Binding of Hydrophobic Guests in a Coordination Cage Cavity is Driven by Liberation of "High-Energy" Water

A Study on Auto-Catalysis and Product Inhibition: A Nucleophilic Aromatic Substitution Reaction Catalysed within the Cavity of an Octanuclear Coordination Cage

The ability of an octanuclear cubic coordination cage to catalyse a nucleophilic aromatic substitution reaction on a cavity-bound guest was studied with 2,4-dinitrofluorobenzene (DNFB) as the guest/substrate. It was found that DNFB undergoes a catalysed reaction with hydroxide ions within the cavity of the cubic cage (in aqueous buffer solution, pH 8.6). The rate enhancement of kcat/kuncat was determined to be 22, with cavity binding of the guest being required for catalysis to occur. The product, 2,4-dinitrophenolate (DNP), remained bound within the cavity due to electrostatic stabilisation and exerts two apparently contradictory effects: it initially auto-catalyses the reaction when present at low concentrations, but at higher concentrations inhibits catalysis when a pair of DNP guests block the cavity. When encapsulated, the UV/Vis absorption spectrum of DNP is red-shifted when compared to the spectrum of free DNP in aqueous solution. Further investigations using other aromatic guests determined that a similar red-shift on cavity binding also occurred for 4-nitrophenolate (4NP) at pH 8.6. The red-shift was used to determine the stoichiometry of guest binding of DNP and 4NP within the cage cavity, which was confirmed by structural analysis with X-ray crystallography; and was also used to perform catalytic kinetic studies in the solution-state.

Free Energy Density of a Fluid and Its Role in Solvation and Binding

The concept that a fluid has a position-dependent free energy density appears in the literature but has not been fully developed or accepted. We set this concept on an unambiguous theoretical footing via the following strategy. First, we set forth four desiderata that should be satisfied by any definition of the position-dependent free energy density, f(R), in a system comprising only a fluid and a rigid solute: its volume integral, plus the fixed internal energy of the solute, should be the system free energy; it deviates from its bulk value, fbulk, near a solute but should asymptotically approach fbulk with increasing distance from the solute; it should go to zero where the solvent density goes to zero; and it should be well-defined in the most general case of a fluid made up of flexible molecules with an arbitrary interaction potential. Second, we use statistical thermodynamics to formulate a definition of the free energy density that satisfies these desiderata. Third, we show how any free energy density satisfying the desiderata may be used to analyze molecular processes in solution. In particular, because the spatial integral of f(R) equals the free energy of the system, it can be used to compute free energy changes that result from the rearrangement of solutes as well as the forces exerted on the solutes by the solvent. This enables the use of a thermodynamic analysis of water in protein binding sites to inform ligand design. Finally, we discuss related literature and address published concerns regarding the thermodynamic plausibility of a position-dependent free energy density. The theory presented here has applications in theoretical and computational chemistry and may be further generalizable beyond fluids, such as to solids and macromolecules.

The preservation of sarin and O,O'-diisopropyl fluorophosphate inside coordination cage hosts

The host-guest chemistry of O,O'-diisopropyl fluorophosphate (DFP), a phosphonofluoridate G-series chemical warfare agent simulant, was investigated in the presence of a number of octanuclear cubic coordination cage hosts. The aim was to demonstrate cage-catalysed hydrolysis of DFP at near neutral pH: however, two octanuclear coordination cages, HPEG (containing water-solubilising PEG groups) and HW (containing water-solubilising hydroxymethyl groups), were actually found to increase the lifetime of DFP in aqueous buffer solution (pH 8.7). Crystallographic analysis of DFP with a structurally related host cage revealed that DFP binds to windows in the cage surface, not in the internal cavity. The phosphorus-fluorine bond is directed into the cavity rather than towards the external environment, with the cage/DFP association protecting DFP from hydrolysis. Initial studies with the chemical warfare agent (CWA) sarin (GB) with HPEG cage in a buffered solution also showed a drastically reduced rate of hydrolysis for sarin when bound in the host cage. The ability of these cages to inhibit hydrolysis of these P-F bond containing organophosphorus guests, by encapsulation, may have applications in forensic sample preservation and analysis.

