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     

Bilayer vesicles of amphiphilic cyclodextrins: host membranes that recognize guest molecules

A family of amphiphilic cyclodextrins (6, 7) has been prepared through 6-S-alkylation (alkyl=n-dodecyl and n-hexadecyl) of the primary side and 2-O-PEGylation of the secondary side of alpha-, beta-, and gamma-cyclodextrins (PEG=poly(ethylene glycol)). These cyclodextrins form nonionic bilayer vesicles in aqueous solution. The bilayer vesicles were characterized by transmission electron microscopy, dynamic light scattering, dye encapsulation, and capillary electrophoresis. The molecular packing of the amphiphilic cyclodextrins was investigated by using small-angle X-ray diffraction of bilayers deposited on glass and pressure-area isotherms obtained from Langmuir monolayers on the air-water interface. The bilayer thickness is dependent on the chain length, whereas the average molecular surface area scales with the cyclodextrin ring size. The alkyl chains of the cyclodextrins in the bilayer are deeply interdigitated. Molecular recognition of a hydrophobic anion (adamantane carboxylate) by the cyclodextrin vesicles was investigated by using capillary electrophoresis, thereby exploiting the increase in electrophoretic mobility that occurs when the hydrophobic anions bind to the nonionic cyclodextrin vesicles. It was found that in spite of the presence of oligo(ethylene glycol) substituents, the beta-cyclodextrin vesicles retain their characteristic affinity for adamantane carboxylate (association constant K(a)=7.1 x 10(3) M(-1)), whereas gamma-cyclodextrin vesicles have less affinity (K(a)=3.2 x 10(3) M(-1)), and alpha-cyclodextrin or non-cyclodextrin, nonionic vesicles have very little affinity (K(a) approximately 100 M(-1)). Specific binding of the adamantane carboxylate to beta-cyclodextrin vesicles was also evident in competition experiments with beta-cyclodextrin in solution. Hence, the cyclodextrin vesicles can function as host bilayer membranes that recognize small guest molecules by specific noncovalent interaction.     

Host–guest complexes between cryptophane‐C and chloromethanes revisited

Cryptophane‐C is composed of two nonequivalent cyclotribenzylene caps, one of which contains methoxy group substituents on the phenyl rings. The two caps are connected by three OCH₂CH₂O linkers in an anti arrangement. Host–guest complexes of cryptophane‐C with dichloromethane and chloroform in solution were investigated in detail by nuclear magnetic resonance techniques and density functional theory (DFT) calculations. Variable temperature proton and carbon‐13 spectra show a variety of dynamic processes, such as guest exchange and host conformational transitions. The guest exchange was studied quantitatively by exchange spectroscopy measurements or by line‐shape analysis. The conformational preferences of the guest‐containing host were interpreted through cross‐relaxation measurements, providing evidence of the gauche+2 and gauche?2 conformations of the linkers. In addition, the mobility of the chloroform guest inside the cavity was studied by carbon‐13 relaxation experiments. Combining different types of evidence led to a detailed picture of molecular recognition, interpreted in terms of conformational selection. Copyright © 2012 John Wiley & Sons, Ltd.     

Supramolecular complexation of homocysteine and cysteine with cucurbit[7]uril

Supramolecular complexation of two bio-thiols, homocysteine (Hcys) and cysteine (Cys), by cucurbit[7]uril (CB[7]) has been fully investigated by ¹H NMR spectroscopy, mass spectrometry, isothermal titration calorimetry, and the results were further verified with computational investigations. NMR titration experimental results obviously indicate that the binding stoichiometry of CB[7] to Hcys is 1:1 and to Cys is 1:2 in aqueous solution. The binding constants and thermodynamic parameters associated with the complexation between CB[7] and the bio-thiols were determined by isothermal titration calorimetry. The energy-minimized structures of the supramolecular complexes of CB[7] with Hcys and Cys were determined and provide good agreement with the experimental results. The CB[7] cavity is sufficient to include the two Cys, but is unable to accommodate two Hcys due to steric hindrance. The differing binding abilities of Hcys and Cys in aqueous solution towards CB[7] host may lead to discriminate them.     

