State-of-the-art and recent progress in resorcinarene-based cavitand

Compartmentalization in the biological world brings excellent efficiency and specificity to the formation of complex compounds, inspiring supramolecular chemists to continuously search for defined spaces that can mimic such natural binding sites. Bowl-shaped cavitands built up from resorcinarenes (RA) present rigid and preorganized concave surfaces, which are capable of mimicking the molecular recognition properties of enzymes. The versatile scaffold of RA endows the cavitand with terrific variety and excellent binding behavior. This review provides a comprehensive overview over the structural modification to date in the high attention field of RA-based cavitands development. Different strategies for synthesizing diverse cavitands, such as small cavity cavitands, wider cavity cavitands, deep cavity cavitands, biscavitands, and asymmetric cavitands, are discussed in details. Furthermore, insights into their applications including catalysis, separations and sensing are provided.     

Specialist gelator for ionic liquids

Cyclo(l-beta-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) and cyclo(L-beta-2-ethylhexylasparaginyl-L-phenylalanyl) (2), prepared from L-asparaginyl-L-phenylalanine methyl ester, have been found to be specialist gelators for ionic liquids. They can gel a wide variety of ionic liquids, including imizazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium, and ammonium salts. The mean minimum gel concentrations (MGCs) necessary to make gels at 25 degrees C were determined for ionic liquids. The gel strength increased at a rate nearly proportional to the concentration of added gelator. The strength of the transparent gel of 1-butylpyridinium tetrafluoroborate ([C(4)py]BF(4)), prepared at a concentration of 60 g L(-1) (gelator 1/[C(4)py]BF(4)), was ca. 1500 g cm(-2). FT-IR spectroscopy indicated that a driving force for gelation was intermolecular hydrogen bonding between amides and that the phase transition from gel to liquid upon heating was brought about by the collapse of hydrogen bonding. The gels formed from ionic liquids were very thermally stable; no melting occurs up to 140 degrees C when the gels were prepared at a concentration of 70 g L(-1) (gelator/ionic liquid). The ionic conductivities of the gels were nearly the same as those of pure ionic liquids. The gelator had electrochemical stability and a wide electrochemical window. When the gels were prepared from ionic liquids containing propylene carbonate, the ionic conductivities of the resulting gels increased to levels rather higher than those of pure ionic liquids. The gelators also gelled ionic liquids containing supporting electrolytes.     

pH-triggered assembly of gold nanorods

The self-assembly of surfactant-protected gold nanorods (aspect ratio 3.3 +/- 0.3, 20.6 +/- 5.5 nm width, and 67.5 +/- 9.0 nm length) into ordered structures using adipic acid is presented. As made, the gold nanorods are coated with cationic surfactant, which gives them a net positive charge in aqueous solution. The pH-dependent assembly is directed by electrostatic interactions between the positively charged nanorods and negatively charged, deprotonated adipic acid. Absorption spectra and light scattering measurements of these nanorods suggest that aggregation is initiated in solution in the presence of adipic acid at pH 7-8, but not at pH 3, to form small assemblies of nanorods. Zeta potential measurements show that the assembly is significantly less positively charged in the presence of deprotonated adipic acid than when adipic acid is fully protonated.     

Film-forming microgels for pH-triggered capture and release

The pH-responsive behavior for a series of lightly cross-linked, sterically stabilized poly(tertiary amine methacrylate)-based latexes adsorbed onto mica and silica was investigated using in situ tapping mode AFM at room temperature. The adsorbed layer structure was primarily determined by the glass transition temperature, T(g), of the latex: poly[2-(diethylamino)ethyl methacrylate]-based particles coalesced to form relatively featureless uniform thin films, whereas the higher T(g) poly[2-(diisopropylamino)ethyl methacrylate] latexes retained their original particulate character. Adsorption was enhanced by using a cationic poly[2-(dimethylamino)ethyl methacrylate] steric stabilizer, rather than a nonionic poly(ethylene glycol)-based stabilizer, since the former led to stronger electrostatic binding to the oppositely charged substrate. Both types of adsorbed latexes acquired cationic microgel character and swelled appreciably at low pH, even those that had coalesced to form films. Fluorescence spectroscopy was used to study the capture of a model hydrophobic probe, pyrene, by these adsorbed latex layers followed by its subsequent release by lowering the solution pH. The repeated capture and release of pyrene through several pH cycles was also demonstrated. Since these poly(tertiary amine methacrylate) latexes are readily prepared by aqueous emulsion polymerization and adsorption occurs spontaneously from aqueous solution, this may constitute an attractive route for the surface modification of silica, mica and other oxides.     