Disentangling contributions to guest binding inside a coordination cage host: analysis of a set of isomeric guests with differing polarities

Binding of a set of three isomeric guests (1,2-, 1,3- and 1,4-dicyanobenzene, abbreviated DCB) inside an octanuclear cubic coordination cage host H (bearing different external substitutents according to solvent used) has been studied in water/dmso (98 : 2) and CD₂Cl₂. These guests have essentially identical molecular surfaces, volumes and external functional groups to interact with the cage interior surface; but they differ in polarity with dipole moments of ca. 7, 4 and 0 Debye respectively. In CD₂Cl₂ guest binding is weak but we observe a clear correlation of binding free energy with guest polarity, with 1,4-DCB showing no detectable binding by NMR spectroscopy but 1,2-DCB having −ΔG = 9 kJ mol⁻¹. In water (containing 2% dmso to solubilise the guests) we see the same trend but all binding free energies are much higher due to an additional hydrophobic contribution to binding, with −ΔG varying from 16 kJ mol⁻¹ for 1,4-DCB to 22 kJ mol⁻¹ for 1,4-DCB: again we see an increase associated with guest polarity but the increase in −ΔG per Debye of dipole moment is around half what we observe in CD₂Cl₂ which we ascribe to the fact the more polar guests will be better solvated in the aqueous solvent. A van't Hoff analysis by variable-temperature NMR showed that the improvement in guest binding in water/dmso is entropy-driven, which suggests that the key factor is not direct electrostatic interactions between a polar guest and the cage surface, but the variation in guest desolvation across the series, with the more polar (and hence more highly solvated) guests having a greater favourable entropy change on desolvation.

The three dicyanobenzene isomers have obvious similarities but differ in their dipole moment: effects on binding in a coordination cage host in different solvents are discussed.

Highly-fluorescent BODIPY-functionalised metallacages as drug delivery systems: synthesis, characterisation and cellular accumulation studies

With the aim of designing new metallosupramolecular architectures for drug delivery, research has focused on porous 3-dimensional (3D)-metallacages able to encapsulate cytotoxic agents protecting them from metabolism while targeting them to cancer sites. Here, two self-assembled [Pd₂L₄]⁴⁺ cages (CG1 and CG2) featuring 3,5-bis(3-ethynylpyridine)phenyl ligands (L) exo-functionalised with dipyrromethene (BODIPY) groups have been synthesised and characterised by different methods, including NMR spectroscopy and mass spectrometry. ¹H NMR spectroscopy studies shows that the cages are able to encapsulate the anticancer drug cisplatin in their hydrophobic cavity, as evidenced by electrostatic potential (ESP) analysis based on XRD studies. The stability of the cages in an aqueous environment, and in the presence of the intracellular reducing agent glutathione, has been confirmed by UV-visible absorption spectroscopy. The luminescence properties of the cages enabled the investigation of their cellular uptake and intracellular localisation in human cancer cells by confocal laser scanning microscopy. In melanoma A375 cells, cage CG1 is taken up via active transport and endocytic trafficking studies show little evidence of transport through the early endosome while the cages accumulated in melanosomes rather than lysosomes. The antiproliferative activity of the lead cage was investigated in A375 together with two breast cancer cell lines, SK-BR-3 and MCF7. While the cage per se is non-cytotoxic, very different antiproliferative effects with respect to free cisplatin were evidenced for the [(cisplatin)₂⊂CG1·BF4] complex in the various cell lines, which correlate with its different intracellular localisation profiles. The obtained preliminary results provide a new hypothesis on how the subcellular localisation of the cage affects the cisplatin intracellular release.

Macrocyclic versus open-chain carbazole receptors for carboxylate binding

Anion recognition properties of six synthetic acyclic and macrocyclic carbazole-based receptors have been studied by 1H-NMR as well as with COSMO-RS calculations towards acetate, benzoate, lactate, sorbate and formate. The...

