Helical Folding of Alkanes in a Self-Assembled, Cylindrical Capsule

Investigation of the effects on proton relaxation times upon encapsulation in a water-soluble synthetic receptor

Sequestration of small molecule guests in the cavity of a water-soluble deep cavitand host has a variety of effects on their NMR properties. The effects of encapsulation on the longitudinal (T1) and transverse (T2) relaxation times of the protons in variably sized guest molecules are analyzed here, using inversion recovery and spin-echo experiments. Sequestration of neutral organic species from the bulk solvent reduces the overall proton relaxation times, but the magnitude of this effect on different protons in the same molecule has a variety of contributors, from the motion of the guest when bound, to the position of the protons in the cavity and the magnetic anisotropy induced by the aromatic walls of the host. These subtle effects can have large consequences on the environment experienced by the bound guest, and this sheds light on the nature of small molecules in enclosed environments.

Bending Enamine Patterns of Stabilized Pentalenes into "Polymethine Ylides"

We report the synthesis and structural study of 2-substituted 1,3-bis(dimethylamino)pentalenes. The two electrons donating substituents shift the formally anti-aromatic pattern toward more suitable polarized structures. A subtle steric trade-off can result either in planar cyclopentadienyl vinamidiniums or in distorted structures featuring a pyramidalized ylidic carbon adjacent to a stabilized π-conjugated iminium (polymethine). This latter pattern mimics a postulated activated distorted geometry for key Breslow intermediates in the active site of thiamine-dependent enzymes. It highlights how the energetic drive to avoid anti-aromaticity can be used to access models for unconventional distorted conformations of organic molecules.

A Twisted Chiral Cavitand with 5-Fold Symmetry and Its Length-Selective Binding Properties

Controlling bottom-up syntheses from chiral seeds to construct architectures with specific chiralities is currently challenging. Herein, a twisted chiral cavitand with 5-fold symmetry was constructed by bottom-up synthesis using corannulene as the chiral seed and pillar[5]arene as the chiral wall. After docking between the seed and the wall, their dynamic chiralities (M and P) are fixed. Moreover, the formed hedges also exhibit M and P chirality. Through dynamic covalent bonding, the thermodynamically stable product is obtained selectively as a pair of enantiomers (MMM and PPP), where all three subcomponents, i.e., the corannulene, hedges, and pillar[5]arene, are tilted in the same direction. Furthermore, the twisted cavitand exhibits length-selective binding to alkylene dibromides, with three maximum binding constants being unexpectedly observed.

Benzene, an Unexpected Binding Unit in Anion–π Recognition: The Critical Role of CH/π Interactions

We report high-level ab initio calculations (CCSD(T)(full)/CBS//SCS-RI-MP2(full)/aug-cc-pwCVTZ) that demonstrate the importance of cooperativity effects when Anion–π and CH/π interactions are simultaneously established with benzene as the π-system. In fact, most of the complexes exhibit high cooperativity energies that range from 17% to 25.3% of the total interaction energy, which is indicative of the strong influence of the CH/π on the Anion–π interaction and vice versa. Moreover, the symmetry-adapted perturbation theory (SAPT) partition scheme was used to study the different energy contributions to the interaction energies and to investigate the physical nature of the interplay between both interactions. Furthermore, the Atoms in Molecules (AIM) theory and the Non-Covalent Interaction (NCI) approach were used to analyze the two interactions further. Finally, a few examples from the Protein Data Bank (PDB) are shown. All results stress that the concurrent formation of both interactions may play an important role in biological systems due to the ubiquity of CH bonds, phenyl rings, and anions in biomolecules.