Anion binding to a tetracopper resorcinarene-based complex

UV–vis and electron paramagnetic resonance (EPR) spectroscopic studies have been carried out on the multinuclear copper(II) complex Cu₄BpaRes. The copper atoms are in a tetragonal distorted geometry with nitrogens from bispicolylamine (Bpa) coordinating in the equatorial plane and water molecules or anions completing four coordination sites. The interaction of anions with this polynuclear copper complex in aqueous solution supports the formation of different complex species, which depend on the type and the concentration of the anions. In the presence of excess anions, frozen solution EPR parameters show the formation of species in which the in-plane coordination is characterised by the presence of three nitrogen atoms coming from the ligand and a donor atom from the specific anion. For the bidentate anion ligands and especially for malonate, UV–vis titrations indicate the formation of a 1:4 (Cu₄BpaRes:anion) species. EPR experiments support the formation of such a species and indicate that the four copper centres are equivalent and reach penta-coordination via the coordination of both oxygens from the bidentate ligand.     

New cholesterol-based gelator with orotate unit

A new cholesterol-based gelator with orotate unit (cholesterol orotate) was synthesised. The morphology and structure of the gels were investigated by scanning electron microscopy and powder X-ray diffractions. It is observed that the addition of melamine modulated the supramolecular self-assembly and aggregation mode, which originated from the intermolecular non-covalent interactions, as revealed by FT-IR and UV–vis absorption spectra.     

pH-triggered strand exchange in coiled-coil heterotrimers

The capacity for pH-triggered strand exchange in designed coiled-coil heterotrimers is demonstrated. Systems employing both hydrophobic core (steric matching) and hydrophilic interface (electrostatic matching) design principles assemble into specific 1:1:1 heterotrimers. Alteration of pH creates electrostatic mismatches, inducing strand exchange in the presence of a suitable replacement peptide. Complexes with one Lys/Lys interface, favored at neutral to high pH, can be transformed to ones with a Glu/Glu contact by lowering pH and adding an appropriate new binding partner. The need to simultaneously maintain matched core alignments enforces specificity in this exchange, such that only a single specific peptide is replaced. These principles have subsequently been applied to the design of dynamically triggered cross-linked structures, in which a bifunctional disulfide-tethered peptide can cross-link two heterotrimers. Both formation and disruption of the cross-link are under pH control.     

Nanosized polymetallic resorcinarene-based host assemblies that strongly bind fullerenes

Polymetallic nanodimensional assemblies have been prepared via metal directed assembly of dithiocarbamate functionalized cavitand structural frameworks with late transition metals (Ni, Pd, Cu, Au, Zn, and Cd). The coordination geometry about the metal centers is shown to dictate the architecture adopted. X-ray crystallographic studies confirm that square planar coordination geometries result in "cagelike" octanuclear complexes, whereas square-based pyramidal metal geometries favor hexanuclear "molecular loop" structures. Both classes of complex are sterically and electronically complementary to the fullerenes (C(60) and C(70)). The strong binding of these guests occurred via favorable interactions with the sulfur atoms of multiple dithiocarbamate moieties of the hosts. In the case of the tetrameric copper(II) complexes, the lability of the copper(II)-dithiocarbamate bond enabled the fullerene guests to be encapsulated in the electron-rich cavity of the host, over time. The examination of the binding of fullerenes has been undertaken using spectroscopic and electrochemical methods, electrospray mass spectrometry, and molecular modeling.     

Novel hybrid vesicles co-assembled from a cationic lipid and PAAc-g-mPEG with pH-triggered transmembrane channels for controlled drug release

This work presents an important example of novel hybrid vesicles with pH-triggered transmembrane channels prepared by co-assembly of poly(acrylic acid)-g-poly(monomethoxy ethylene glycol) (PAAc-g-mPEG) with a cationic lipid, didodecyldimethylammonium bromide (DDAB), via electrostatic interaction for effective doxorubicin (DOX) release.     

