Ultrastable Host–Guest Complexes and Their Applications

This review describes monovalent synthetic receptor–ligand (or host–guest) pairs with extremely high binding affinity, comparable to that of the biotin–avidin pair, and their applications. Cucurbit[7]uril (CB[7]), a member of the host family cucurbit[n]uril (CB[n], n=5–8, 10), forms ultrastable host–guest complexes with ferrocene-, adamantane- or bicyclo[2.2.2]octane-based molecules having ammonium groups properly positioned to interact with the carbonyl oxygens at the portals of CB[7]. The extremely high affinity is achieved by a large enthalpic gain arising from the near perfect size/shape complementarity between the rigid CB cavity and the rigid core of the guest molecules, with the critical assistance of the positive entropy change due to the extensive dehydration of the host and guest. The high stability of the complexes allowed us and others to explore several biological applications such as immobilization of biomolecules on a solid surface, protein isolation, triggering intracellular events, and regulating enzymatic activities. These complexes with their exceptional affinity, chemical robustness, simple preparation, biocompatibility, and easy handling may replace the biotin–(strept)avidin system in diverse areas of research, including affinity chromatography, high throughput biochemical assays, imaging, and sensor technologies.     

Small Molecule Crystallography in the Laboratory of Organic Chemistry at ETH Zürich

The development of small-molecule crystallography in the Laboratory of Organic Chemistry (LOC) at ETH since its foundation by Jack Dunitz in 1957 is reported. We briefly review the scientific path of Dunitz before coming to ETH and subsequently illustrate research highlights until his retirement in 1990. Together with his doctoral students and postdoctoral fellows, Dunitz developed X-ray crystallography as a tool for studying chemical reactivity and reaction mechanisms, stereochemical, and in particular, conformational analysis, and supramolecular interactions. He hosted numerous leaders in the field in Zurich. After retirement, Dunitz remained highly research-active. The crystallography facilities were run in 1991–2013 by his former coworkers Bernd Schweizer and Paul Seiler. In 2013, the Small Molecule Crystallography Center (SMoCC) was founded by merging the X-ray facilities in inorganic and organic chemistry and its research-driven service is illustrated.     

Terminal Protection of Small Molecule-Linked DNA for Small Molecule–Protein Interaction Assays

Methods for the detection of specific interactions between diverse proteins and various small-molecule ligands are of significant importance in understanding the mechanisms of many critical physiological processes of organisms. The techniques also represent a major avenue to drug screening, molecular diagnostics, and public safety monitoring. Terminal protection assay of small molecule-linked DNA is a demonstrated novel methodology which has exhibited great potential for the development of simple, sensitive, specific and high-throughput methods for the detection of small molecule–protein interactions. Herein, we review the basic principle of terminal protection assay, the development of associated methods, and the signal amplification strategies adopted for performance improving in small molecule–protein interaction assay.     

Atomic Scale Observations of the Chemistry at the Metal–Oxide Interface in Heterogeneous Catalysts

Here we present a preliminary analysis of the atomic scale interface chemistry in a model heterogeneous catalyst system, Pt/SiO₂. We show that the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) exhibits a sensitivity to surface oxidation and changes in the interfacial chemistry that is higher than that of conventional analytical methods such as energy-dispersive X-ray spectroscopy (EDS), chemisorption, and X-ray absorption near edge structure analysis (XANES). In particular, the presence of a few monolayers of platinum oxide can be clearly seen, and changes in the chemistry of the SiO₂ support within ∼1 nm of the metal–oxide interface can be characterized as a function of the catalyst preparation conditions. These results demonstrate that this combination of novel techniques can provide unparalleled information that is potentially the key to understanding the activity and selectivity of heterogeneous catalyst systems.     

