Guest Recognition with Micelle-Bound Cavitands

Selective Molecular Recognition and Indicator Displacement Sensing of Neurotransmitters in Cellular Environments

Flexible, water-soluble hosts are capable of selective molecular recognition in cellular environments and can detect neurotransmitters such as choline in cells. Both cationic and anionic water-soluble self-folded deep cavitands can recognize suitable styrylpyridinium dyes in cellular interiors. The dyes selectively accumulate in nucleotide-rich regions of the cell nucleus and cytoplasm. The hosts bind the dyes and promote their relocation to the outer cell membrane: the lipophilic cavitands predominantly reside in membrane environments but are still capable of binding suitable targets in other cellular organelles. Incubating the cells with structurally similar biomarkers such as choline, cholamine, betaine, or butyrylcholine illustrates the selective recognition. Choline and butyrylcholine can be bound by the hosts, but minimal binding is seen with betaine or cholamine. Varying the dye allows control of the optical detection method, and both "turn-on" sensing and "turn-off" sensing are possible.

Host-assisted delivery of a model drug to genomic DNA: Key information from ultrafast spectroscopy and in silico study

Intended drug delivery to a target without adverse effect is one of the major criteria for its acceptance in real use. Herein, we have made an attempt to explore the delivery efficacy of SDS surfactant in a monomer and micellar stage during the delivery of model drug, Toluidine Blue (TB) from micellar cavity to DNA. Molecular recognition of pre-micellar SDS encapsulated TB with DNA occurs at a rate constant (k1~652 s-1). On the contrary, no significant release of encapsulated TB at micellar concentration was observed within the experimental time frame. This originated from the higher binding affinity of TB towards the nano cavity of SDS at micellar concentration which doesn't allow the delivery of TB from the nano cavity of SDS micelle to DNA. Thus, molecular recognition controls the extent of DNA recognition by TB which in turn modulates the rate of delivery of TB from SDS in a concentration dependent morphology.

Embedding calix[4]resorcinarenes in liposomes: Experimental and computational investigation of the effect of resorcinarene inclusion on liposome properties and stability

Two calix[4]resorcinarenes, which differ in the length of the alkyl chain on the methylene bridge between the aromatic rings, have been embedded in unilamellar liposomes prepared from 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine in three host/guest ratios, following two different procedures. The effect of the insertion of the guests has been evaluated through the measurements of the viscosity and the kinetic stability of the liposomal systems by means of the fluorescent probes pyrene and 5(6)-carboxyfluorescein. The presence of the guests reduces the viscosity of the liposomes, suggesting a modification of the bilayer structure. However, this does not affect liposome stability. A calix[4]resorcinarene cavitand with a more rigid conformation compared to the parent resorcinarene, has been also synthetized and embedded in liposomes. The free energy of the insertion of the substrates in the lipid bilayer has been evaluated through Molecular Dynamics simulations.

Lipid bilayer environments control exchange kinetics of deep cavitand hosts and enhance disfavored guest conformations

The effects on the molecular recognition properties of water-soluble deep cavitand hosts upon embedding them in phosphocholine lipid bilayer environments have been studied by 2D NMR experiments. By employing suitable guests containing 19F or 13C nuclei that can be encapsulated inside the host, 2D EXSY NMR experiments can be used to analyze and compare the in/out guest exchange rates in aqueous solution, isotropically tumbling micelles, or magnetically ordered bicelles. These analyses show that embedding the deep cavitands in lipid bilayers slows the guest exchange rate, due to the lipids acting as a "compression sleeve" around the host, restricting guest egress. This effect also enhances guest conformations in the host that are not observed in free solution, such as axial cyclohexane conformers and ketone hydrates.

Calixarene-mediated liquid membrane transport of choline conjugates 3: The effect of handle variation on neurotransmitter transport

Upper rim phosphonic acid functionalized calix[4]arene affects selective transport of multiple molecular payloads through a liquid membrane. The secret is in the attachment of a receptor-complementary handle to the payload. We find that the trimethylammonium ethylene group present in choline is one of several general handles for the transport of drug and drug-like species. Herein we compare the effect of handle variation against the transport of serotonin and dopamine. We find that several ionizable amine termini handles are sufficient for transport and identify two ideal candidates. Their performance is significantly enhanced in HEPES buffered solutions. This inquiry completes a series of 3 studies aimed at optimization of this strategy. In completion a new approach towards synthetic receptor mediated selective small molecule transport has emerged; future work in vesicular and cellular systems will follow.

