Fei Ding

A heterowheel [3]pseudorotaxane by integrating β-cyclodextrin and cucurbit[8]uril inclusion complexes.

A heterowheel [3]pseudorotaxane was prepared by integrating two binary inclusion complexes of β-cyclodextrin-hydroxynaphthalene (β-CD·3) with a cucurbit[8]uril-viologen derivative (CB[8]·2), in which simultaneous molecular recognition of the adamantine moiety in 2 by β-CD and the charge-transfer interaction of 3 with the viologen nucleus of 2 in the cavity of CB[8] are two crucial factors for the formation of the quaternary complex.

Cyclodextrins as carriers for cinchona alkaloids: a pH-responsive selective binding system

A series of cyclodextrin-cinchona alkaloid inclusion complexes were prepared from beta-cyclodextrin, heptakis(2,6-di-O-methyl)-beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin and four cinchona alkaloids in ca. 90% yields, and their inclusion complexation behavior was investigated at pH 7.2 and 1.5 by means of fluorescence, UV/Vis and 2D NMR spectroscopy. The results showed that the cinchona alkaloids can be efficiently encapsulated in the cyclodextrin cavity in an acidic environment and sufficiently released in a neutral environment, which makes these cyclodextrin derivatives the potential carriers for cinchona alkaloids. The binding ability and molecular selectivity of cyclodextrins toward cinchona alkaloids were discussed from the viewpoint of the size-fit concept and multiple recognition mechanism between host and guest.

Thermodynamic origins of selective binding affinity between p-sulfonatocalix(4,5)arenes with biguanidiniums†

The binding geometries, abilities and thermodynamic parameters for the intermolecular complexation of two water-soluble calixarenes, p-sulfonatocalix[4]arene (SC4A) and p-sulfonatocalix[5]arene (SC5A), with biguanidinium guests, metformin (MFM) and phenformin (PFM), were investigated by (1)H and 2D NMR spectroscopy, X-ray crystallography, and isothermal titration calorimetry (ITC). The obtained results show that biguanidinium guests are captured by calixarenes with the alkyl or aromatic portion immersed into the cavities and the guanidinium portion fixed at the upper-rims. At both acidic and neutral conditions, SC4A always presents stronger binding affinities to biguanidinium guests than SC5A. Moreover, SC4A prefers to include MFM rather than PFM. As a result, the binding selectivity of MFM is up to 44.7 times for the SC4A/SC5A hosts. The intrinsic relationship between binding structures and selectivities were comprehensively analyzed and discussed from the viewpoint of thermodynamics. Finally, the ITC measurements were further performed in phosphate buffer instead of aqueous solution, to examine the buffer effects, counterion effect, and the differences between thermodynamic and apparent association constants.

Residue- and Sequence-Selective Binding of Nonaromatic Dipeptides by Bis(β-cyclodextrin) with a Functional Tether

Peptide recognition by synthetic receptors is known to mediate versatile protein±protein interactions and give specific biochemical functions. Among them, the development of cyclodextrin (CD)-based receptors for residueand sequence-selective recognition of peptides is one of the most challenging tasks, which may also lead us to deeper understanding and smarter mimicking of vital biological functions. Recent investigations have demonstrated that bis(b-CD)s linked with a short tether possess binding abilities and selectivities for specific guests much higher than native b-CD. This was made possible through the cooperative two-point recognition that mimics the highly substrate-specific binding of enzymes. Thus, a variety of bis(b-CD)s with considerable structural diversity have been prepared in order to elucidate their complexation behavior as well as the factors and mechanisms governing the multipoint recognition upon inclusion complexation by bis-CDs. However, the work on molecular recognition by bis(b-CD)s has been concentrated mostly on the complexation of rather simple organic guests and amino acids, and practically no attempt has been made on the recognition of nonaromatic oligopeptides by bis(b-CD)s, outside the elegant work on oligopeptides carrying two aromatic amino acid residues for a simultaneous complexation by CD cavities and on testing a library of aromatic and nonaromatic tripeptides for binding to a bisas well as a mono-CD receptor. Here, we report the unique molecular-recognition behavior of a newly synthesized dithiobenzoylamino-bridged bis(b-CD) (1) with representative nonaromatic dipeptides and the remarkable fluorescence enhancement by coinclusion of a dipeptide guest with the tether moiety into the cavity.