Formation of Supramolecular Gels from Host–Guest Interactions between PEGylated Chitosan and α-Cyclodextrin

Chitosan-based hydrogels are prepared via the formation of polypseudorotaxanes (PPR), by selectively threading α-cyclodextrin (α-CD) macrocycles onto polymeric chains, which, through the formation of microcrystalline domains, act as junction points for the network. Specifically, host–guest inclusion complexes are formed between α-CD and PEGylated chitosan (PEG-Ch), resulting in the formation of supramolecular gels. PEG-grafted chitosan is obtained with a reaction yield of 79.8%, a high degree of grafting (50.9% GW) and water solubility (≈16 mg mL⁻¹), as assessed by turbidimetry. A range of compositions for mixtures of PEG-Ch solutions (0.2–0.8% w/w) and α-CD solutions (2−12% w/w, or 0.04–0.2% mol) are studied. Regardless of PEG content, gels are not formed at low α-CD concentrations (<4%). Dynamic rheology measurements reveal stiff gels (G’ above 15k) and a narrow linear viscoelastic region, reflecting their brittleness. The highest elastic modulus is obtained for a hydrogel composition of 0.4% PEG-Ch and 6% α-CD. Steady-state measurements, cycling between low and high shear rates, confirm the thixotropic nature of the gels, demonstrating their capacity to fully recover their mechanical properties after being exposed to high stress, making them good candidates to use as in-situ gel-forming materials for drug delivery to topical or parenteral sites.     

Construction and Properties of Double-Crosslinked Hydrogels Based on Host–Guest Interactions/UV Polymerization

Supramolecular hydrogels based on host–guest interactions have inherent flaw that the host molecules easily slide on or fall off the linear guest molecules, causing collapse of the networks. Hence, a double-crosslinking strategy is introduced in this study. The primary crosslinking formed via host–guest interactions between α-cyclodextrin (α-CD) and poly(ethylene glycol) dimethacrylate (PEGDMA) or between α-CD and four-arm poly(ethylene glycol) methacrylate (4arm PEG-MA). Then, secondary networks among PEGDMA or 4arm PEG-MA formed via UV-induced crosslinking. Results show that the fracture stress and fracture strain of PEGDMA-α-CD double-crosslinked hydrogels (P-C-U) increases up to 0.63 MPa and 71%, respectively, which significantly affected by molecular weight of PEGDMA. The double-crosslinking strategy helps increase the toughness up to 12.9 MJ m⁻³ (P_6k-0.025M-C-U) and 17.23 MJ m⁻³ (4P_10k-0.025M-C-U), as well as impart a certain degree of fatigue resistance to both PEGDMA hydrogels and 4arm-PEG-MA hydrogels, which is believed to be due to the energy dissipation mechanism introduced in the structure. The swelling capacity of double-crosslinked hydrogels is decreased compared to that with single-UV-crosslinked hydrogels, may be because the double-crosslinking strategy increases the crosslinking density of the hydrogel structure. In addition, both the molecular weight and concentration of PEGDMA and 4arm-PEG-MA influences the swelling capacity of the double-crosslinked hydrogels.     

Deep Inside Cucurbiturils: Physical Properties and Volumes of their Inner Cavity Determine the Hydrophobic Driving Force for Host–Guest Complexation

