Molecular recognition studies on supramolecular systems (XXIV)

Circular dichroism spectral and fluorescence decay methods have been employed to determine the conformations of mono[6-(p-tolylseleno)-6-deoxy]-β-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 (logK _s) and Gibbs free en-ergy change (?ΔAG °) 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.     

Cooperative multipoint recognition of organic dyes by bis(beta-cyclodextrin)s with 2,2'-bipyridine-4,4'-dicarboxy tethers

A series of novel 6,6'-bis(beta-cyclodextrin)s linked by 2,2'-bipyridine-4,4'-dicarboxy tethers; that is, 2,2'-bipyridine-4,4'-dicarboxy-bridged bis(6-O-beta-cyclodextrin) (2) and N,N'-bis(2-aminoethyl )-2,2'-bipyridine-4,4'-dicarboxamide-bridged (3), N,N'-bis(5-amino-3-azapentyl)-2,2'-bipyridine-4,4'-dicarboxamide-bridged (4) and N,N'-bis(8-amino-3,6-diazaoctyl)-2,2'-bipyridine-4,4'-dicarboxamide-bridged bis(6-amino-6-deoxy-beta-cyclodextrin) (5), has been synthesized as cooperative multipoint-recognition receptor models. The inclusion complexation behavior of 2-5 with organic dyes; that is, ammonium 8-anilino-1-naphthalenesulfonate, Brilliant Green, Methyl Orange, Acridine Red, and Rhodamine B, has been investigated in aqueous phosphate buffer solutions (pH 7.20) at 25 degrees C by means of ultraviolet, fluorescence, and circular dichroism spectrometry as well as by fluorescence lifetime measurements. The spectral titrations gave the complex stability constants (Ks) and Gibbs' free energy changes (deltaG degrees) for the inclusion complexation of 2-5 with the organic dyes and other thermodynamic parameters (deltaH degrees and deltaS degrees) for the inclusion complexation of 2-4 with the fluorescent dyes Acridine Red and Rhodamine B. Bis(beta-cyclodextrin)s 2-5 displayed higher binding abilities toward most of the examined dye molecules than native beta-cyclodextrin 1; this is discussed from the viewpoints of the size/shape-fit concept, the induced-fit interaction, and cooperative, multipoint recognition by the bridging chain and the dual hydrophobic cavities. Thermodynamically, the inclusion complexation of 2-4 with Acridine Red is totally enthalpy driven with a negative or minor positive entropic contribution, but the inclusion complexation with Rhodamine B is mainly entropy-driven with a mostly positive, but occasionally negative, enthalpic contribution; in some cases this determines the complex stability.     

Molecular recognition on supramolecular systems (XXXV)

A novel β-cyclodextrin derivative4 bearing a pyridinio group on the primary side was synthesized by the reaction of 2-aminopyridine with 6-β-cyclodextrin monoaldehyde3, and its complexation stability constants with several aliphatic amino acids have been determined in phosphate buffer solution ( pH = 7.2, 0.1 mol·L⁻¹) at 25 °C by using spectrofluormetric titrations. The stoichiometry is 1:1 for the inclusion complexation of amino acids with compound4. Circular dichroism study indicates that the aromatic moiety was embedded shallowly into the cyclodextrin cavity. As a spectral probe, the pyridinio group in the modified cyclodextrin can recognize not only differences of the size and shape of amino acid molecules, but also theL/D-amino acid chiral isomer. As compared with mono-[6-(1-pyridinio)-6-deoxy]-β-cyclodextrin5, compound4 switched the enantiomer preference forL- toD-isomer, and showed the highest enantioselectivity of 5.4 forD/L-serine. These results are discussed from the viewpoints of geometric compensation, induced-fit concept and cooperation of several weak interactions.     

Molecular recognition thermodynamics and structural elucidation of interactions between steroids and bridged bis(beta-cyclodextrin)s