Narrow disperse polymers using amine functionalized dithiobenzoate RAFT agent and easy removal of thiocarbonyl end group from the resultant polymers

A novel amine functionalized RAFT agent, 2‐cyanoprop‐2‐yl(4‐N,N‐dimethylaminophenyl) dithiobenzoate has been synthesized and used to control the polymerization of vinyl monomers. This dithiobenzoate RAFT agent, although air sensitive, controlled the polymerization of MMA and St very well in an inert atmosphere and the polymerization results obtained were marginally better than using the most popular 2‐cyanoprop‐2‐yl dithiobenzoate RAFT agent. The living nature of these polymerizations was confirmed by kinetics study and chain extension reactions to yield narrow disperse di‐block copolymers. Most importantly, use of this novel RAFT agent simplified the removal procedure of the color causing end thiocarbonyl group from the RAFT derived polymers and thereby leading to polymers with improved appearance. The removal of end group from the polymer was confirmed by ¹H NMR and UV‐vis spectroscopic techniques. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Characterization of molecularly imprinted polymers employing crosslinkers with nonsymmetric polymerizable groups

Crosslinking monomers have been developed with a combination of methacrylamide and methacrylate or vinyl ketone polymerizable groups that provide molecularly imprinted polymers (MIPs) with improved binding and selectivity. The differential reactivity rates of the polymerizable groups prompted an investigation into the time‐dependent behavior of the crosslinkers, which suggests a new mechanism for MIP formation. The mechanism involves the formation of long sections of linear poly(vinyl ketone) with pendant methacrylamide groups that form a highly crosslinked network in a subsequent step. This has implications for the sequence morphology of polymers, affecting the structure and improving the binding properties of MIPs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3668–3675, 2004     

以苯并杂环为端基的非环多醚化合物的合成

本文合成了8种以2(3H)-苯并恶唑酮和苯并三唑为端基的非环多醚化合物. 它们的组成和结构经元素分析,IR,MS和^1H NMR波谱分析得到证实. 初步反应揭示了这些化合物可与UO~2^2^+ 等离子形成稳定的配合物.     

Molecular recognition studies on supramolecular systems (XXIV)——Conformations and complexation abilities of chemically modified cyclodextrins in aqueous solution

Circular dichroism spectral and fluorescence decay methods have been employed to determine the conformations of mono[6-(p-tolylseleno)-6-deoxy]-p-CD(1), mono(6-anilino-6-deoxy) β -CD (2) and mono[6-(L-tryptophan)-6-deoxy]-β-CD (3) in phosphate buffer solution (pH 7.2, 0.1 mol dm-3) at 298.15 K. The results indicate that compounds 2 and 3 formed self-inclusion complexes in aqueous buffer solution, while the substituent of compound 1 was not included into cyclodextrin cavity at all. Furthermore, the complex stability constant (logKs) and Gibbs free energy change (-ΔG° ) of these three cylcodextrin derivatives with several cycloalkanols have been determined by circular dichroism spectral titration in phosphate buffer solution at 298.15 K. It is found that the location of the substituent affects the stability of host-guest complex in aqueous solution.     

Molecular recognition of sialic acid end groups by phenylboronates

A multinuclear NMR study of the interaction between phenylboronic acid (PBA) and sialic acid (Neu5 Ac) has been performed. The latter compound is known to be overexpressed on the cell surface of tumor cells. The results of this investigation suggest that the binding of PBA to sialic acid is pH dependent. 17O NMR experiments with glycolic acid as the model compound prove that an interaction at the alpha-hydroxycarboxylate occurs at pH < 9, while a study with threonic and erythronic acids shows that the PBA group interacts selectively with the vicinal diol functions at higher pH. Similarly, Neu5 Ac binds PBA through its alpha-hydroxycarboxylate at low pH (< 9) and through its glycerol side chain at higher pH values. The conditional stability constant of the phenylboronate ester at pH 7.4 is 11.4. On cell surfaces, sialic acid is connected to the neighboring sugar unit through the 2-hydroxy group. To mimic this the 2-alpha-O-methyl derivative of Neu5 Ac was included in this study. The erythro configuration of the hydroxy substituents prevents stable-complex formation at positions C7 and C8 and, consequently, the strongest interaction is observed at positions C8 and C9, leading to a five-membered 2-boron-1,3-dioxalate. In addition, a relatively small amount of the C7-C9 six-membered complex was observed. Molecular modeling studies confirm that the C8-C9 boronate complex has the lowest energy.     

