aminobutanol‐bis(trifluoromethane‐sulfon)imidate) (EtCholNTf₂) as chiral selectors. Using 2 mM γ‐CD and 5 mM EtCholNTf₂ in 50 mM borate buffer (pH 9), FMOC‐Hcy enantiomers were separated with a resolution value of 3.8. A reversal in the enantiomer migration order in comparison with the single use of γ‐CD in the separation buffer was obtained. Then, NMR experiments were carried out to elucidate the interactions taking place in the enantiomeric separation of FMOC‐Hcy. NMR analyses highlighted the formation of an inclusion complex since the hydrophobic group of FMOC‐Hcy was inserted into the γ‐CD cavity. Moreover, interactions between EtCholNTf₂ and γ‐CD were also observed, suggesting that the chiral ionic liquid would also enter the cavity of the γ‐CD.     

Structure selective complexation of cyclodextrins with five polyphenols investigated by capillary electrokinetic chromatography

The complexation of five polyphenols, namely trans‐resveratrol, astilbin, taxifolin, ferulic acid, and syringic acid (guest molecules) with α‐, β‐, and γ‐cyclodextrin (host molecules), was investigated by capillary electrokinetic chromatography. The binding constants were calculated based on the effective electrophoretic mobility change of guests with the addition of cyclodextrins into the background electrolyte. Because of cavity size, cyclodextrins showed structure‐selective complexation property to different guest. The stability of the trans‐resveratrol complexes was in the order of β‐ > α‐ > γ‐cyclodextrin. The cavity size of α‐cyclodextrin was too small for astilbin and taxifolin molecules, and thus they could not form complexes. The molecular size of syringic acid was too big for all cyclodextrins cavity, and no cyclodextrin could form complexes with it. Temperature studies showed that the binding constants decreased with the rise of temperature. Enthalpy and entropy values were calculated and the negative values of these parameters indicated that the complexation process was enthalpy‐controlled. Van der Waals force and release of high‐enthalpy water molecules from the cyclodextrins cavity played important roles in the process.     

Studies on the non‐covalent interactions between cyclodextrins and aryl alkanol piperazine derivatives by mass spectrometry and fluorescence spectroscopy

The non‐covalent complexes of α‐ and β‐cyclodextrins (α‐, β‐CDs) with two aryl alkanol piperazine derivatives (Pipe I and Pipe II) have been studied by electrospray ionization mass spectrometry (ESI‐MS) and fluorescence spectroscopy. The ESI‐MS experimental results demonstrated that Pipe I can conjugate to β‐CD and form 1:1 or 1:2 stoichiometric non‐covalent complexes, and Pipe II can only form 1:1 complexes with α‐ or β‐CD. Fluorescence spectra indicated that the fluorescence intensities of Pipe I and Pipe II can be enhanced by increasing the content of β‐CD. The mass spectrometric titration experiments showed that the dissociation constants K_d1 were 5.77 and 9.52 × 10^?4 mol L^?1 for the complexes of α‐CD with Pipe I and Pipe II, respectively, revealing that the binding of α‐CD‐Pipe I was stronger than α‐CD‐Pipe II. The K_d1 and K_d2 values were 9.81 × 10^?4 mol L^?1 and 1.11 × 10^?7 (mol L^?1)² for 1:1 and 1:2 complexes of Pipe I with β‐CD, respectively. The K_d values obtained from fluorescence spectroscopy were in agreement with those from ESI‐MS titration. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal decomposition of amide and imide derivatives of maleated polyethylene