A series of bridged bis(beta-cyclodextrin(CD))s (2-7) were synthesized, i.e., bridged bis(beta-CD)s 2 and 3 bearing binaphthyl or biquinoline tethers and bridged bis(beta-CD)s 4-7 possessing dithiobis(benzoyl) tether, and their complex stability constants (KS), enthalpy (DeltaH degrees), and entropy changes (DeltaS degrees) for the 1:2 inclusion complexation with representative steroids, deoxycholate, cholate, glycocholate, and taurocholate, have been determined in an aqueous phosphate buffer solution of pH 7.20 at 298.15 K by means of titration microcalorimetry. The original conformations of bridged bis(beta-cyclodextrin)s were investigated by circular dichroism and 1H ROESY spectroscopy. Structures of the inclusion complexes between steroids and bridged bis(beta-CD)s in solution were elucidated by 2D NMR experiments, indicating that anionic groups of two steroid molecules penetrate, respectively, into the two hydrophobic CD cavities in one 6,6'-bridged bis(beta-CD) molecule from the secondary rim to give a 1:2 binding mode upon inclusion complexation. The results obtained from titration microcalorimetry and 2D NMR experiments jointly demonstrate that bridged bis(beta-CD)s 2, 3 and 5-7 tethered by protonated amino group possessing different substituted groups can enhance not only the molecular binding ability toward steroids by electrostatic interaction but also molecular selectivity. Thermodynamically, the resulting 1:2 bis(beta-CD)-steroid complexes are formed by an enthalpy-driven process, accompanied by smaller entropy loss. The increased complex stability mainly results from enthalpy gain, accompanied by large conformational change and extensive desolvation effects for the 1:2 inclusion complexation between bis(beta-CD)s and steroids.     

Synthesis of novel bis(beta-cyclodextrin)s and metallobridged bis(beta-cyclodextrin)s with 2,2'-diselenobis(benzoyl) tethers and their molecular multiple recognition with model substrates

To investigate quantitatively the cooperative binding ability of beta-cyclodextrin dimers, a series of bridged bis(beta-cyclodextrin)s with 2,2'-diselenobis(benzoyl) spacer connected by different lengths of oligo(ethylenediamine)s (2-5) and their platinum(IV) complexes (6-9) have been synthesized and their inclusion complexation behavior with selected substrates, such as Acridine Red, Neutral Red, Brilliant Green, Rhodamine B, ammonium 8-anilino-1-naphthalenesulfonate, and 6-p-toluidino-2-naphthalenesulfonic acid, were investigated by means of ultraviolet, fluorescence, fluorescence lifetime, circular dichroism, and 2D-NMR spectroscopy. The spectral titrations have been performed in aqueous phosphate buffer solution (pH 7.20) at 25 degrees C to give the complex stability constants (K(S)) and Gibbs free energy changes (-DeltaG degrees ) for the inclusion complexation of hosts 2-9 with organic dyes and other thermodynamic parameters (DeltaH degrees and TDeltaS degrees ) for the inclusion complexation of 2-5with fluorescent dyes ANS and TNS. The results obtained indicate that beta-cyclodextrin dimers 2-5 can coordinate with one or two platinum(IV) ions to form 1:1 or 1:2 stoichiometry metallobridged bis(beta-cyclodextrin)s. As compared with parent beta-cyclodextrin (1) and bis(beta-cyclodextrin)s 2-5, metallobridged bis(beta-cyclodextrin)s 6-9 can further switch the original molecular binding ability through the coordinating metal to orientate two beta-cyclodextrin cavities and an additional binding site upon the inclusion complexation with model substrates, giving the enhanced binding constants K(S) for both ANS and TNS. The tether length between two cyclodextrin units plays a crucial role in the molecular recognition with guest dyes. The binding constants for TNS decrease linearly with an increase in the tether length of dimeric beta-cyclodextrins. The Gibbs free energy change (-DeltaG degrees ) for the unit increment per ethylene is 0.32 kJ.mol(-)(1) for TNS. Thermodynamically, the higher complex stabilities of both ANS and TNS upon the inclusion complexation with 2-5 are mainly contributed to the favorable enthalpic gain (-DeltaH degrees ) by the cooperative binding of one guest molecule in the closely located two beta-cyclodextrin cavities as compared with parent beta-cyclodextrin. The molecular binding ability and selectivity of organic dyes by hosts 1-9 are discussed from the viewpoints of the multiple recognition mechanism and the size/shape-fitting relationship between host and guest.     