Interaction of anions with the surface of a coordination cage in aqueous solution probed by their effect on a cage-catalysed Kemp elimination

An octanuclear M8L12 coordination cage catalyses the Kemp elimination reaction of 5-nitro-1,2-benzisoxazole (NBI) with hydroxide to give 2-cyano-4-nitrophenolate (CNP) as the product. In contrast to the previously-reported very efficient catalysis of the Kemp elimination reaction of unsubstituted benzisoxazole, which involves the substrate binding inside the cage cavity, the catalysed reaction of NBI with hydroxide is slower and occurs at the external surface of the cage, even though NBI can bind inside the cage cavity. The rate of the catalysed reaction is sensitive to the presence of added anions, which bind to the 16+ cage surface, displacing the hydroxide ions from around the cage which are essential reaction partners in the Kemp elimination. Thus we can observe different binding affinities of anions to the surface of the cationic cage in aqueous solution by the extent to which they displace hydroxide and thereby inhibit the catalysed Kemp elimination and slow down the appearance of CNP. For anions with a -1 charge the observed affinity order for binding to the cage surface is consistent with their ease of desolvation and their ordering in the Hofmeister series. With anions that are significantly basic (fluoride, hydrogen carbonate, carboxylates) the accumulation of the anion around the cage surface accelerates the Kemp elimination compared to the background reaction with hydroxide, which we ascribe to the ability of these anions to participate directly in the Kemp elimination. This work provides valuable mechanistic insights into the role of the cage in co-locating the substrate and the anionic reaction partners in a cage-catalysed reaction.

A Family of Externally-Functionalised Coordination Cages

New synthetic routes are presented to derivatives of a (known) M8L12 cubic coordination cage in which a range of different substituents are attached at the C4 position of the pyridyl rings at either end of the bis(pyrazolyl-pyridine) bridging ligands. The substituents are (i) –CN groups (new ligand LCN), (ii) –CH2OCH2–CCH (containing a terminal alkyne) groups (new ligand LCC); and (iii) –(CH2OCH2)3CH2OMe (tri-ethyleneglycol monomethyl ether) groups (new ligand LPEG). The resulting functionalised ligands combine with M2+ ions (particularly Co2+, Ni2+, Cd2+) to give isostructural [M8L12]16+ cage cores bearing 24 external functional groups; the cages based on LCN (with M2+ = Cd2+) and LCC (with M2+ = Ni2+) have been crystallographically characterised. The value of these is twofold: (i) exterior nitrile or alkene substituents can provide a basis for further synthetic opportunities via ‘Click’ reactions allowing in principle a diverse range of functionalisation of the cage exterior surface; (ii) the exterior –(CH2OCH2)3CH2OMe groups substantially increase cage solubility in both water and in organic solvents, allowing binding constants of cavity-binding guests to be measured under an increased range of conditions.

Orthogonal binding and displacement of different guest types using a coordination cage host with cavity-based and surface-based binding sites

The octanuclear Co(ii) cubic coordination cage system H (or HW if it bears external water-solubilising substituents) has two types of binding site for guests. These are (i) the partially-enclosed central cavity where neutral hydrophobic organic species can bind, and (ii) the six 'portals' in the centres of each of the faces of the cubic cage where anions bind via formation of a network of CH⋯X hydrogen bonds between the anion and CH units on the positively-charged cage surface, as demonstrated by a set of crystal structures. The near-orthogonality of these guest binding modes provides the basis for an unusual dual-probe fluorescence displacement assay in which either a cavity-bound fluorophore (4-methyl-7-amino-coumarin, MAC; λ_em = 440 nm), or a surface-bound anionic fluorophore (fluorescein, FLU; λ_em = 515 nm), is displaced and has its emission ‘switched on’ according to whether the analyte under investigation is cavity-binding, surface binding, or a combination of both. A completely orthogonal system is demonstrated based using a Hw/MAC/FLU combination: addition of the anionic analyte ascorbate displaced solely FLU from the cage surface, increasing the 515 nm (green) emission component, whereas addition of a neutral hydrophobic guest such as cyclooctanone displaced solely MAC from the cage central cavity, increasing the 440 nm (blue) emission component. Addition of chloride results in some release of both components, and an intermediate colour change, as chloride is a rare example of a guest that shows both surface-binding and cavity-binding behaviour. Thus we have a colourimetric response based on differing contributions from blue and green emission components in which the specific colour change signals the binding mode of the analyte. Addition of a fixed red emission component from the complex [Ru(bipy)₃]²⁺ (Ru) provides a baseline colour shift of the overall colour of the luminescence closer to neutral, meaning that different types of guest binding result in different colour changes which are easily distinguishable by eye.

Orthogonal binding of neutral or anionic fluorophores to the cavity or surface, respectively, of a coordination cage host allows a dual-probe displacement assay which gives a different fluorescence colorimetric response according to where analyte species bind.