Characterization of end groups in nylon 6 by MALDI-TOF mass spectrometry

Four samples of Ny6, each terminated by different end groups, i.e., diamino terminated, monoamino terminated (monocapped), dicarboxyl terminated, and amino-carboxyl terminated, were synthesized and analyzed by MALDI-TOF Mass Spectrometry, in order to accurately characterize their structure by direct identification of mass resolved chains. A self-calibrating method for the MALDI-TOF mass spectra of polymeric samples was used in order to distinguish the end groups existing in the four samples of Ny6. The MALDI-TOF spectra showed the presence of protonated, sodiated, and potassiated ions that were assigned to Ny6 chains containing the expecteted end groups. Furthermore, the MALDI-TOF spectra made possible the simultaneous detection of the cyclic oligomers of Ny6 present in these samples, thus achieving the full structural characterization of the molecular species present in these polyamides. © 1996 John Wiley & Sons, Inc.     

The formation of host–guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host–guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host–guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD–surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature.     

Some connections between viscoelastic properties of PVC and plasticized PVC and molecular kinetics

A coupling model that has been shown in the past to be capable of relating macroscopically measured relaxation parameters to molecular ones has been presented. In this article the coupling model is applied to the analysis of stress relaxation data collected by Cama and Sternstein on PVC and plasticized PVC. The Kohlrausch-Williams-Watts form, exp — (t/τ*)^1?n, using n = 0.77 is found to be capable of describing the stress relaxation master curve at temperatures below the glass transition, T_g. From the temperature-independent apparent activation energy found by Cama and Sternstein, the primitive activation energy of the α-relaxation was calculated to be 7.5 kcal/mol, which is a reasonable value for the energy barrier to internal rotational isomerism in PVC. Support for this value is found from the data on two plasticized PVCs with different T_gs and apparent activation energies. By applying the coupling model in a similar manner, the primitive activation energies were found to be 8.5 kcal/mol for PVC plasticized with 6 pph dioctylphthalate and 7.7 kcal/mol for PVC plasticized with 6 pph tricresyl phosphate. Within experimental uncertainties, the three primitive activation energies can be considered to be the same. This finding is consistent with the physical basis for primitive activation energy and its identification with the internal rotation barrier, which should be independent of the type and amount of plasticizer in the system. Analysis of Cama and Sternstein's data on the effect of repeated stress aging on stress relaxation of quenched samples of PVC and plasticized PVC show that the coupling constant n increases systematically with each successive stress-aging cycle until it approaches the value for slow-cooled samples. These results are consistent with the notion that stress-aging changes the structural state of the glass in ways similar to physical aging.     

Effect of hydroxypropylmethyl cellulose on the complexation of diclofenac with cyclodextrins

The effect of a hydrophilic polymer, hydroxypropylmethyl cellulose K4M, on the complexation of diclofenac sodium with b- and hydroxypropyl-b-cyclodextrins has been studied. Multicomponent systems were prepared with the drug, both cyclodextrin and the polymer. Phase solubility diagrams revealed the positive effect of the polymer on the complexation of the drug but this effect was found after autoclaving the solutions. Solid inclusion complexes were prepared by freeze-drying and characterized by thermal analysis (DSC) and X-ray diffractometry. In solid state, binary inclusion complexes enhance the dissolution behaviour of diclofenac but, from the b-cyclodextrin multicomponent complex, the polymer controls the release of the drug. In the case of hydroxy- propyl-b-cyclodextrin multicomponent system, the solubility of the drugs increases significantly compared with the binary complex. This revised version was published online in July 2006 with corrections to the Cover Date.     