Spectrophotometric study on the controlling factor of molecular selective binding of dyes by bridged bis(beta-cyclodextrin)s with diselenobis(benzoyl) linkers

A series of beta-cyclodextrin (beta-CD) dimers with 4,4'-diselenobis(benzoyl) linkers, that is, 6,6'-[4,4'-diselenobis(benzoyloxyl)]-bridged bis(beta-CD) (1a), 6,6'-[4,4'-diselenobis[2-(benzoylamino)ethyleneamino]]-bridged bis(beta-CD) (2a), and 6,6'-[4,4'-diselenobis[2-(benzoylamino)-3,6-diazaoctylamino]]-bridged bis(beta-CD) (3a), were synthesized in moderate yields by the reaction of 4,4'-diselenobis(benzoic acid) with beta-CD or oligo(ethylenediamino)-6-deoxy-beta-CD. Their binding behaviors with some structure-related substrates, such as acridine red (AR), neutral red (NR), rhodamine B (RhB), ammonium 8-anilino-1-naphthalenesulfonate (ANS), and 6-p-toluidino-2-naphthalenesulfonic acid (TNS), were investigated in aqueous phosphate buffer solution (pH 7.20) at 298.15 K by means of fluorescence, NMR, as well as circular dichroism spectroscopy and compared with those of their 2,2'-diselenobis(benzoyl)-linked analogues, that is, 6,6'-[2,2'-diselenobis(benzoyloxyl)]-bridged bis(beta-CD) (1b), 6,6'-[2,3'-diselenobis[2-(benzoylamino)ethyleneamino]]-bridged bis(beta-CD) (2b), and 6,6'-[2,2'-diselenobis[2-(benzoylamino)-3,6-diazaoctylamino]]-bridged bis(beta-CD) (3b). The results showed that bis(beta-CD)s 1a-3a, whose Se-Se bonds were located at the para position of the carboxyl group, gave stronger binding abilities toward nonlinear guests (RhB and ANS) than their analogues 1b-3b, whose Se-Se bonds were located at the ortho position relative to the carboxyl group. The molecular binding ability and selectivity of model substrates by these ditopic hosts were sufficiently discussed to reveal not only the cooperative contributions of the linker group and CD cavities upon inclusion complexation with dye guest molecules but also the controlling factors for the molecular selective binding.     

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.     

Bridged bis(beta-cyclodextrin)s possessing coordinated metal center(s) and their inclusion complexation behavior with model substrates: enhanced molecular binding ability by multiple recognition.

To investigate quantitatively the cooperative binding ability of several beta-cyclodextrin oligomers bearing single or multiligated metal center(s), the inclusion complexation behavior of four bis(beta-cyclodextrin)s (2-5) linked by 2,2'-bipyridine-4,4'-dicarboxy tethers and their copper(II) complexes (6-9) with representative dye guests, i.e., methyl orange (MO), acridine red (AR), rhodamine B (RhB), ammonium 8-anilino-1-naphthalenesulfonic acid (ANS), and sodium 6-(p-toludino)-2-naphthalenesulfonate (TNS), have been examined in aqueous solution at 25 degrees C by means of UV-vis, circular dichroism, fluorescence, and 2D NMR spectroscopy. The results obtained indicate that bis(beta-cyclodextrin)s 2-5 can associate with one or three copper(II) ion(s) producing 2:1 or 2:3 bis(beta-cyclodextrin)-copper(II) complexes. These metal-ligated oligo(beta-cyclodextrin)s can bind two model substrates to form intramolecular 2:2 host-guest inclusion complexes and thus significantly enhance the original binding abilities of parent beta-cyclodextrin and bis(beta-cyclodextrin) toward model substrates through the cooperative binding of two guest molecules by four tethered cyclodextrin moieties, as well as the additional binding effect supplied by ligated metal center(s). Host 6 showed the highest enhancement of the stability constant, up to 38.3 times for ANS as compared with parent beta-cyclodextrin. The molecular binding mode and stability constant of substrates by bridged bis- and oligo(beta-cyclodextrin)s 2-9 are discussed from the viewpoint of the size/shape-fit interaction and molecular multiple recognition between host and guest.     

Synthesis and molecular recognition of novel oligo(ethylenediamino) bridged bis(beta-cyclodextrin)s and their copper(II) complexes: enhanced molecular binding ability and selectivity by multiple recognition

Four bridged bis(beta-cyclodextrin)s tethered by different lengths of oligo(ethylenediamine)s have been synthesized and their inclusion complexation behavior with selected substrates elucidated by circular dichroism spectroscopy and fluorescence decay. In order to study their binding ability quantitatively, inclusion complexation stability constants with four dye guests, that is, brilliant green (BG), methyl orange (MO), ammonium 8-anilino-1-naphthalenesulfonic acid (ANS), and sodium 6-(p-toluidino)-2-naphthalenesulfonate (TNS), have been determined in aqueous solution at 25 degrees C with spectrophotometric, spectropolarimetric, or spectrofluorometric titrations. The results obtained indicate that the two tethered cyclodextrin units might cooperatively bind to a guest, and the molecular binding ability toward model substrates, especially linear guests such as TNS and MO, could be extended. The tether length plays a crucial role in the molecular recognition, the binding constants for ANS and TNS decrease linearly with an increase in the tether length of dimeric cyclodextrin. The Gibbs free energy changes (-deltaGo) for the unit increment per ethylene are 0.99 kJ mol(-1) for ANS and 0.44 kJmol(-1) for TNS, respectively. On the other hand, the presence of a copper(II) ion in metallobis(beta-cyclodextrin)s oligo(ethylenediamino) tethers enhances not only the original binding ability, but also the molecular selectivity through triple or multiple recognition, as compared with the parent bis(beta-cyclodextrin)s.     

