Conformational changes of functionalised indole receptors upon their interaction with anions

The conformational analysis of four C2-amido and C7-ureido functionalised indole anion receptors was performed by a combination of heteronuclear NMR spectroscopy and ab initio quantum mechanical calculations. NOE experiments showed that anti–anti conformation across C2–C2α and C7–N7α bonds is predominant in acetone solution in the absence of anions. Upon anion binding to receptors, syn–syn conformation becomes predominant. The conformational changes upon anion binding are in good agreement with energetic preferences established by ab initio calculations. Chemical shift changes induced by interaction of anions suggest that binding of chloride and bromide anions occurs primarily to H1 and H7α protons. Nitrate anions favour interaction with H7α and H7γ ureido protons, whereas acetate anions interact strongly with all four available hydrogen bond donor groups.     

Live‐cell MRI with Xenon Hyper‐CEST Biosensors Targeted to Metabolically Labeled Cell‐Surface Glycans

The targeting of metabolically labeled glycans with conventional MRI contrast agents has proved elusive. In this work, which further expands the utility of xenon Hyper‐CEST biosensors in cell experiments, we present the first successful molecular imaging of such glycans using MRI. Xenon Hyper‐CEST biosensors are a novel class of MRI contrast agents with very high sensitivity. We designed a multimodal biosensor for both fluorescent and xenon MRI detection that is targeted to metabolically labeled sialic acid through bioorthogonal chemistry. Through the use of a state of the art live‐cell bioreactor, it was demonstrated that xenon MRI biosensors can be used to image cell‐surface glycans at nanomolar concentrations.     

Cyclodextrins in Polymer Synthesis: Crosslinking Water‐Soluble Unsaturated Polyester Resins Using a Cyclodextrin/Styrene Complex in Aqueous Medium

A new method was developed to crosslink water‐soluble unsaturated polyester resins prepared from maleic anhydride and poly(ethylene glycol) in water as the solvent. Crosslinking was carried out with various molar ratios of the host‐guest complex consisting of styrene as the guest and methylated β‐cyclodextrin as the host. Polymerizations were performed in water with K₂S₂O₈/Na₂S₂O₅ as free radical initiator at 25 °C. Thermal properties of the networks obtained depend on the amount of styrene incorporated into the polymer.

Acceleration effect of me‐β‐CD during crosslinking of an unsaturated polyester with styrene in water: (a) monomer complexed with me‐β‐CD, and (b) with uncomplexed monomer.     

Cucurbit[7]uril: A High‐Affinity Host for Encapsulation of Amino Saccharides and Supramolecular Stabilization of Their α‐Anomers in Water

Cucurbit[7]uril (CB[7]), an uncharged and water‐soluble macrocyclic host, binds protonated amino saccharides (D ‐glucosamine, D ‐galactosamine, D ‐mannosamine and 6‐amino‐6‐deoxy‐D ‐glucose) with excellent affinity (K_a=10³ to 10⁴ M ^?1). The host–guest complexation was confirmed by NMR spectroscopy, isothermal titration calorimetry (ITC), and MALDI‐TOF mass spectral analyses. NMR analyses revealed that the amino saccharides, except D ‐mannosamine, are bound as α‐anomers within the CB[7] cavity. ITC analyses reveal that CB[7] has excellent affinity for binding amino saccharides in water. The maximum affinity was observed for D ‐galactosamine hydrochloride (K_a=1.6×10⁴ M ^?1). Such a strong affinity for any saccharide in water using a synthetic receptor is unprecedented, as is the supramolecular stabilization of an α‐anomer by the host.     

Cyclodextrin‐Based Bimodal Fluorescence/MRI Contrast Agents: An Efficient Approach to Cellular Imaging

A novel bimodal fluorescence/MRI probe based on a cyclodextrin scaffold has been synthesized and characterized. The final agent employs the fluorescein (F) functionality as a fluorescence marker and the Gd^III complex of a macrocyclic DOTA‐based ligand (GdL) having one aminobenzyl‐phosphinic acid pendant arm as an MRI probe, and has a statistical composition of (GdL)_6.9‐F_0.1‐β‐CD. Slow rotational dynamics (governed by a very rigid cyclodextrin scaffold) combined with fast water exchange (ensured by the chosen macrocyclic ligand) resulted in a high relaxivity of ～22 s^?1 mM ^?1 per Gd^III or ～150 s^?1 mM ^?1 per molecule of the final conjugate (20 MHz, 25 °C). In vitro labelling of pancreatic islets (PIs) and rat mesenchymal stem cells has been successfully performed. The agent is not cytotoxic and is easily internalized into cells. The labelled cells can be visualized by MRI, as proved by the detection of individual labelled PIs. A fluorescence study performed on mesenchymal stem cells showed that the agent stays in the intracellular space for a long time.     