Exploring the relation between amplification and binding in dynamic combinatorial libraries of macrocyclic synthetic receptors in water

Herein we describe an extensive study of the response of a set of closely related dynamic combinatorial libraries (DCLs) of macrocyclic receptors to the introduction of a focused range of guest molecules. We have determined the amplification of two sets of diastereomeric receptors induced by a series of neutral and cationic guests, including biologically relevant compounds such as acetylcholine and morphine. The host–guest binding affinities were investigated using isothermal titration calorimetry. The resulting dataset enabled a detailed analysis of the relationship between the amplification of selected receptors and host–guest Gibbs binding energies, giving insight into the factors affecting the design, simulation and interpretation of DCL experiments. In particular, two questions were addressed: Is amplification by a given guest selective for the best receptor? And does the best guest induce the largest amplification of a given receptor? Our experimental results and computer simulations showed that the relative levels of amplification of hosts by a guest are well‐correlated with their relative affinities, and simulations have confirmed previous observations that amplification can be selective for the best receptor when only modest amounts of guest are used. In contrast, the correlation between guest binding and the extent of amplification of a given receptor across a wide range of guests tends to be poorer, because every guest has its own unique set of affinities for competing receptors in the DCL. This implies that the results of screening a DCL for selective receptors by comparing the response of the mixture to two different guests should be interpreted with caution. DCLs are complex mixtures in which all compounds are connected through a set of equilibria. Obtaining quantitative information about all host–guest binding constants from such systems will require the explicit and simultaneous consideration of all of the main equilibria within a DCL.     

Kinetics and equilibrium of the complexation of Al3+ with poly(maleic, acrylic) acid

Kinetics and equilibrium of the complexation of Al³⁺ with a polycarboxylic acid (PCA, random copolymer of maleic and acrylic acid with a mean molecular weight of 92 kDa) are investigated by the stopped flow technique and potentiometric titration. The complexation proceeds according to the Eigen–Tamm mechanism, i.e. in first diffusion-controlled step an outer sphere complex is formed. The second rate determining step is the formation of the inner sphere complex, controlled by the exchange rate of hydration water. For this second step the rate constant is k ₁=3 s^-1. It is in the order of magnitude of the water exchange at the Al³⁺ ion as expected for the Eigen–Tamm mechanism. The activation parameters are also determined. Parallel to this direct reaction path a base catalyzed path is found, typical for complexation reactions of hydrolyzable metal ions. Stable complexes are formed for which the overall association constant K _ass=Q _o(1+K _i) is determined by two parts: a chemical (intrinsic) part, described by the inner sphere association constant K _i=3 and an electrostatically controlled part described by the outer-sphere association quotient Q _o. The evaluation of the kinetic experiments allows to determine the value of log(Q _o) as a function of pH: 3.3<log Q _o<4.6. From these data the potential is calculated in the range −67 to ∝93 mV at pH values between 2 and 4. For comparison, analogous experiments with the monomeric subunits of the polyacid, glutarate (GA), and tricarballylate (TCA), are performed. The complexation with the monomeric subunits glutaric- and tricarballylic acid can be explained within the classical view of a discrete outer sphere association constant Q _o. Received: 13 November 1997 Accepted: 24 March 1998     

Supramolecular catalytic systems based on calixarenes and cyclodextrins

A number of supramolecular metal complex catalitic system based on cyclodextrins and calixarenes have been developed. Particular attention has been given to design of new metal complexes with molecular recognition abilities. Data on application catalytic systems based on receptor molecules capable to formation of inclusion “host-guest” complexes in hydroformylation and Wacker-oxidation are given. Influence of different factors on activity, stability, selectivity of metal complex supramolecular catalytic systems is discussed.     

Effect of montmorillonite on kinetics of polyurethane preparation reaction

The prepolymerization and curing reaction kinetics of polyurethane/montmorillonite have been studied with end group analysis and FTIR respectively. It was found that the prepolymerization and curing reaction followed the 2nd-order kinetics. But the activation energy of prepolymerization increased from 42.7 kJ/mol to 56.5 kJ/rnol after the montmorillonite was added in the reaction system, and activation energy of curing reaction decreased from 64.4 kJ/mol to 17.5 kJ/mol. 2007 Bing Liao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

Synthesis of novel modified p‐cyclodextrins and their fluorescence studies of inclusion complexation with Rhodamine B

Three novel β‐cyclodextrin derivatives have been synthesized and their inclusion complexation behavior with Rhodamine B (RhB) was investigated by the fluorescence spectroscopy.