Environmental Modulation of Chiral Prolinamide Catalysts for Stereodivergent Conjugate Addition

Synthetic chiral catalysts generally rely on proximal functional groups or ligands for chiral induction. Enzymes often employ environmental chirality to achieve stereoselectivity. Environmentally controlled catalysis has benefits such as size and shape selectivity but is underexplored by chemists. We here report molecularly imprinted nanoparticles (MINPs) that utilized their environmental chirality to either augment or reverse the intrinsic selectivity of a chiral prolinamide cofactor. The latter ability allowed the catalyst to produce products otherwise disfavored in the conjugate addition of aldehyde to nitroalkene. The catalysis occurred in water at room temperature and afforded γ-nitroaldehydes with excellent yields (up to 94%) and ee (>90% in most cases). Up to 25:1 syn/anti and 1:6 syn/anti ratios were achieved through a combination of catalyst-derived and environmentally enabled selectivity. The high enantioselectivity of the MINP also made it possible for racemic catalysts to perform asymmetric catalysis, with up to 80% ee for the conjugate addition.

Size-dependent compression of threaded alkyldiphosphate in head to head cyclodextrin [3]pseudorotaxanes

The encapsulation of guests in a confined space enables unusual conformations and reactivities. In particular, the compression of akyl chains has been obtained by self-assembled molecular capsules but such an effect has not been reported in solution for pseudorotaxane architectures. By exploiting the tendency of cyclodextrin (CD) to form head to head [3]pseudorotaxanes and the hydrogen bonding abilities of phosphate groups, we have studied the effect of the CD dimer cavity on the conformation of threaded α,ω-alkyl-diphosphate axles. The formation of [2]pseudorotaxanes and [3]pseudorotaxanes was investigated by a combination of NMR, ITC and X-ray diffraction techniques. In the solid state, the [3]pseudorotaxane with a C₈ axle presents a fully extended conformation with both terminal phosphate groups interacting with hydroxyl groups of the primary rim of CDs. Such hydrogen bonding interactions are also present with the C₉ and C₁₀ axles resulting in a compression of the alkyl chain with gauche conformations in the solid state. NMR studies have shown that this effect is maintained in solution resulting in a size-dependent progressive compression of the alkyl chain by the CD [3]pseudorotaxane architecture for C₉, C₁₀ and C₁₁ axles.

Alkyl chain compression of alkanediphosphate guests was achieved by head-to-head cyclodextrin [3]pseudorotaxanes in a mechanostereoselective self-assembly process.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Cavitand Complexes in Aqueous Solution: Collaborative Experimental and Computational Studies of the Wetting, Assembly, and Function of Nanoscopic Bowls in Water

Water is the dominant liquid on Earth. Despite this, the main focus of supramolecular chemistry research has been on binding and assembly events in organic solvents. This arose because it is more straightforward to synthesize organic-media-soluble hosts and because of the relative simplicity of organic solvents compared to water. Nature, however, relies on water as a solvent, and spurred by this fact, supramolecular chemists have recently been making forays into the aqueous domain to understand water-mediated non-covalent interactions. These studies can benefit from the substantial understanding of the hydrophobic effect and electrostatic interactions developed by physical chemists. Nearly 20 years ago, the Gibb group first synthesized a class of water-soluble host molecules, the deep-cavity cavitands, that possess non-polar pockets that readily bind non-polar moieties in aqueous solution and are capable of assembling into a wide range of complexes with distinct stoichiometries. As such, these amphipathic host species are ideal platforms for studying the role of negatively curved features on guest complexation and the structural requirements for guided assembly processes driven by the hydrophobic effect. Here we review the collaborative experimental and computational investigations between Gibb and Ashbaugh over the past 10 years exploring questions including the following: How does water wet/solvate the non-polar surfaces of non-polar pockets? How does this wetting control the binding of non-polar guests? How does wetting affect the binding of anionic species? How does the nature and size of a guest size impact the assembly of cavitand hosts into multimeric capsular complexes? What are the conformational motifs of guests packed within the confines of capsular complexes? How might the electrostatic environment engendered by hosts impact the properties and reactivity of internalized guests?