Site selective reading of epigenetic markers by a dual-mode synthetic receptor array

Variably functionalized self-folding deep cavitands form an arrayed, fluorescent indicator displacement assay system for the detection of post-translationally modified (PTM) histone peptides. The hosts bind trimethyllysine (KMe3) groups, and use secondary upper rim interactions to provide more sensitive discrimination between targets with identical KMe3 binding handles. The sensor array uses multiple different recognition modes to distinguish between miniscule differences in target, such as identical lysine modifications at different sites of histone peptides. In addition, the sensor is affected by global changes in structure, so it is capable of discriminating between identical PTMs, at identical positions on amino acid fragments that vary only in peptide backbone length, and can be applied to detect non-methylation modifications such as acetylation and phosphorylations located multiple residues away from the targeted binding site. The synergistic application of multiple variables allows dual-mode deep cavitands to approach levels of recognition selectivity usually only seen with antibodies.

A Tetraferrocenyl-Resorcinarene Cavitand as a Redox-Switchable Host of Ammonium Salts

Tetraannulation of a resorcinarene-octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole-resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of (1) H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl-resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox-addressable conformational and binding properties of the resorcinarene-tetraferrocenyl cavitand constitute all the necessary features of a redox-switchable molecular gripper. By means of mass-spectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host-guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase.

Molecular containers assembled through the hydrophobic effect

This review focuses on molecular containers formed by assembly processes driven by the hydrophobic effect, and summarizes the progress made in the field over the last ten years. This small but growing facet of supramolecular chemistry discusses three classes of molecules used by researchers to investigate how self-assembly can be applied to form discrete, mono-dispersed, and structurally well-defined supramolecular entities. The approaches demonstrate the importance of preorganization of arrays of rigid moieties to define a specific form predisposed to bind, fold, or assemble. As the examples demonstrate, studying these systems and their properties is teaching us how to control supramolecular chemistry in water, shedding light on aspects of aqueous solutions chemistry, and illustrating novel applications that harness the unique properties of the hydrophobic effect.

Calixarene-Mediated Liquid Membrane Transport of Choline Conjugates 2: Transport of Drug-Choline Conjugates and Neurotransmitters

Lower rim carboxylic acid calix[n]arenes and upper rim phosphonic acid functionalized calix[4]arenes effect selective transport of distinct molecular payloads through a liquid membrane. The secret to this success lies in the attachment of a receptor-complementary handle. We find that the trimethylammonium ethylene group present in choline is a general handle for the transport of drug and drug-like species. Furthermore, neurotransmitters possessing ionizable amine termini are also transported. Some limitations to this strategy have been uncovered as payloads become increasingly lipophilic. These developments reveal new approaches to synthetic receptor-mediated selective small molecule transport in vesicular and cellular systems.

Fluoride binding in water with the use of micellar nanodevices based on salophen complexes

Uranyl-salophen complexes incorporated into micelles are evaluated as supramolecular nanosystems for the binding of fluoride in water.

Labeled protein recognition at a membrane bilayer interface by embedded synthetic receptors

Self-folding deep cavitands embedded in a supported lipid bilayer are capable of recognizing suitably labeled proteins at the bilayer interface. The addition of a choline derived binding "handle" to a number of different proteins allows their selective noncovalent recognition, with association constants on the order of 10(5) M(-1). The proteins are displayed at the water:bilayer interface, and a single binding handle allows recognition of the large, charged protein by a small molecule synthetic receptor via complementary shape and charge interactions.

Calixarene-Mediated Liquid-Membrane Transport of Choline Conjugates

A series of supramolecular calixarenes efficiently transport distinct molecular species through a liquid membrane when attached to a receptor-complementary choline handle. Calix-[6]arene hexacarboxylic acid was highly effective at transporting different target molecules against a pH gradient. Both carboxylic- and phosphonic-acid-functionalized calix[4]arenes effect transport without requiring a pH or ion gradient. NMR binding studies, two-phase solvent extraction, and three-phase transport experiments reveal the necessary and subtle parameters to effect the transport of molecules attached to a choline "handle". On the other hand, rescorin[4]arene cavitands, which have similar guest recognition profiles, did not transport guest molecules. These developments reveal new approaches towards attempting synthetic-receptor-mediated selective small-molecule transport in vesicular and cellular systems.