Substituent effects on the structure and supramolecular assembly of bis(dioxaborole)s

Solid-state and solution analysis shows that dialkyl substituents on the central phenyl ring of bis(dioxaborole)s, such as , do not have an appreciable effect on the planarity but do significantly alter the supramolecular assembly of these compounds.     

Molecular recognition. Electrostatic effects in supramolecular self-assembly

A di-positively charged metal-based receptor is shown to form 1:1 or 2:1 association complexes with rigid, linear two-site guests depending on the site separation, suggesting that electrostatic repulsion controls the association nuclearity.     

Fluorescence response mechanism of D-glucose selectivity for supramolecular probes composed of phenylboronic-acid-modified beta-cyclodextrin and styrylpyridinium dyes.

Supramolecular complex formation of phenylboronic-acid-modified beta-cyclodextrin (1) with 1-methyl-4-(4-dimethylaminostyryl)pyridinium (C1SP) in aqueous solutions containing saccharides was fully clarified to gain an insight into the observed D-glucose (D-glc) selectivity of a supramolecular fluorescent probe composed of 1 and the 1-heptyl analogue of C1SP (Chem. Commun., 2006, 4319). At pH 9.6, where 1 was in its anionic form, both the stability and the fluorescence of the 1/C1SP complex were reduced by the formation of boronate esters of 1 with saccharides. Among the saccharides, D-glc had the smallest effect on destabilization of the 1/C1SP complex, almost completely retaining the fluorescence of the 1/C1SP complex that was reduced by other saccharides by approximately 2/3. Under neutral conditions, D-glc enhanced the fluorescence of the 1/C1SP complex by increasing the fraction of anionic 1 while minimally decreasing the stability and fluorescence of the 1/C1SP complex. Although other saccharides also increased the fraction of the anionic 1, their relatively large effects on the destabilization and reduction of fluorescence of the 1/C1SP complex limited the enhancement of the fluorescence of the 1-C1SP system under neutral conditions.     

Novel synthesis of polyethers by the polyaddition of bis(epoxide)s with bis(phosphinate)s

The polyaddition of bisphenol A diglycidyl ether with bis[4‐(P,P‐diphenylphosphinyloxy)phenyl] sulfone catalyzed by quaternary onium salt, such as tetrabutylammonium chloride afforded a new phosphorus‐containing polyether with good solubility in common organic solvents. Having studied various factors affecting the reaction, such as temperature, catalyst concentration, reaction time, etc., an appropriate polyaddition condition was suggested as using 5 mol % of suitable quaternary ammonium or phosphonium salt in polar solvent at 150°C within 25 h in an ampule for producing high molecular weight polymer. A number of polyethers bearing pendent phosphinate ester groups from the polyaddition of certain bis(epoxide)s and bis(phosphinate)s were synthesized under the above condition and characterized by GPC, IR, and NMR. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1009–1016, 1999     

Self-assembly of supramolecular complexes of cyanine dyes containing terminal ammonium groups with bis(18-crown-6)stilbene

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Interfacial supramolecular self-assembled monolayers of C(60) by thiolated beta-cyclodextrin on gold surfaces via monoanionic C(60)

The supramolecular self-assembled monolayers (SAMs) of C(60) by thiolated beta-cyclodextrin (CD) on gold surfaces were constructed for the first time using C(60) monoanion. The results indicate that monoanionic C(60) plays a crucial role in the formation of the C(60)-containing self-assembled monolayers. The generation of C(60) monoanion and the formation process of C(60) SAMs were monitored in-situ by UV-visible and near-IR spectroscopy. The resulting C(60) SAMs were fully characterized by spectroscopic ellipsometry (SE), cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and water contact angle measurements. After the immobilization of C(60) by the SAMs of thiolated beta-CD, the film thickness increased by approximately 1 nm from 0.8 to 1.8 nm as determined by SE, demonstrating the formation of the supramolecular self-assembled monolayers of thiolated beta-CD/C(60). The new C(60) SAMs exhibited one quasi-reversible redox couple at half wave potential of -0.57 V vs SCE in aqueous solution containing 0.1 M KCl. The surface coverage of C(60) on the gold surfaces was estimated to be 1.1 x 10(-10) mol cm(-2). The XPS showed the assembly of C(60) over the thiolated beta-CD SAMs. The surface hydrophobicity increased greatly upon the formation of the C(60)-containing SAMs as analyzed by water contact angle measurements. The results are in agreement with the formation of 1:1 complex of C(60) and cyclodextrin on gold surfaces, though it also reveals some non-homogeneous features of the monolayers.     