Cucurbit[n]urils (CBn) bind guest molecules through a combination of electrostatic interactions with the carbonyl rims and hydrophobic interactions with the inner cavity. Investigations with solvatochromic probes in CB7 reveal that the polarity of the cavity resembles that of alcohols (e.g., n-octanol), while its polarizability (P=0.12) and apparent refractive index (n_D=1.10±0.12) are extremely low, close to the gas phase. The calculated molecular quadrupole moments of CBs are extremely large (Θ_zz=−120 to −340 Buckingham). A survey of reported binding constants of neutral guests and hydrophobic residues that form 1 : 1 inclusion complexes with CB6, reveals a preferential inclusion of C3–C5 residues in its cavity. The largest guests which show non-negligible binding contain 7 heavy atoms (excluding hydrogen). For CB7, the strongest binding is observed for guests with adamantyl (10 heavy atoms) and ferrocenyl groups (11 heavy atoms), while the largest guests known to be complexed are carborane and the adduct of two pyridine derivatives (12 heavy atoms). The evaluation of different volumes shows that the most meaningful cavity, namely that responsible for binding of hydrophobic residues, is confined by the planes through the oxygen carbonyls. The volume of this inner cavity follows the formula V/ų=68+62(n−5)+12.5(n−5)², affording representative cavity volumes of 68 ų for CB5, 142 ų for CB6, 242 ų for CB7, and 367 ų for CB8. The volume of the 2 bond dipole regions is comparably smaller, amounting, for example, to 2×35 ų for CB6. The analysis of packing coefficients for representative sets of known guests with clearly defined hydrophobic binding motifs reveals average values of 47 % for CB5, 58 % for CB6, 52 % for CB7, and 53 % for CB8, which are well in line with the preferred packing (“55 % solution”, see S. Mecozzi, J. Rebek, Chem. Eur. J. 1998 , 4, 1016–1022) in related supramolecular host–guest assemblies. The driving force for binding of hydrophobic guests and residues by CBs is interpreted in terms of the unimportance of dispersion interactions (owing to the low polarizability of their cavity) and the dominance of classical and nonclassical hydrophobic effects related to the removal of very-high-energy water molecules (2 for CB5, 4 for CB6, 8 for CB7, and 12 for CB8) from the cavity.     

Interfacial Dynamic Properties and Dilational Rheology of Sulfonate Gemini Surfactant and its Mixtures with Quaternary Ammonium Bromides at the Air–Water Interface

The dynamic interfacial properties and dilational rheology of gemini sulfonate surfactant (SGS) and its mixtures with quaternary ammonium bromides (DTAB, CTAB) at the air–water interface were investigated using drop shape analysis. Results suggest that the adsorption process of these surfactants is diffusion-controlled at dilute concentrations, whereas the adsorption mechanism gradually shifts to a mixed kinetic-diffusion control with increasing surfactant concentration. The mixed surfactant system possesses the best surface activity when the molar ratios of SGS/DTAB and SGS/CTAB mixtures are 9:10. The formation of catanionic complexes shields the electrostatic repulsion between surfactant molecules and lowers the electrostatic adsorption barrier. Therefore, SGS/DTAB and SGS/CTAB mixtures exhibit higher adsorption rates than either component alone. The effects of oscillating frequency and surfactant concentration on the surface dilational properties of SGS, DTAB, CTAB, SGS/DTAB, and SGS/CTAB mixtures were also determined. As the oscillating frequency increases, the dilational elasticity of these surfactants gradually increases. The dilational elasticity peaks at a certain concentration, which is less than the critical micelle concentration (CMC). Results show that the dilational elasticity of SGS/DTAB and SGS/CTAB mixtures is higher than that of either component, resulting from the formation of a denser monomolecular adsorption layer at the air–water interface. Our study provides a basis for understanding the interaction mechanism of catanionic surfactant mixtures containing Gemini surfactant at the air–water interface.     

Thermoresponsive Interplay of Water Insoluble Poly(2-alkyl-2-oxazoline)s Composition and Supramolecular Host–Guest Interactions

A series of water insoluble poly[(2-ethyl-2-oxazoline)-ran-(2-nonyl-2-oxazoline)] amphiphilic copolymers was synthesized and their solubility properties in the presence of different supramolecular host molecules were investigated. The resulting polymer-cavitand assemblies exhibited a thermoresponsive behavior that could be modulated by variation of the copolymer composition and length. Interestingly, the large number of hydrophobic nonyl units across the polymer chain induced the formation of kinetically-trapped nanoparticles in solution. These nanoparticles further agglomerate into larger aggregates at a temperature that is dependent on the polymer composition and the cavitand type and concentration. The present research expands the understanding on the supramolecular interactions between water insoluble copolymers and supramolecular host molecules.     