The thermal decomposition behavior of six derivatives of maleated polyethylene was investigated by high‐resolution pyrolysis gas chromatography–mass spectrometry. The results revealed that substituents attached to maleated polyethylene as amides formed from secondary amines were significantly less stable than imides formed from primary amines. Morpholine amide and N‐methylaniline amide derivatives of maleated polyethylene underwent significant decomposition at 160 °C and substantial decomposition at 200 °C. In contrast, the imide derivatives of maleated polyethylene were stable for long periods of time at elevated temperatures. Following 2 min of heating, the first traces of decomposition were detected at 200 °C for the 2‐aminoanthrancene imide derivative, at 255 °C for the 2‐phenethylamine imide, and at 280 °C for the 9‐aminomethylphenanthrene imide. With the exception of the 9‐aminomethylphenanthrene imide, all other derivatives decomposed to form the corresponding amine as the single most significant volatile product. The most likely explanation for this result is that the polymer contained small amounts of succinamic acid that did not close to form the imide. We concluded that the imide was stable even to 315 °C and that the amine was lost from β‐carboxyamide groups present in the sample. In the 9‐aminomethylphenanthrene imide derivative, we observed no loss of amine. Instead, we observed an alternative fragmentation process yielding 9‐methyl phenanthrene. The dependence of the thermal stability of these various derivatives of maleated polyethylene has important implications for the design of reactive‐blending strategies for polyolefins with other functional polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 730–740, 2000     

Polyrotaxanes based on polyethers and β‐cyclodextrin

A polyrotaxane in which β‐cyclodextrins (β‐CDs) are threaded onto a polyether chain was prepared by polycondensation of a β‐CD/bisphenol A (BPA) inclusion complex with aromatic dihalides. Two dihalides, with and without a side chain, were used. This polycondensation results in a polyrotaxane (or pseudopolyrotaxane for polymers without stoppers) with a 1:1 threading ratio when the side chain is present and 2:3 when there is none. The long side chain prevents dethreading of the macrocycles. The best yield and a good threading ratio were obtained when the polycondensation was performed by liquid?solid phase transfer catalysis without solvent (L/S PTC) using 2,5‐bi(iodomethyl)‐4‐methoxy‐(1‐octyloxy)benzene as dihalide. The ¹H NMR and FTIR spectra show that the products consist of β‐CD and polyether. The 2D NOESY NMR spectrum shows that the polyether chains are included in the β‐CD cavity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4391–4399, 2009     

Asymmetric cyclopolymerization of N‐tert‐butyl‐N‐allylacrylamide in the presence of β‐cyclodextrin

The radical polymerization of N‐tert‐butyl‐N‐allylacrylamide (t‐BAA) was carried out in a dimethyl sulfoxide/H₂O mixture in the presence of β‐cyclodextrin (β‐CD). The polymerization proceeded with the complete cyclization of the t‐BAA unit and yielded optically active poly(t‐BAA). The IR spectrum of the obtained polymer showed that the cyclic structure in the polymer was a five‐membered ring. The optical activity of poly(t‐BAA) increased with an increasing molar ratio of β‐CD to the t‐BAA monomer. The interaction of β‐CD with t‐BAA was confirmed by ¹H NMR and ¹³C NMR analyses of the polymerization system. It is suggested that interaction of the t‐BAA monomer with the hydrophobic cavity of β‐CD plays an important role in the asymmetric cyclopolymerization of t‐BAA. The radical copolymerization of t‐BAA with styrene (St), methyl methacrylate, ethyl methacrylate, or benzyl methacrylate (BMA) also produced optically active copolymers with a cyclic structure from the t‐BAA unit. St and BMA carrying a phenyl group were predicted to compete with t‐BAA for interaction with β‐CD in the copolymerization system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2098–2105, 2000     

NMR spectra and structures of oridonin derivatives complexes with β‐cyclodextrin

Complexations between three oridonin derivatives and β‐cyclodextrin (βCD) were studied by nuclear magnetic resonance (NMR) method. Job's plots for complexes were depicted by ¹H NMR spectra chemical shifts, which proved the 1:1 stoichiometry inclusion complex formation between each derivative and βCD. Two‐dimensional rotating frame overhauser effect spectroscopy (2D ROESY) support the above conclusion and also proved that ring A of each oridonin derivative deeply enters into hydrophobic cavity from the wider rim and the other parts are outside the cavity. Apparent formation constants (K_a) of complexes between three oridonin derivatives and two CDs are calculated according to Scott's equation. Copyright © 2011 John Wiley & Sons, Ltd.