Non-covalent self assembly controls the relaxivity of magnetically active guests

The relaxivity of a magnetically responsive Gd complex can be controlled by non-covalent molecular recognition with a water-soluble deep cavitand. Lowered relaxivity is conferred by a self-assembled micellar "off state", and the contrast can be regenerated by addition of a superior guest.

Paramagnetic relaxation enhancement experiments: a valuable tool for the characterization of micellar nanodevices

Micellar incorporation of hydrophobic molecular receptors is a promising strategy to obtain efficient nanodevices that work in water. In order to fully evaluate the potential of this approach, information on the localization and orientation of the receptor inside the micelle are necessary. Systematic studies undertaken on a uranyl-salophen receptor incorporated into CTABr and CTACl micelles show that nuclear magnetic resonance paramagnetic relaxation enhancement (NMR-PRE) experiments are particularly suitable to provide this type of information. The effect on the measurements of surfactant concentration, nature of the surfactant polar head, and ionic strength is also reported. Notably the normalization procedure applied to the obtained data can be considered of general application, thus enabling the comparison of information collected for different types of supramolecular micelle/receptor systems.

Selective cavitand-mediated endocytosis of targeted imaging agents into live cells

A water-soluble synthetic receptor molecule is capable of selective, controlled endocytosis of a specifically tagged target molecule in different types of living human cells. The presence of suitable choline-derived binding handles is essential for the molecular recognition and transport process, allowing selective guest transport and imaging of cancer cells.

Deep cavitand vesicles--multicompartmental hosts

The synthesis and characterization of vesicles assembled from deep cavitands in water is reported. These vesicles act as hosts for three different types of guests: the cavitands bind small guest molecules, the bilayer attracts larger hydrophobic guests and the inner aqueous compartment contains hydrophilic molecules.

Embedding resorcinarene cavitands in lipid vesicles()

A fluorescently labeled resorcinarene cavitand has been successfully embedded in DLPC lipid vesicles and imaged using confocal microscopy. The cavitand resides exclusively in the bilayer.

A carbohydrate-conjugated deep cavitand permits observation of caviplexes in human serum

A deep cavitand was covalently modified with carbohydrates to provide solubility in biologically relevant environments and to investigate its receptor function. Specifically, a tetrakis(β-D-glucosyl) cavitand (1) that was soluble in neutral water or acid/base-buffered solutions was synthesized, and it formed complexes with hydrophobic small molecules. Extraction of the cavitand into aqueous sodium dodecyl sulfate micelles as simple membrane mimetics increased the scope of guests bound by 1 beyond that observed in only aqueous media. Complex formation was also detected in human serum. The findings show the functional compatibility of the receptor in both micelle-bound and serum-soluble forms.

Guest recognition in a partially bridged deep cavitand

The synthesis of a new deep cavitand partially bridged via disulfide bonds is described. Its thermodynamic and kinetic complexation parameters are compared with those of an unbridged analogue. The disulfide bridges cause deviations in ΔH and ΔS but result in only small differences in ΔG of complexation. The bridges increase the activation barrier for guest dissociation and lead to complexes with enhanced kinetic stability.

Protein and small molecule recognition properties of deep cavitands in a supported lipid membrane determined by calcination-enhanced SPR spectroscopy

This paper details the incorporation of a water-soluble deep cavitand into a membrane bilayer assembled onto a nanoglassified surface for study of molecular recognition in a membrane-mimicking setting. The cavitand retains its host properties, and real-time analysis of the host:guest properties of the membrane:cavitand complex via surface plasmon resonance and fluorescence microscopy is described. The host shows selectivity for choline-derived substrates, and no competitive incorporation of substrate is observed in the membrane bilayer. A variety of trimethylammonium-derived substrates are suitable guests, displaying varied binding affinities in a millimolar range. The membrane:cavitand:guest complexes can be subsequently used to capture NeutrAvidin protein at the membrane surface if a biotin-derived guest molecule is used. The surface coverage of NeutrAvidin is affected by the spacer used to derivatize the biotin. Increased distance from the bilayer allows a higher concentration of protein to be immobilized, suggesting a diminishing detrimental steric effect when the binding event is shifted away from the surface.