Historical Background of the Interface—Studies and Theories

This paper is a historical review of the many theories of the interface which were proposed during the period 1947 up to 1964. The evidence for and against these theories are discussed.     

A Novel Approach to Molecular Recognition Surface of Magnetic Nanoparticles Based on Host–Guest Effect

A novel route has been developed to prepared β-cyclodextrin (β-CD) functionalized magnetic nanoparticles (MNPs). The MNPs were first modified with monotosyl-poly(ethylene glycol) (PEG) silane and then tosyl units were displaced by amino-β-CD through the nucleophilic substitution reaction. The monotosyl-PEG silane was synthesized by modifying a PEG diol to form the corresponding monotosyl-PEG, followed by a reaction with 3-isocyanatopropyltriethoxysilane (IPTS). The success of the synthesis of the monotosyl-PEG silane was confirmed with ¹H NMR and Fourier transform infrared (FTIR) spectroscopy. The analysis of FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the immobilization of β-CD onto MNPs. Transmission electron microscopy (TEM) indicated that the β-CD functionalized MNPs were mostly present as individual nonclustered units in water. The number of β-CD molecules immobilized on each MNP was about 240 according to the thermogravimetric analysis (TGA) results. The as-prepared β-CD functionalized MNPs were used to detect dopamine with the assistance of a magnet.     

One-Pot Formation of a Metallosupramolecularly Assembled and Redox-Active Adlayer at the Solid–Liquid Interface

A redox-active adlayer consisting of cobalt ions and terpyridine ligands, 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), was prepared on a Au(111) surface by a stepwise coordination method. The obtained adlayer produced a well-defined, stable redox wave associated with cobalt ions coordinated to 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), as compared to a tetra-2-pyridinyl-pyrazine adlayer, for which no redox wave was observed. In situ scanning tunneling microscopy revealed a structural change in the redox-active adlayer consisting of 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine) and cobalt ions. It was found that the ability of cobalt ions to coordinate on Au(111) was clearly dependent on the chemical structure of the ligand, suggesting that ligand coordination with metal ions on the Au surface is determined by the molecular orientation and configuration of the ligand when the ligand is adsorbed on a Au substrate.     

Modeling and Analysis of the Irradiance of Incipient Curing Light Reaching the Adhesive–Workpiece Interface Within a LAAG Joint

Light Activated Adhesive Gripper (LAAG) workholding is a technology used to hold workpieces for manufacturing processes. This paper presents a modeling technique for predicting the irradiance of incipient curing light reaching the adhesive–workpiece interface. It also describes an experimental methodology that can be used to derive the model coefficients for any arbitrary selection of curing lamp, gripper pin and adhesive. Lastly it provides experimental evidence that optical transmission loss increases dramatically during photo-polymerization. This increase is most likely due to increased ray scattering due to shrinkage void formation and densification of the adhesive matrix.     

First Principles Study of the Aluminum–Cubic Boron Nitride Interface

A plane wave density functional methodology, with the local density approximation for the elemental constituents, was used to investigate the structure, bonding, and adhesion of atomic-scale interfaces between aluminum and cubic-boron nitride (c-BN). Two fully periodic interfaces, Al(110)–c-BN(110) and Al(001)–c-BN(110), were constructed for this purpose. Interfacial bonding, examined with contours of the charge density difference and electron localization function, was found to be stronger between Al–N pairs than Al–B pairs. The computed work of separation (?W_s?) values were 2.25 J/m² for Al(110)–c-BN(110) and 2.65 J/m² for Al(001)–c-BN(110). The higher adhesion in the latter interface is attributed to a higher planar density of interfacial Al atoms. The computed W_s values were compared with values from first principles calculations on other aluminum–ceramic interfaces. The possibility of adhesive transfer during tensile debonding was qualitatively investigated.     