Folding and Unfolding of Acetoxy Group-Terminated Alkyl Chains Inside a Size-Regulable Hemicarcerand

A resorcinarene-based hemicarcerand, which consists of two cavitands covalently linked to each other by four alkyl chains, allows structural expansion and contraction by demetalation and metalation of Cu(I) cations with a size change of approximately 12 Å. This metal-mediated switching of the two states regulates the conformations of acetoxy group-terminated alkyl chains. A guest binding study reveals the encapsulation of heptyl to undecyl chains in metal-free and Cu(I)-coordinated capsules. The chemical shifts of the acetoxy groups of the bound guests are the same in the metal-free capsule, while those in the Cu(I)-coordinated one differ from each other. This indicates that the metal-free capsule regulates its size to the bound guests, while the bound guests adopt their conformations to the cavity of the Cu(I)-coordinated capsules. ¹H NMR measurements and molecular mechanics calculations suggest that the bound guests have extended conformations in the metal-free capsule, while the Cu(I)-coordinated capsule forces the bound guests to adopt folded conformations. The presence of folded conformations is supported by the conformational study with structurally similar capsules and a nonsymmetric guest, allowing us to observe nuclear Overhauser effects stemming from the folded conformations of the guest in the cavity.

Selective alkane co-inclusion by methylene- and adamantane-bridged macrocycle with biphenyl units

A methylene- and adamantane-bridged macrocycle with biphenyl units showed selective co-inclusion of n-hexane among the hexane isomers through crystallization. In the use of n-pentane or n-heptane, inclusion crystals with alkanes were not formed.

Binding-Induced Unfolding of 1-Bromopropane in α-Cyclodextrin

Raman multivariate curve resolution vibrational spectroscopy and X-ray crystallography are used to quantify changes in the gauche-trans conformational equilibrium of 1-bromopropane (1-BP) upon binding to α-cyclodextrin (α-CD). Both conformers of 1-BP are found to bind to α-CD, although binding favors the unfolded trans conformation. Temperature-dependent measurements of the binding-induced change in the 1-BP conformation equilibrium constant indicate that the trans conformer is both enthalpically and entropically stabilized in the host cavity.

Pagoda[4]arene and i-Pagoda[4]arene

A new type of macrocyclic arenes, named pagoda[4]arene (P4) and i-pagoda[4]arene (i-P4), were conveniently synthesized by the TFA-catalyzed one-pot condensation of 2,6-dimethoxylanthracene and paraformaldehyde in dichloromethane at room temperature. P4 and i-P4 showed unique square pagoda structures and fixed conformations in solution and also exhibited strong blue fluorescence. Moreover, P4 and i-P4 with deep and rich-electron cavities could not only encapsulate n-hexane and one or two dichloromethane molecules in the solid state but also showed strong binding abilities toward neutral dinitriles with different chain lengths and various nitrogen-containing heterocyclic salts to form 1:1 stable host-guest complexes in both solution and the solid state. In particular, it was also found that with the increase in the alkyl chain length of the dinitriles, the association constants for their complexes with both P4 and i-P4 were markedly increased from glutaronitrile to octanedinitrile as a result of the deep cavities of the macrocycles and multiple intermolecular interactions. Since P4 and i-P4 had stable planar chirality, their efficient resolutions were further achieved by HPLC with a chiral column. Interestingly, the two enantiomers showed mirror-imaged CD signals and excellent CPL properties, which could allow them to have potential applications in chiral luminescent materials.

Symmetrical-Tetramethyl-Cucurbit[6]uril-Driven Movement of Cucurbit[7]uril Gives Rise to Heterowheel [4]Pseudorotaxanes

Two novel heterowheel [4]pseudorotaxanes consisting of cucurbit[7]uril (Q[7]) and symmetrical-tetramethyl-cucurbit[6]uril (TMeQ[6]) were constructed via the multirecognition mechanism, in which Q[7] can rotate freely around the horizontal axis, while TMeQ[6] cannot. In the construction process, due to strong repulsive forces between carbonyl portals of two neighboring wheels, the dethreading and movement of the wheels along the axle was observed. The dissociation of the [4]pseudorotaxanes was also discussed.