Non-spherical micro- and nanoparticles for drug delivery: Progress over 15 years

Shape of particulate drug carries has been identified as a key parameter in determining their biological outcome. In this review, we analyze the field of particle shape as it shifts from fundamental investigations to contemporary applications for disease treatment, while highlighting outstanding remaining questions. We summarize fabrication and characterization methods and discuss in depth how particle shape influences biological interactions with cells, transport in the vasculature, targeting in the body, and modulation of the immune response. As the field moves from discoveries to applications, further attention needs to be paid to factors such as characterization and quality control, selection of model organisms, and disease models. Taken together, these aspects will provide a conceptual foundation for designing future non-spherical drug carriers to overcome biological barriers and improve therapeutic efficacy.

Structural Flexibility in Metal-Organic Cages

Metal-organic cages (MOCs) have emerged as a diverse class of molecular hosts with potential utility across a vast spectrum of applications. With advances in single-crystal X-ray diffraction and economic methods of computational structure optimisation, cavity sizes can be readily determined. In combination with a chemist's intuition, educated guesses about the likelihood of particular guests being bound within these porous structures can be made. Whilst practically very useful, simple rules-of-thumb, such as Rebek's 55% rule, fail to take into account structural flexibility inherent to MOCs that can allow hosts to significantly adapt their internal cavity. An often unappreciated facet of MOC structures is that, even though relatively rigid building blocks may be employed, conformational freedom can enable large structural changes. If it could be exploited, this flexibility might lead to behavior analogous to the induced-fit of substrates within the active sites of enzymes. To this end, in-roads have already been made to prepare MOCs incorporating ligands with large degrees of conformational freedom. Whilst this may make the constitution of MOCs harder to predict, it has the potential to lead to highly sophisticated and functional synthetic hosts.

Hierarchically Porous Organic Cages

Imparting mesopores to organic cages of an intrinsic microporous nature to build up hierarchically porous cage soft materials is a grand challenge and will reshape the property and application scope of traditional organic cage molecules. Herein, we discovered how to engineer mesopores into microporous organic cages via their host-guest interactions with long chain ionic surfactants. Equally important, the ionic head of surfactants equips the supramolecularly assembled porous structures with charge-selective uptake and release function in solution. Interestingly, such hierarchically porous organic cage can serve as a nanoreactor once trapping enzymes within the cavity, which show 5-fold enhanced activity of enzymatic catalysis when compared with the free enzymes.

Outside the box: quantifying interactions of anions with the exterior surface of a cationic coordination cage

We describe a study of the binding of anions to the surface of an octanuclear coordination cage HW, which carries a 16+ charge, in aqueous solution. Anionic aromatic fluorophores such as fluorescein (and derivatives) and hydroxypyrene tris-sulfonate (HPTS) bind strongly to an extent depending on their charge and hydrophobicity. Job plots indicated binding of up to six such fluorescent anions to HW, implying that one anion can bind to each face of the cubic cage, as previously demonstrated crystallographically with small anions such as halides. The quenching of these fluorophores on association with the cage provides the basis of a fluorescence displacement assay to investigate binding of other anions: addition of analyte (organic or inorganic) anions in titration experiments to an HW/fluorescein combination results in displacement and restoration of the fluorescence from the bound fluorescein, allowing calculation of 1 : 1 binding constants for the HW/anion combinations. Relative binding affinities of simple anions for the cage surface can be approximately rationalised on the basis of ease of desolvation (e.g. F- < Cl- < Br-), electrostatic factors given the 16+ charge on the cage (monoanions < dianions), and extent of hydrophobic surface. The interaction of a di-anionic pH indicator (bromocresol purple) with HW results in a pKa shift, with the surface-bound di-anionic form stabilised by approximately 1 pKa unit compared to the non-bound neutral form due to the charge on the cage.