Multiple conformations of benzil in resorcinarene-based supramolecular host matrixes

Six supramolecular complexes incorporating benzil as a guest, CMCR*bipy*benzil (alpha) 1 (CMCR = C-methylcalix[4]resorcinarene), CMCR*bipy*benzil (beta) 2, CMCR*2bpe*benzil*ethanol 3 (bpe = trans-1,4-bis(pyridyl)ethylene), CMCR*2bpe*benzil*2H2O 4, CMCR.2bpeh*benzil*ethanol 5 (bpeh = bis-(1-pyridin-4-yl-ethylidene)-hydrazine), and CECR*2bpe.benzil 6 (CECR = C-ethylcalix[4]resorcinarene), have been synthesized by hydrothermal and conventional methods and characterized by X-ray diffraction. Resorcinarene adopts a boat conformation in 1-4 and a bowl conformation in 5 and 6. Compounds 1-4 show a brick-wall-like framework, in which two benzil molecules are incorporated. For 5, bpeh spacers link CMCR molecules to give a one-dimensional wavelike polymer in which one benzil guest is embedded within the polymer cavity. Complex 6 forms a carcerand-like capsule in which two benzil guests are encapsulated. The O=C-C=O torsion angles vary from 91.8 to 139.3 degrees and correlate with the length of the central C-C bond. The benzil concentration, which is approximately 6.2 mol/L in the neat crystals, varies between 1.01 and 1.51 mol/L in the structures studied, corresponding to a 6-fold dilution. The benzil molecules are disordered in the larger cavities of 4 and 5. The two benzoyl fragments are almost perpendicular in 3, which has the next largest cavity size when solvent volume is excluded, whereas a nearly trans-coplanar conformation occurs for the cavity with the smallest volume in 6.     

Morphology transformation via pH-triggered self-assembly of peptides

Three flexible peptides (P1: (C(17)H(35)CO-NH-GRGDG)(2)KG; P2: (Fmoc-GRGDG)(2)KG; P3: (CH(3)CO-NH-GRGDG)(2)KG) self-assembled to form a variety of morphologically distinct assemblies at different pHs. P1 formed nanofibers at pH 3, then self-assembled into nanospheres with pH up to 6 and further changed to lamellar structures when the pH value was further increased to 10. P2 aggregated into an entwined network structure at pH 3, and then self-assembled into well-defined nanospheres, lamellar structures, and vesicles via adjusting the pH value. However, P3 did not self-assemble into well-ordered nanostructures, presumably due to the absence of a large hydrophobic group. The varying self-assembly behaviors of the peptides at different pHs are attributed to molecular conformational changes. These self-assembled supramolecular materials might contribute to the development of new peptide-based biomaterials.     

A novel reversible pH-triggered release immobilized enzyme system

A novel immobilized enzyme system supported by poly(acrylic acid/N,N’-methylene-bisacryl-amide) hydrogel microspheres was prepared. This system exhibited characteristics of reversible pH-triggered release. The morphology, size, and chemical structure were examined through optical microscopy, particle size analyzer, and Fourier transform infrared spectrometer. Immobilization and release features were further investigated under different conditions, including pH, time, and microsphere quantity. Results showed the microspheres were regularly spherical with 3.8 ~ 6.6 μm diameter. Loading efficiencies of bovine serum albumin immobilized by gel entrapment and adsorption methods were 93.9% and 56.2%, respectively. The pH-triggered protein release of the system occurred when medium pH was above 6.0, while it was hardly detected when medium pH was below 6.0. Release efficiencies of entrapped and adsorbed protein were 6.38% and 95.0%, respectively. Hence, adsorption method was used to immobilize trypsin. Loading efficiency of 77.2% was achieved at pH 4.0 in 1 h. Release efficiency of 91.6% was obtained under optimum pH catalysis condition set at 8.0 and trypsin was free in solutions with retention activity of 63.3%. And 51.5% of released trypsin could be reloaded in 10 min. The results indicate this kind of immobilized enzyme system offers a promising alternative for enzyme recovery in biotechnology.     