Self-Assembly and Molecular Recognition in Water: Tubular Stacking and Guest-Templated Discrete Assembly of Water-Soluble, Shape-Persistent Macrocycles

Supramolecular chemistry in aqueous media is an area with great fundamental and practical significance. To examine the role of multiple noncovalent interactions in controlled assembling and binding behavior in water, the self-association of five water-soluble hexakis(m-phenylene ethynylene) (m-PE) macrocycles, along with the molecular recognition behavior of the resultant assemblies, is investigated with UV-vis, fluorescence, CD, and NMR spectroscopy, mass spectrometry, and computational studies. In contrast to their different extents of self-aggregation in organic solvents, all five macrocycles remain aggregated in water at concentrations down to the micromolar (μM) range. CD spectroscopy reveals that 1-F₆ and 1-H₆, two macrocycles carrying chiral side chains and capable of H-bonded self-association, assemble into tubular stacks. The tubular stacks serve as supramolecular hosts in water, as exemplified by the interaction of macrocycles 1-H₆ and 2-H₆ and guests G1 through G4, each having a rod-like oligo(p-phenylene ethynylene) (p-PE) segment flanked by two hydrophilic chains. Fluorescence and ¹H NMR spectroscopy revealed the formation of kinetically stable, discrete assemblies upon mixing 2-H₆ and a guest. The binding stoichiometry, determined with fluorescence, ¹H NMR, and ESI-MS, reveals that the discrete assemblies are novel pseudorotaxanes, each containing a pair of identical guest molecules encased by a tubular stack. The two guest molecules define the number of macrocyclic molecules that comprise the host, which curbs the "infinite" stack growth, resulting in a tubular stack with a cylindrical pore tailoring the length of the p-PE segment of the bound guests. Each complex is stabilized by the action of multiple noncovalent forces including aromatic stacking, side-chain H-bonding, and van der Waals interactions. Thus, the interplay of multiple noncovalent forces aligns the molecules of macrocycles 1 and 2 into tubular stacks with cylindrical inner pores that, upon binding rod-like guests, lead to tight, discrete, and well-ordered tubular assemblies that are unprecedented in water.

Substituent-controlled racemization of dissymmetric coordination capsules

We report the effect of substituents (methyl, isopropyl, methoxy, and methoxyphenyl) at the 6'-position of the 2,2'-bipyridyl arms on the racemization of dissymmetric coordination capsules 1a-d. When the capsules included (R)-4,4'-diacetoxy-2,2'-benzyloxycarboxyl-biphenyl ((R)-3), the (M)-helical conformer was enriched with a diastereomeric excess (de%) of >98% for 1a, 31% for 1b, 81% for 1c and 75% for 1d. The entrapped guests in 1a, 1c and 1d can be removed by washing the solid containing the host-guest complexes with diethyl ether. The rate of racemization in THF follows the order of 1c > 1d ≫ 1a. X-ray crystal structural analysis and density functional theory calculation of model complex 4c indicate a distorted tetrahedral coordination of the Cu(i) center, and UV-vis absorption spectroscopy indicates similar coordination environments in 1c and 4c. A series of experiments demonstrates that the racemization rate depends on the dihedral angles of the bipyridyl arms, and the angles are regulated by the substituents. The methoxy and methoxyphenyl substituents in 1c and 1d enlarge the dihedral angles of the bipyridyl arms. This facilitates the access of solvent molecules to the Cu(i) centers and promotes racemization. The slower racemization of 1d can be ascribed to the steric protection of the Cu(i) centers from incoming solvent molecules by the p-methoxyphenyl group.

Size-Selective Hydroformylation by a Rhodium Catalyst Confined in a Supramolecular Cage

Size-selective hydroformylation of terminal alkenes was attained upon embedding a rhodium bisphosphine complex in a supramolecular metal-organic cage that was formed by subcomponent self-assembly. The catalyst was bound in the cage by a ligand-template approach, in which pyridyl-zinc(II) porphyrin interactions led to high association constants (>105  m-1 ) for the binding of the ligands and the corresponding rhodium complex. DFT calculations confirm that the second coordination sphere forces the encapsulated active species to adopt the ee coordination geometry (i.e., both phosphine ligands in equatorial positions), in line with in situ high-pressure IR studies of the host-guest complex. The window aperture of the cage decreases slightly upon binding the catalyst. As a result, the diffusion of larger substrates into the cage is slower compared to that of smaller substrates. Consequently, the encapsulated rhodium catalyst displays substrate selectivity, converting smaller substrates faster to the corresponding aldehydes. This selectivity bears a resemblance to an effect observed in nature, where enzymes are able to discriminate between substrates based on shape and size by embedding the active site deep inside the hydrophobic pocket of a bulky protein structure.