Molecular Recognition in Water Using Macrocyclic Synthetic Receptors

Molecular recognition in water using macrocyclic synthetic receptors constitutes a vibrant and timely research area of supramolecular chemistry. Pioneering examples on the topic date back to the 1980s. The investigated model systems and the results derived from them are key for furthering our understanding of the remarkable properties exhibited by proteins: high binding affinity, superior binding selectivity, and extreme catalytic performance. Dissecting the different effects contributing to the proteins' properties is severely limited owing to its complex nature. Molecular recognition in water is also involved in other appreciated areas such as self-assembly, drug discovery, and supramolecular catalysis. The development of all these research areas entails a deep understanding of the molecular recognition events occurring in aqueous media. In this review, we cover the past three decades of molecular recognition studies of neutral and charged, polar and nonpolar organic substrates and ions using selected artificial receptors soluble in water. We briefly discuss the intermolecular forces involved in the reversible binding of the substrates, as well as the hydrophobic and Hofmeister effects operating in aqueous solution. We examine, from an interdisciplinary perspective, the design and development of effective water-soluble synthetic receptors based on cyclic, oligo-cyclic, and concave-shaped architectures. We also include selected examples of self-assembled water-soluble synthetic receptors. The catalytic performance of some of the presented receptors is also described. The latter process also deals with molecular recognition and energetic stabilization, but instead of binding ground-state species, the targets become elusive counterparts: transition states and other high-energy intermediates.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

An isolated water droplet in the aqueous solution of a supramolecular tetrahedral cage

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid-solid phase transitions, but with structural and dynamical signatures that map onto known regions of water's phase diagram. Using terahertz (THz) absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a dynamically distinct network of 9 ± 1 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the Ga₄L₆ ^12- host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water. A further inference is that the release of the highly unusual encapsulated water droplet creates a strong thermodynamic driver for the high-affinity binding of guests in aqueous solution for the Ga₄L₆ ^12- supramolecular construct.

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (1H, proton-coupled 13C, selective 1H-decoupling 13C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin-crossover complexes to supramolecular chemistry through the characterisation of CoII and high-spin FeII mononuclear complexes as well as a Co4L6 cage.

Characterization of Host-Guest Complexes of Supramolecular Self-Assembled Cages Using EPR Spectroscopy

Host-guest interactions between nitroxide stable radicals and supramolecular coordination cages were investigated using electron paramagnetic resonance (EPR) spectroscopy in water and acetonitrile. TEMPO showed negligible association with the cages in water, while 4-oxo-TEMPO bound with a strength comparable to that previously reported for related ketones. Carboxylic acid-functionalized nitroxides bound strongly to the acetonitrile-soluble coordination cages. In all cases, host-guest complex formation resulted in significant decreases in the molecular tumbling rate of the guests, with tumbling becoming strongly anisotropic. The polarity of the cage environment in both solvents was found to be intermediate between water and acetonitrile.

Interactions of Small-Molecule Guests with Interior and Exterior Surfaces of a Coordination Cage Host

Coordination cages are well-known to act as molecular containers that can bind small-molecule guests in their cavity. Such cavity binding is associated with interactions of the guests with the surrounding set of surfaces that define the cavity; a guest that is a good fit for the cavity will have many favourable interactions with the interior surfaces of the host. As cages have exterior as well as interior surfaces, possibilities also exist for ‘guests’ that are not well-bound in the cavity to interact with the exterior surface of the cage where spatial constraints are fewer. In this paper, we report a combined solid-state and solution study using an octanuclear cubic M8L12 coordination cage which illustrates the occurrence of both types of interaction. Firstly, crystallographic studies show that a range of guests bind inside the cavity (either singly or in stacked pairs) and/or interact with the cage exterior surface, depending on their size. Secondly, fluorescence titrations in aqueous solution show how some flexible aromatic disulfides show two separate types of interaction with the cage, having different spectroscopic consequences; we ascribe this to separate interactions with the exterior surface and the interior surface of the host cage with the former having a higher binding constant. Overall, it is clear that the idea of host/guest interactions in molecular containers needs to take more account of external surface interactions as well as the obvious cavity-based binding.

Discrete Supracrystalline Heterostructures from Integrative Assembly of Nanocrystals and Porous Organic Cages

Although self-assembly across multiple length scales has been well recognized and intensively investigated in natural biological system, the design of artificial heterostructures enabled by integrative self-assembly is still in its infancy. Here we report a strategy toward the growth of discrete supracrystalline heterostructures from inorganic nanocrystals and porous organic cages (CC3-R), which in principle relies on the host-guest interactions between alkyl chains coated on nanocrystals and the cavity of cage molecules. Density functional theory calculation indicates that an attractive energy of ∼-2 k_BT is present between an alkyl chain and the cavity of a CC3-R molecule, which is responsible for the assembly of nanocrystal superlattices on the CC3-R octahedral crystals. Of particular interest is that, determined by the shape of the nanocrystals, two distinct assembly modes can be controlled at the mesoscale level, which eventually produce either a core/shell or heterodimer supracrystalline structure. Our results highlight opportunities for the development of such a noncovalent integrative self-assembly not limited to a particular length scale and that could be generally applicable for flexible integration of supramolecular systems.