Energy migration in novel pH-triggered self-assembled beta-sheet ribbons

Energy migration between tryptophan residues has been experimentally demonstrated in self-assembled peptide tapes. Each peptide contains 11 amino acids with a Trp at position 6. The peptide self-assembly is pH-sensitive and forms amphiphilic tapes, which further stack in ribbons (double tapes) and fibrils in water depending on the concentration. Fluorescence spectra, quenching, and anisotropy experiments showed that when the pH is lowered from 9 to 2, the peptide self-assembly buries the tryptophan in a hydrophobic and restricted environment in the interior of stable ribbons as expected on the basis of the peptide design. These fluorescence data support directly and for the first time the presence of such ribbons which are characterized by a highly packed and stable hydrophobic interior. In common with Trp in many proteins, fluorescence lifetimes are nonexponential, but the average lifetime is shorter at low pH, possibly due to quenching with neighboring Phe residues. Unexpectedly, time-resolved fluorescence anisotropy does not change significantly with self-assembly when in water. In highly viscous sucrose-water mixtures, the anisotropy decay at low pH was largely unchanged compared to that in water, whereas at high pH, the anisotropy decay increased significantly. We concluded that depolarization at low pH was not due to rotational diffusion but mainly due to energy migration between adjacent tryptophan residues. This was supported by a master equation kinetic model of Trp-Trp energy migration, which showed that the simulated and experimental results are in good agreement, although on average only three Trp residues were visited before emission.     

pH-triggered assembly of organometallic receptors for lithium ions

The reaction of half-sandwich complexes of ruthenium, rhodium, and iridium with amino-substituted 3-hydroxy-2-pyridone ligands in aqueous solution gives monomeric O,O'-chelate complexes. Upon addition of base, the complexes assemble to form trimeric metallamacrocycles, as evidenced by NMR spectroscopy and single-crystal X-ray analyses. The macrocycles are able to act as highly selective receptors for lithium ions. The binding constants depend on the nature of the half-sandwich complex, the ligand, and the pH. With a commercially available (cymene)Ru complex, a receptor with a Li+ binding constant of K(a) = 5.8 (+/-1.0) x 10(4) M(-1) and a Li+-Na+ selectivity of 10 000:1 can be obtained. The fact that the assembly process of the receptor is pH-dependent can be used to detect the presence of lithium ions by a pH measurement. Furthermore, it is possible to transduce the binding of Li+ into a change of color by means of a chemical reaction with FeCl(3). This allows the detection of Li+ in the pharmacologically relevant concentration range of 0.5-1.5 mM by the "naked eye".     

New fluorescent amide-functionalized phenylethynylthiophene low molecular weight gelator

[reaction: see text]. An amide-functionalized phenylethynylthiophene gelator has been synthesized. Self-assembly of this molecule via cooperative hydrogen bonding and pi-stacking induced gelation of a variety of organic solvents. The presence of aggregates was confirmed by concentration-dependent absorption and fluorescence properties. SEM and TEM studies reveal the formation of fiberlike nanostructure.     

A physical gel made from hyperbranched polymer gelator

A novel hyperbranched polymer gelator has been synthesized, which can self-assemble into the thermoreversible physical gel in DMF, DMAC, pyridine, DMSO or NMP with the driving force of hydrogen bonds among amide and amine groups of the highly branched macromolecules.     

Fluorescent 'two-faced' polymer wafers with embedded pyrene-functionalised gelator nanofibres

Pyrene-functionalised gelators self-assemble into nano-fibrillar organogels in DMSO/styrene/divinylbenzene mixtures, which when polymerised yield polymer wafers with two distinct faces, only one of which is fluorescent and has embedded gelator nanofibres.     

Attogram sensing of trinitrotoluene with a self-assembled molecular gelator

Detection of explosives is of utmost importance due to the threat to human security as a result of illegal transport and terrorist activities. Trinitrotoluene (TNT) is a widely used explosive in landmines and military operations that contaminates the environment and groundwater, posing a threat to human health. Achieving the detection of explosives at a sub-femtogram level using a molecular sensor is a challenge. Herein we demonstrate that a fluorescent organogelator exhibits superior detection capability for TNT in the gel form when compared to that in the solution state. The gel when coated on disposable paper strips detects TNT at a record attogram (ag, 10(-18) g) level (～12 ag/cm(2)) with a detection limit of 0.23 ppq. This is a simple and low-cost method for the detection of TNT on surfaces or in aqueous solutions in a contact mode, taking advantage of the unique molecular packing of an organogelator and the associated photophysical properties.