Tunable enforced cavities inside self-assembled capsules

Controlling and tuning the molecular recognition properties is a crucial task in host–guest chemistry.

Confinement Effect on Molecular Conformation of Alkanes in Water-Filled Cavitands: A Combined Quantum/Classical Density Functional Theory Study

The depletion force exerted on an alkane molecule from surrounding solvent may greatly alter its conformation. Such a behavior is closely related to the selective molecular recognition, molecular sensors, self-assembly, and so on. Herein, we report a multiscale theoretical study on the conformational change of a single alkane molecule confined in water-filled cavitands, in which the quantum and classical density functional theories (DFTs) are combined to determine the grand potential of alkane-water system. Specifically, the intrinsic free energy of the alkane molecule is tackled by quantum DFT, while the solvent effect arising from the solvent density inhomogeneity in confined space is addressed by classical DFT. By varying the alkane chain length, pore size, and wettability of inner pore surface, we find that pore confinement and hydrophilic inner surface facilitate the alkane conformational change from extended state to helical state, which becomes more significant as the alkane chain length increases. Our findings, which are in line with previous experimental observations, provide not only the microscopic mechanism but also theoretical guidance for elaborately manipulating molecular conformation at the nanoscale.

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.

Mixed Explicit-Implicit Solvation Approach for Modeling of Alkane Complexation in Water-Soluble Self-Assembled Capsules

The host-guest binding properties of a water-soluble resorcinarene-based cavitand are examined using density functional theory methodology. Experimentally, the cavitand has been observed to self-assemble in aqueous solution into both 1:1 and 2:1 host/guest complexes with hydrophobic guests such as n-alkanes. For n-decane, equilibrium was observed between the 1:1 and 2:1 complexes, while 1:1 complexes are formed with shorter n-alkanes and 2:1 complexes are formed with longer ones. These findings are used to assess the standard quantum chemical methodology. It is first shown that a rather advanced computational protocol (B3LYP-D3(BJ)/6-311+G(2d,2p) with COSMO-RS and quasi-rigid-rotor-harmonic-oscillator) gives very large errors. Systematic examination of the various elements of the methodology shows that the error stems from the implicit solvation model. A mixed explicit-implicit solvation protocol is developed that involves a parametrization of the hydration free energy of water such that water cluster formation in water is predicted to be thermoneutral. This new approach is demonstrated to lead to a major improvement in the calculated binding free energies of n-alkanes, reproducing very well the 1:1 versus 2:1 host/guest binding trends.

HYDROPHOBE Challenge: A Joint Experimental and Computational Study on the Host-Guest Binding of Hydrocarbons to Cucurbiturils, Allowing Explicit Evaluation of Guest Hydration Free-Energy Contributions

The host-guest complexation of hydrocarbons (22 guest molecules) with cucurbit[7]uril was investigated in aqueous solution using the indicator displacement strategy. The binding constants (10³-10⁹ M^-1) increased with guest size, pointing to the hydrophobic effect and dispersion interactions as driving forces. The measured affinities provide unique benchmark data for the binding of neutral guest molecules. Consequently, a computational blind challenge, the HYDROPHOBE challenge, was conducted to allow a comparison with state-of-the-art computational methods for predicting host-guest affinity constants. In total, three quantum-chemical (QM) data sets and two explicit-solvent molecular dynamics (MD) submissions were received. When searching for sources of uncertainty in predicting the host-guest affinities, the experimentally known hydration energies of the investigated hydrocarbons were used to test the employed solvation models (explicit solvent for MD and COSMO-RS for QM). Good correlations were obtained for both solvation models, but a rather constant offset was observed for the COSMO data, by ca. +2 kcal mol^-1, which was traced back to a required reference-state correction in the QM submissions (2.38 kcal mol^-1). Introduction of the reference-state correction improved the predictive power of the QM methods, particularly for small hydrocarbons up to C5.