Catalysis of an Aldol Condensation Using a Coordination Cage

The aldol condensation of indane-1,3-dione (ID) to give ‘bindone’ in water is catalysed by an M8L12 cubic coordination cage (Hw). The absolute rate of reaction is slow under weakly acidic conditions (pH 3–4), but in the absence of a catalyst it is undetectable. In water, the binding constant of ID in the cavity of Hw is ca. 2.4 (±1.2) × 103 M−1, giving a ∆G for the binding of −19.3 (±1.2) kJ mol−1. The crystal structure of the complex revealed the presence of two molecules of the guest ID stacked inside the cavity, giving a packing coefficient of 74% as well as another molecule hydrogen-bonded to the cage’s exterior surface. We suggest that the catalysis occurs due to the stabilisation of the enolate anion of ID by the 16+ surface of the cage, which also attracts molecules of neutral ID to the surface because of its hydrophobicity. The cage, therefore, brings together neutral ID and its enolate anion via two different interactions to catalyse the reaction, which—as the control experiments show—occurs at the exterior surface of the cage and not inside the cage cavity.

Embedding alkenes within an icosahedral inorganic fullerene {(NH4)42[Mo132O372(L)30(H2O)72]} for trapping volatile organics

Eight alkene-functionalized molybdenum-based spherical Keplerate-type (inorganic fullerene) structures have been obtained via both direct and multistep synthetic approaches. Driven by the opportunity to design unique host-guest interactions within hydrophobic, π-electron rich confined environments, we have synthesised {(NH₄)₄₂[Mo₁₃₂O₃₇₂(L)₃₀(H₂O)₇₂]}, where L = (1) acrylic acid, (2) crotonic acid, (3) methacrylic acid, (4) tiglic acid, (5) 3-butenoic acid, (6) 4-pentenoic acid, (7) 5-hexenoic acid, and (8) sorbic acid. The compounds, which are obtained in good yield (10-40%), contain 30 carboxylate-coordinated alkene ligands which create a central cavity with hydrophobic character. Extensive Nuclear Magnetic Resonance (NMR) spectroscopy studies contribute significantly to the complete characterisation of the structures obtained, including both 1D and 2D measurements. In addition, single-crystal X-ray crystallography and subsequently-generated electron density maps are employed to highlight the distribution in ligand tail positions. These alkene-containing structures are shown to effectively encapsulate small alkyl thiols (1-propanethiol (A), 2-propanethiol (B), 1-butanethiol (C), 2-butanethiol (D) and 2-methyl-1-propanethiol (E)) as guests within the central cavity in aqueous solution. The hydrophobically driven clustering of up to 6 equivalents of volatile thiol guests within the central cavity of the Keplerate-type structure results in effective thermal protection, preventing evaporation at elevated temperatures (ΔT ≈ 25 K).

Neurotransmitter selection by monoamine oxidase isoforms, dissected in terms of functional groups by mixed double mutant cycles

Double mutant cycles were constructed using neurotransmitters and synthetic substrates that measure their selective binding to one monoamine oxidase (MAO) enzyme isoform over another as a function of structural change. This work measures a reduction in selectivity for the MAOB isoform of 3 to 9.5 kJ mol^-1 upon the addition of hydroxy functional groups to a phenethylamine scaffold. Replacement of hydroxy functional groups on the phenethylamine scaffold by hydrophobic substituents measures an increase in selectivity for MAOB of -1.1 to -6.9 kJ mol^-1. The strategies presented here can be applied to the development of competitive reversible inhibitors of MAO enzymes and other targets with structurally related isoforms.

Quantification of the hydrophobic effect using water-soluble super aryl-extended calix[4]pyrroles

We report the quantification of the hydrophobic effect using a model system based on water-soluble super aryl-extended calix[4]pyrrole receptors and a series of pyridyl N-oxide derivatives, bearing a non-polar para-substituent, as guests.

Efficient hydrogen bonding recognition in water using aryl-extended calix[4]pyrrole receptors

For a series of six-membered neutral polar guests, the cavity of 1oo provides a better hydrogen-bond donor environment than water.