Reactive oligomers. II. Polymerization of glycol bis(allyl phthalate)s and glycol bis(allyl succinate)s

Seven glycol bis(allyl phthalate)s (GBAP) and four glycol bis(allyl succinate)s (GBASu) as reactive oligomers were prepared and their polymerization behaviors were investigated in detail in terms of cyclopolymerization and gelation as compared with diallyl dicarboxylates. Thus, the rates of polymerization of GBAPs were reduced compared to diallyl phthalate, being attributed to the steric effect on the intermolecular propagation of the uncyclized radical, whereas those of GBASus were enhanced as a consequence of intermolecular association by dipole–dipole interaction in polar GBASu monomers. Cyclization was enhanced in the following order: diallyl aliphatic dicarboxylates series < GBASu series < GBAP series. Gelation was discussed according to Gordon's theory; the actual gel-point conversions increased with an increase in the molecular weight of monomers, although the discrepancy between actual and theoretical gel-point conversion inversely tended to be decreased. The decreased delay in gelation with an increase of the molecular weight of monomers is ascribed to the reduction of excluded volume effects on crosslinking.     

Molecular recognition on supramolecular systems (XXXV)-- Synthesis of novel b-cyclodextrin derivative bearing pyridinio group and its chiral discrimination of amino acids

A novel b-cyclodextrin derivative 4 bearing a pyridinio group on the primary side was synthesized by the reaction of 2-aminopyridine with 6-b-cyclodextrin monoaldehyde 3, and its complexation stability constants with several aliphatic amino acids have been determined in phosphate buffer solution ( pH = 7.2, 0.1 mol·L-1) at 25 ℃ by using spectrofluormetric titrations. The stoichiometry is 1︰1 for the inclusion complexation of amino acids with compound 4. Circular dichroism study indicates that the aromatic moiety was embedded shallowly into the cyclodextrin cavity. As a spectral probe, the pyridinio group in the modified cyclodextrin can recognize not only differences of the size and shape of amino acid molecules, but also the L/D-amino acid chiral isomer. As com-pared with mono-[6-(1-pyridinio)-6-deoxy]-b-cyclodextrin 5, compound 4 switched the enantiomer preference for L- to D-isomer, and showed the highest enantioselectivity of 5.4 for D/L-serine. The-se results are discussed from the viewpoints of geometric compensation, induced-fit concept and cooperation of several weak interactions.     

Electrochemical recognition of cations by bis(pyrrolo)tetrathiafulvalene macrocycles

[structure: see text] Tetrathiafulvalene redox-responsive ligands devoid of cis/trans isomerism containing the electroactive bis(pyrrolo[3,4-d])tetrathiafulvalene moiety and polyether subunits have been synthesized. One ligand exhibits high binding affinities for Pb2+ and Ba2+ cations as shown by independent methods (1H NMR, UV-vis spectroscopy, and cyclic voltammetry). The ability of this receptor to electrochemically recognize Pb2+ and Ba2+ is shown by cyclic voltammetry.     

Formation of supramolecular systems via directed Nucleoside-Lipid recognition

We report the synthesis of a new series of Ketal Nucleoside Lipids (KNLs) featuring saturated hydrophobic double chains and either adenosine or uridine as nucleosides (KNL(A) and KNL(U), respectively). Physicochemical studies (differential scanning calorimetry, small angle X ray scattering, transmission electronic microscopy, atomic force microscopy, Langmuir isotherm, infrared spectroscopy) show that the KNLs form hydrogels below the main phase transition temperature (Tm), whereas fluid lamellar phases are obtained above T(m). Mixing complementary KNLs affords a new stable Combined Supramolecular Systems (CSSs) due to complementary A-U recognition. Molecular modeling calculations of the bilayers in a fluid state exhibit a merging of the bilayers partially due to base-base interactions.