Endo/exo binding of alkyl and aryl diammonium ions by cyclopentanocucurbit[6]uril

Binding interactions of cyclopentanocucurbit[6]uril (CyP₆Q[6]) with a series of alkyl and aryl diammonium ions were investigated. CyP₆Q[6] exhibits endo and exo binding modes with the guests, which depends on the size and shape complementarity between the host and the guests.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Host-guest complexation of di-cyclohexanocucurbit[6]uril and hexa-cyclohexanocucurbit[6]uril with alkyldiammonium ions: a comparative study

The host-guest complexation of symmetrical di-cyclohexanocucurbit[6]uril (Cy2Q[6]) and hexa-cyclohexanocucurbit[6]uril (Cy6Q[6]) with a series of alkyldiammonium ions (H(3+)N(CH(2))nNH(3+), n = 2-8) has been studied both in solution and in the gas phase. (1)H NMR data indicate that all alkyldiammonium ions have inclusion interactions with both hosts except for the ethanediammonium ion. In addition, if the alkyl chain of the alkyldiammonium ion is longer than n = 5 methylene groups, compressed conformation may occur, which depends on the cavity shape of the hosts and the length of the alkyl chain. Isothermal titration calorimetry (ITC) studies point out that the host-guest complexations of both hosts with the latter five alkyldiammonium ions are enthalpically driven. The comparison of the thermodynamic data reveals that the enthalpies of the van der Waals interactions contribute more to the host-guest complexation enthalpy than the ion-dipole interactions. The enthalpic gain arises from the van der Waals interactions and the reduction of entropy upon the host-guest complexation is strongly affected by the cavity shape of the host. Gas phase structures of long alkyldiammonium guests within both hosts are completely different from those in aqueous solution.

Host–guest complexation of HMeQ[7] with alkyldiammonium ions and alkyldiamines: a comparative study

The host–guest complexations of HMeQ[7] with a series of alkyldiammonium ions and alkyldiamines have been investigated, indicating that the driving forces strongly depend on the features of the guests.

A thorough experimental study of CH/π interactions in water: quantitative structure-stability relationships for carbohydrate/aromatic complexes

CH/π interactions play a key role in a large variety of molecular recognition processes of biological relevance. However, their origins and structural determinants in water remain poorly understood. In order to improve our comprehension of these important interaction modes, we have performed a quantitative experimental analysis of a large data set comprising 117 chemically diverse carbohydrate/aromatic stacking complexes, prepared through a dynamic combinatorial approach recently developed by our group. The obtained free energies provide a detailed picture of the structure-stability relationships that govern the association process, opening the door to the rational design of improved carbohydrate-based ligands or carbohydrate receptors. Moreover, this experimental data set, supported by quantum mechanical calculations, has contributed to the understanding of the main driving forces that promote complex formation, underlining the key role played by coulombic and solvophobic forces on the stabilization of these complexes. This represents the most quantitative and extensive experimental study reported so far for CH/π complexes in water.

Soft templates in encapsulation complexes

Template effects are an inevitable feature of supramolecular chemistry and were prominent in the discovery of crown ethers, carcerands and catenanes. Templates can act as guests or hosts, but in either role they must be structurally persistent - rigid or "hard" - on the timescale needed to form the final complexes. This report explores a peculiar effect encountered with self-assembled container molecules: soft templates. In these cases neither the guest nor the host has an independent existence, but they do coexist as complexes. The host and guest are prevented from collapsing into more familiar, stable structures by the forces that hold the complex together. The complexes represent emergent phenomena and offer a look at structures otherwise unknown in free solution.

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.

Modulating weak interactions for molecular recognition: a dynamic combinatorial analysis for assessing the contribution of electrostatics to the stability of CH-π bonds in water

Electrostatic and charge-transfer contributions to CH-π complexes can be modulated by attaching electron-withdrawing substituents to the carbon atom. While clearly stabilizing in the gas phase, the outcome of this chemical modification in water is more difficult to predict. Herein we provide a definitive and quantitative answer to this question employing a simple strategy based on dynamic combinatorial chemistry.