The spectrophotometric study of a fluorescence‐enhanced inclusion complex threaded with β‐cyclodextrin: Synthesis and characterization

A novel achiral monomer end‐capped with a phenyl‐[1,3,4]oxadiazolyl group and threaded through β‐cyclodextrin was synthesized to investigate the host‐guest interactions in the inclusion complex. ¹H NMR studies revealed that one or two cyclodextrin molecules were threaded onto the synthesized achiral monomer, leading to the formation of a fibrous construction of self‐assembled inclusion complexes. The formation of a self‐assembled inclusion complex was identified using SEM and TEM. The highly ordered alignment of self‐assembled supramolecules was confirmed using polarized optical microscopy. We demonstrate an easy process for the fabrication of nano‐structured self‐assembled inclusion complexes in pyridine/ethanol (1 mL/10 mL) as well as the enhancement of photo‐induced fluorescence via monomers end‐capped with a phenyl‐[1,3,4]oxadiazolyl moiety threaded with β‐cyclodextrins. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3368–3374, 2010     

Photoluminescence of bridged polysilsesquioxanes containing urea groups: Doping with Safranine‐O to generate red‐light emission

Bridged polysilsesquioxanes containing urea groups (ureasils) exhibit a broad photoluminescent emission in the visible spectrum. For a particular bridged polysilsesquioxane synthesized by the hydrolytic condensation of a precursor prepared by reaction of 4,4′‐[1,3 phenylenebis‐(1‐methylethylidene)]bis(aniline) (BSA) with 3‐(isocyanatepropyl)triethoxysilane (IPTES), we show that the broad photoluminescent band could be transformed into a relatively narrow emission band located in the red region of the spectra, by doping with Safranine‐O. The spectral overlap between the host emission and the dye absorption generated an efficient energy transfer process (antenna effect). Absorption, emission, and triplet–triplet absorption spectra revealed an efficient dispersion of the dye in the ureasil as well as the presence of strong host‐guest specific interactions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 289–296, 2008     

Three‐Component Cucurbit[6]uril Framework with 1:2 Host‐Guest Motif and Dimeric Boric Acid

Three‐component framework of cucurbit[6]uril, 3‐(1‐methylimidazolium‐3‐yl)propane‐1‐sulfonate and boric acid has been constructed. The crystal structure reveals 1:2 host‐guest motif of cucurbit[6]uril and 3‐(1‐methyl‐imidazolium‐3‐yl)propane‐1‐sulfonate, demonstrating both cation binding of imidazolium moiety and anion binding of sulfonate moiety for the first time. Incorporation of dimeric boric acid facilitates the formation of metal‐free three‐dimensional framework.     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

The accessibility of a unimolecular micelle's core to environmental ions: Exploration with a xanthene dye

To learn the impact of aqueous environmental species on the property of the isolated core of a water‐soluble unimolecular micelle (UIM), a guest dye of erythrosine B (EB) is used as a probe to map the dynamic microenvironment of the UIM. PEGylation of branched polyethylenimine (PEI) with oxirane‐functionalized polyethylene glycol (PEG) leads to a UIM of PEI@PEG, and the core is further chemically engineered. The resulting UIMs can irreversibly encapsulate EB exclusively by the core. It is found that the stacking of EB molecules is dependent on the electronic microenvironment of the UIMs, where a polar and ionic core favors a twist stacking of EB, but a less polar core results in an unprecedented parallel stacking of EB. Spectral analysis shows that EB is encapsulated along with its counter ion of Na+, and an exterior ion can cause dehydration of the UIMs but can hardly enter the UIMs; moreover, ion exchange through the PEG shell is actually allowed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 566–573     

Towards Green Routes for Polymer Synthesis: Polymerization of Cyclodextrin Host‐Guest Complexed Diethyl Fumarate and Copolymerization with Complexed Styrene in Homogenous Aqueous Solution

Summary: The radical homo‐ and copolymerization of styrene ( 1 ) and diethyl fumarate (DEF, 2 ) in the presence of methylated β‐cyclodextrin (β‐CD) in water is described. It has been shown for the first time that homopolymerization of CD‐complexed DEF and its copolymerization with CD‐complexed styrene occur readily in aqueous solution. In the absence of CD, or in organic solvents, the homopolymerization of DEF is strongly retarded.

     

Competitive Molecular‐/Ion‐Recognition Responsive Characteristics of Poly(N‐isopropylacrylamide‐co‐benzo‐12‐crown‐4‐acrylamide) Copolymers with Benzo‐12‐crown‐4 as Both Guest and Host Units

Novel dual molecular‐ and ion‐recognition responsive poly(N‐isopropylacrylamide‐co‐benzo‐12‐crown‐4‐acrylamide) (PNB₁₂C₄) linear copolymers with benzo‐12‐crown‐4 (B12C4) as both guest and host units are prepared. The copolymers exhibit highly selective sensitivities toward γ‐cyclodextrin (γ‐CD) and Na⁺. The presence of γ‐CD induces the lower critical solution temperature (LCST) of PNB₁₂C₄ copolymer to shift to a higher value due to the formation of 1:1 γ‐CD/B12C4 host‐guest inclusion complexes, while Na⁺ causes a negative shift in LCST due to the formation of 2:1 “sandwich” B12C4/Na⁺ host‐guest complexes. Regardless of the complexation order, when γ‐CD and Na⁺ coexist with PNB₁₂C₄, competitive complexation actions of B12C4 as both guest and host units toward γ‐CD and Na⁺ finally form equilibrium 2:2:1 γ‐CD/B12C4/Na⁺ composite complexes, and the final LCST values of PNB₁₂C₄ copolymer reach almost the same level. The results provide valuable guidance for designing and applying PNB₁₂C₄‐based smart materials in various applications.