Molecular binding behaviors of triazole-bridged bis(β-cyclodextrin)s towards cinchona alkaloids

Three bridged bis(β-cyclodextrin)s 3–5 possessing 1,2,3-triazole moieties and polyether chains of different lengths have been synthesized by click reactions in high yields. Their binding affinities towards four cinchona alkaloids, namely, cinchonine, cinchonidine, quinine, and quinidine, have been quantitatively investigated by means of spectrophotometric titrations and 2D NMR spectroscopy. Spectroscopic analyses demonstrated that, compared with native β- and γ-cyclodextrins, bridged bis(β-cyclodextrin)s with a rigid spacer give enhanced molecular binding abilities towards the selected substrates. Moreover, the factors resulting in the significant differences in photophysical behaviors of bridged bis(β-cyclodextrin)s towards cinchona alkaloids are discussed from the viewpoint of the binding geometry of host–guest complexes, revealing that the aromatic ring containing the nitrogen atom of quinine is accommodated in the cavity of 3, whereas the rings of cinchonine, cinchonidine, and quinidine are located out of the cavity of the cyclodextrin and are exposed to aqueous media.

Correlation between Thermodynamic Behavior and Structure in the Complexation of Modified β-Cyclodextrins and Bile Salts

Complex stability constants (K S), standard molar enthalpy changes (ΔH 0) and entropy changes (ΔS 0) for the inclusion complexation of two cyclodextrin dimers, 6,6′-{2,2′-diselenobis[2-(benzoylamino)ethylamino]}-bridged bis(β-cyclodextrin) (1) and o-phenylenediseleno bridged bis(β-cyclodextrin)s (3), and their monomer analogs, 6-deoxy-6-{[2-(2,3-dihydro-3-oxo-1,2-benzisoselenazol-2-yl)ethyl]amino}-β-cyclodextrin (2) and mono[6-(phenylseleno)-6-deoxy]-β-cyclodextrin (4), with two bile salt guests, sodium cholate (CA) and sodium deoxycholate (DCA), were determined at 25°C in Tris buffer solutions (pH 7.4) at 298.15 K by means of isothermal titration microcalorimetry (ITC). The interactions and binding modes between the host cyclodextrins and the guest bile salts were further studied by ROESY spectroscopy. The thermodynamic parameters obtained, together with the ROESY spectra, were used to examine the correlations between thermodynamic behavior and binding modes of the host–guest complexation. The results indicate that the length, structure and conformation of the tethers linked to the cyclodextrins determine the binding modes and the binding abilities between hosts and guests to a great extent, leading to a reversion in binding ability when comparing the corresponding dimer and its monomer analog.

Molecular recognition of amphiphilic p-sulfonatocalix[4]arene with organic ammoniums

The binding abilities and thermodynamic origin for the intermolecular complexation of two water-soluble calixarenes, p-sulfonatocalix[4]arene (SC4A) and 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(n-butyl)-calix[4]arene (SC4A-Bu), with six organic cations: 1,4-diazabicyclo[2,2,2]octane (G1), 3,5,6,8,-tetrahydropyrazino[1,2,3,4-Imn][1,10]phenanthroline (G2), diquat (G3), paraquat (G4), 1-methylpyridin-1-ium (G5) and 1,3-dimethylimidazolium (G6), have been determined by means of isothermal titration calorimetry in aqueous solutions at pH 7.0, 298.15 K, and their binding modes have been investigated by NMR spectroscopy. The obtained results indicate that the binding modes of SC4A-Bu and SC4A change a little but their binding affinities show great difference, resulting from the distinguishable binding thermodynamics. The binding selectivity of G1 is up to 688 times for the SC4A/SC4A-Bu hosts, and SC4A-Bu prefers to include planer molecules of large π system with low electron density. The aggregation behaviours of SC4A-Bu before and after complexation with G3 were then investigated, showing that G3 is able to induce the aggregation of SC4A-Bu.