We describe the synthesis of four water-soluble aryl-extended calix[4]pyrrole receptors and report their binding properties with multiple neutral polar guests in water. The prepared receptors present different functionalization at their upper rims and have in common the placement of water solubilizing pyridinium groups at their lower rims. We investigate the interaction of the receptors with a guest series in water solution using ¹H NMR titrations and ITC experiments. Despite the known competitive nature of water for hydrogen-bonding interactions, we demonstrate the formation of thermodynamically highly stable 1 : 1 inclusion complexes stabilized by hydrogen-bonding interactions. We show that increasing the hydrogen-bond acceptor character of the guest has a positive impact on binding affinity. This result suggests that the receptor's cavity is indeed a better hydrogen-bond donor to interact with the guests than water molecules. We also assess the important contribution of the hydrophobic effect to binding by comparing the binding affinities of analogous inclusion complexes in water and chloroform solutions. The more polar guests are bound with similar affinities in the two solvents. We compare the binding properties of the different complexes in order to derive general trends.

Induced-fit expansion and contraction of a self-assembled nanocube finely responding to neutral and anionic guests

Induced-fit or conformational selection is of profound significance in biological regulation. Biological receptors alter their conformation to respond to the shape and electrostatic surfaces of guest molecules. Here we report a water-soluble artificial molecular host that can sensitively respond to the size, shape, and charged state of guest molecules. The molecular host, i.e. nanocube, is an assembled structure consisting of six gear-shaped amphiphiles (GSAs). This nanocube can expand or contract its size upon the encapsulation of neutral and anionic guest molecules with a volume ranging from 74 to 535 Å3 by induced-fit. The responding property of this nanocube, reminiscent of a feature of biological molecules, arises from the fact that the GSAs in the nanocubes are connected to each other only through the hydrophobic effect and very weak intermolecular interactions such as van der Waals and cation-π interactions.

Cramming versus threading of long amphiphilic oligomers into a polyaromatic capsule

Oligo(ethylene oxide)s are known as widely useable yet not very interactive amphiphilic compounds. Here we report that the long amphiphilic oligomers are bound by a polyaromatic capsule in two different manners, depending on the chain length. For instance, the shorter pentamer is crammed into the isolated cavity of the capsule, whereas the longer decamer is threaded into the capsule to form a 1:1 host-guest complex in a pseudo-rotaxane fashion. These unusual bindings occur instantly, spontaneously, and quantitatively even in water at room temperature, with relatively high binding constants (K_a > 10⁶ M^-1). Isothermal titration calorimetry (ITC) studies reveal that enthalpic stabilization is a dominant driving force for both of the complexations through multiple host-guest CH-π and hydrogen-bonding interactions. Furthermore, long oligomers with an average molecular weight of 1000 Da (e.g., 22-mer) are also threaded into the capsules to give pseudo-rotaxane-shaped 2:1 host-guest complexes in water, selectively.

Coordination Cages Based on Bis(pyrazolylpyridine) Ligands: Structures, Dynamic Behavior, Guest Binding, and Catalysis