Behavior of Anionic Surfactants and Short Chain Alcohols Mixtures in the Monolayer at the Water–Air Interface

Measurements of the surface tension of aqueous solution of mixtures of sodium dodecyl sulfate (SDDS) with methanol and ethanol in SDDS concentration range from 10⁻⁵ to 10⁻² M and mixtures of sodium hexadecyl sulfonate (SHS) with methanol and ethanol at SHS concentration from 10⁻⁵ to 8 × 10⁻⁴ M and for methanol and ethanol from 0 to 21.1 and, 11.97 M, respectively, were carried out at 293 K. Moreover, the surface tension of aqueous solution mixtures of SDDS with propanol in the concentration range from 0 to 6.67 M taken from the literature was also considered. The results obtained indicate that it is possible to describe the relationship between the surface tension and molar concentration or molar fraction of alcohol by Szyszkowski and Connors equations. However, the Fainerman and Miller equation allows us to predict the isotherm of the surfactant tension at constant anionic surfactants concentration at which their molecules are present in the solution in the monomeric form if the molar area of surfactants and alcohols can be determined. Based on the surface tension isotherms, the Gibbs surface excess of anionic surfactants and alcohols concentration at water–air interface was determined and then recalculated for Guggenheim-Adam surface excess concentration of these substrates, and next the molar fraction of alcohols and surfactants in the surface layer was determined. These molar fractions were discussed with regard to surfactant and alcohol standard free energy of adsorption at the water–air interface determined from Langmuir and Aronson and Rosen equations. The standard free energy of adsorption determined in these ways was compared to that deduced on the basis of pC₂₀ and Lifshitz van der Waals-components of the anionic surfactant and alcohol tails.     

Study of the Dynamic Modes of the Operation of Flowsheets for the Extractive Distillation of a Benzene–Cyclohexane–Toluene Mixture

Theoretical Foundations of Chemical Engineering - This work considers four extractive distillation flowsheets of a benzene–cyclohexane–toluene mixture with different structures: one of...     

An Oxidation System for Green Chemistry: The Oxidation of Alcohols and Hydrocarbons in the Presence of Metal–Polymer Complexes

Environmental and economic factors make the use of harmful oxidants increasingly unacceptable except on a small scale. Accordingly, we have investigated the use of dioxygen and hydrogen peroxide as oxidants. For this oxidation reaction of hydrocarbons, transition metal complexes and polymer-bound transition metal complexes were effective as catalysts. Earlier investigations indicated that the catalytic site was a bi- or multinuclear complex. Thus, several binuclear complexes of Cu and Mn [Eqs. (9)–(11)] were designed and their effectiveness in oxidizing phenols to biphenols and benzoquinones and in monooxygenase activity was demonstrated. In the oxidation of phenols, the system did not produce poly(phenylene oxide) since the intermediate phenoxy radical underwent C–C coupling to biphenol or underwent attack by hydroxyl radical to a hydroquinone that could be oxidized to a quinone. A reaction mechanism involving the binuclear complex for the oxidation is proposed.     

Chemistry,morphology and structural characteristics of synthetic Al–Ni and Al–Co-lizardites

Al–Ni and Al–Co lizardites have been synthesized at hydrothermal conditions. The two phases display different morphologies: thin, dominantly curved platelets (Al–Ni); and stacks of very thin platy particles (Al–Co). The X-ray diffraction and transmission/analytical electron microscopy studies have been used to make accurate distinctions among different structural types and chemical populations. Al–Ni bearing lizardite includes a 1M polytype and probably very subordinate amounts of chlorite. Al–Co bearing lizardite includes two 1M polytypes with different cell-parameters, one of these consisting of an apparently modulated structure. Chemically, Al–Ni- and specially Al–Co lizardite consist of a mixture of two populations with tetrahedral compositions ~ Si_1.8Al_0.2 and Si_2.0. In contrast with previously described Al-rich serpentines, our serpentines are characterized by an asymmetrical Al distribution, with ^VIAl on the order of 1.0 atom per formula unit.