We describe here a family of coordination cages with interesting structural, guest-binding, and catalytic properties. Flexible bridging ligands containing two bidentate pyrazolylpyridine termini assemble with transition-metal dications to afford coordination cages containing a metal ion at each vertex, a bridging ligand spanning each edge, and a 2:3 metal:ligand ratio. This stoichiometry is expressed in structures ranging from M₄L₆ tetrahedra to M₁₆L₂₄ tetracapped truncated tetrahedra, which are stabilized by the formation of π-stacked arrays between electron-rich and electron-poor ligand segments that form around the cage periphery. In some cases concentration- and/or temperature-dependent equilibria between multiple cage structures occur, arising from a balance between entropy, which favors the formation of a larger number of smaller assemblies, and enthalpy, which maximizes both interligand aromatic stacking and solvophobic effects in the larger assembles. The cages are hollow and can accommodate guests-often anions or solvent molecules-in the central cavity. For one cage family, M₈L₁₂ species with an approximately cubic structure and a ca. 400 ų cavity, the guest binding properties have been studied extensively. This cage can accommodate a wide range of neutral organic guests, with binding in water being driven principally by the hydrophobic effect, which leads to binding constants of up to 10⁸ M^-1. The accumulation of a large amount of empirical data on guest binding in the M₈L₁₂ cage in water provided the basis for a predictive tool for in silico screening of potential guests using the molecular docking program GOLD; this methodology has allowed the identification of numerous new guests with accurately predicted binding constants and provides a transformative new approach to exploring the host/guest chemistry of cages. Binding of benzisoxazole inside the M₈L₁₂ cage results in substantial rate enhancements-by a factor of up to 2 × 10⁵-of the Kemp elimination, in which benzisoxazole reacts to give 2-cyanophenolate. Catalysis arises because the 16+ cage cation accumulates anions around the surface by ion pairing, leading to a high effective concentration of hydroxide ions surrounding the guest even when the bulk pH is modest. Thus, the catalysis relies on the operation of two orthogonal interactions that bring the reaction partners together: hydrophobic guest binding in the cavity, which is lined with CH groups from the ligands, and ion pairing around the highly cationic cage surface. A consequence of this is that under some conditions the product of the cage-catalyzed Kemp elimination (the 2-cyanophenolate anion) itself accumulates around the cage surface and deprotonates another benzisoxazole guest, perpetuating the reaction in an autocatalytic manner. Thus, different anions accumulating around the cage can act as partners for reaction with a cavity-bound guest, opening up the possibility that the M₈L₁₂ cage can act as a general catalyst for reactions of electrophilic guests with surface-bound anions.

Catalysis in a Cationic Coordination Cage Using a Cavity-Bound Guest and Surface-Bound Anions: Inhibition, Activation, and Autocatalysis

The Kemp elimination (reaction of benzisoxazole with base to give 2-cyanophenolate) is catalyzed in the cavity of a cubic M₈L₁₂ coordination cage because of a combination of (i) benzisoxazole binding in the cage cavity driven by the hydrophobic effect, and (ii) accumulation of hydroxide ions around the 16+ cage surface driven by ion-pairing. Here we show how reaction of the cavity-bound guest is modified by the presence of other anions which can also accumulate around the cage surface and displace hydroxide, inhibiting catalysis of the cage-based reaction. Addition of chloride or fluoride inhibits the reaction with hydroxide to the extent that a new autocatalytic pathway becomes apparent, resulting in a sigmoidal reaction profile. In this pathway the product 2-cyanophenolate itself accumulates around the cationic cage surface, acting as the base for the next reaction cycle. The affinity of different anions for the cage surface is therefore 2-cyanophenolate (generating autocatalysis) > chloride > fluoride (which both inhibit the reaction with hydroxide but cannot deprotonate the benzisoxazole guest) > hydroxide (default reaction pathway). The presence of this autocatalytic pathway demonstrates that a reaction of a cavity-bound guest can be induced with different anions around the cage surface in a controllable way; this was confirmed by adding different phenolates to the reaction, which accelerate the Kemp elimination to different extents depending on their basicity. This represents a significant step toward the goal of using the cage as a catalyst for bimolecular reactions between a cavity-bound guest and anions accumulated around the surface.

Binding of Hydrophobic Guests in a Coordination Cage Cavity is Driven by Liberation of "High-Energy" Water

The cavity of an M8 L12 cubic coordination cage can accommodate a cluster of ten water molecules in which the average number of hydrogen bonds per water molecule is 0.5 H-bonds less than it would be in the bulk solution. The presence of these "hydrogen-bond frustrated" or "high-energy" water molecules in the cavity results in the hydrophobic effect associated with guest binding being predominantly enthalpy-based, as these water molecules can improve their hydrogen-bonding environment on release. This contrasts with the classical form of the hydrophobic effect in which the favourable entropy change associated with release of ordered molecules from hydrophobic surfaces dominates. For several guests Van't Hoff plots showed that the free energy of binding in water is primarily enthalpy driven. For five homologous pairs of guests related by the presence or absence of a CH2 group, the incremental changes to ΔH and TΔS for guest binding-that is, ΔΔH and TΔΔS, the difference in contributions arising from the CH2 group-are consistently 5(±1) kJ mol-1 for ΔΔH and 0(±1) kJ mol-1 for TΔΔS. This systematic dominance of ΔH in the binding of hydrophobic guests is consistent with the view that guest binding is dominated by release of "high energy" water molecules into a more favourable solvation environment, as has been demonstrated recently for some members of the cucurbituril family.