Rigid molecular tweezers as hosts for the complexation of neutral guests

Buckybowl Molecular Tweezers for Recognition of Fullerenes

Buckybowl tweezers are a relatively young research area closely associated with the development of non-planar polycyclic aromatic systems and supramolecular chemistry. Since the appearance of the first prototypes in the early 2000s, the tweezers have undergone evolutionary changes. Nowadays they are able to effectively interact with fullerenes and the results opened up prospects for development in the field of sensing, nonlinear optics, and molecular switchers. In the present study, examples of corannulene-based and other buckybowl tweezers for the recognition of C60 and C70 fullerenes were summarized and analyzed. The main structural components of the tweezers were also reviewed in detail and their role in the formation of complexes with fullerenes was evaluated. The revealed structural patterns should trigger the development of novel recognition systems and materials with a wide range of applications.

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

"Lock and Key" and "Induced-Fit" Host-Guest Models in Two Digold(I)-Based Metallotweezers

Two different metallotweezers, each with two pyrene-imidazolylidene-gold(I) arms, were used as hosts for a series of planar aromatic guests. The metallotweezer with a dibenzoacridinebis(alkynyl) spacer (1) orients the two pyrene-imidazolylidene-gold(I) arms in a parallel disposition, with an interpanel distance of about 7 Å. The second metallotweezer (2) contains a carbazolylbis(alkynyl) spacer that directs the two pyrene panels in a diverging orientation. Determination of the association constants via ¹H NMR titrations demonstrates that the binding strength shown by 1 is significantly larger than that found by 2, with binding affinities as large as 10⁴ M^-1 (in CDCl₃), for the encapsulation of N,N'-dimethylnaphthalenetetracarboxydiimide with 1. The differences in the binding affinities are due to binding models associated with formation of the related host-guest complexes. While 1 operates via a "lock and key" model, in which the host does not suffer distortions upon formation of the inclusion complex, 2 operates via a guest-induced fit model. The large association constants shown by 1 with two planar guests were used for promotion of the template-directed synthesis of 1, which in the absence of an external template is produced in an equimolecular mixture with its self-aggregated congener, clippane [1₂]. This observation strongly suggests that the mechanically interlocked clippane is formed through a self-template-directed mechanism, while bonds are broken/formed during the synthetic protocol.

Photoswitchable molecular tweezers: isomerization to control substrate binding, and what about vice versa?

The linkage of two identical binding motifs by a molecular photoswitch has proven to be a straightforward and versatile strategy to control substrate binding affinity by light. Stimulus control of binding properties in artificial receptors is partly inspired by the dynamic behavior of proteins and is highly attractive as it could, for example, improve extraction processes and allow (de)activation of membrane transport on demand. This feature article summarizes the development and design principles of molecular tweezers containing a molecular photoswitch as the core unit. Besides the control of binding affinity by isomerization, the effect of substrate binding on the isomerization behavior is discussed where data is available. While the latter often receives less attention, it could be of benefit in the future creation of multi-stimuli-controlled molecular switching and machine-like systems.

The chaotropic effect of ions on the self-aggregating propensity of Whitlock's molecular tweezers

Molecular tweezers feature the first class of artificial receptors to pique the interest of researchers and emerge as an effective therapeutic candidate. The exceptional structure and exquisite binding specificity of tweezers establish this overall class of receptors as a promising tool, with abundant applications. However, their inclination to self-aggregate by mutual π-π stacking interactions of their aromatic arms diminishes their efficacy as a therapeutic candidate. Therefore, following up on sporadic studies, since the discovery of the Hofmeister series, on the ability of ions to either solvate (salting-in) or induce aggregation (salting-out) of hydrophobic solutes, the notions of ion-specificity effects are utilized on tweezer moieties. The impacts of three different aluminum salts bearing anions Cl^-, ClO₄^- and SCN^- on the self-association propensity of Whitlock's caffeine-pincered molecular tweezers are investigated, with a specific emphasis placed on elucidating the varied behavior of the ions on the hydration ability of tweezers. The comparative investigation is conducted employing a series of all-atom molecular dynamics simulations of five tweezer molecules in pure water and three salt solutions, at two different concentrations each, maintaining a temperature of 300 K and a pressure of 1 atm, respectively. Radial distribution functions, coordination numbers, and SASA calculations display a steady reduction in the aggregation proclivity of the receptor molecules with an increase in salt concentration, as progressed along the Hofmeister series. Orientational preferences between the tweezer arms reveal a disruptive effect in the regular π-π stacking interactions, in the presence of high concentrations of ClO₄^- and SCN^- ions, while preferential interactions and tetrahedral order parameters unveil the underlying mechanism, by which the anions alter the solubility of the hydrophobic molecules. Overall, it is observed that SCN^- exhibits the highest salting-in effect, followed by ClO₄^-, with both anions inhibiting tweezer aggregation through different mechanisms. ClO₄^- ions impart an effect by moderately interacting with the solute molecules as well as modifying the water structure of the bulk solution promoting solvation, whereas, SCN^- ions engage entirely in interaction with specific tweezer sites. Cl^- being the most charge-dense of the three anionic species experiences stronger hydration and therefore, imparts a very negligible salting-in effect.

Fluorescence Response and Self-Assembly of a Tweezer-Type Synthetic Receptor Triggered by Complexation with Heme and Its Catabolites

There is increasing interest in the development and applications of synthetic receptors that recognize target biomolecules in aqueous media. We have developed a new tweezer-type synthetic receptor that gives a significant fluorescence response upon complexation with heme in aqueous solution at pH 7.4. The synthetic receptor consists of a tweezer-type heme recognition site and sulfo-Cy5 as a hydrophilic fluorophore. The receptor-heme complex exhibits a supramolecular amphiphilic character that facilitates the formation of self-assembled aggregates, and both the tweezer moiety and the sulfo-Cy5 moiety are important for this property. The synthetic receptor also exhibits significant fluorescence responses to biliverdin and bilirubin, but shows very weak fluorescence responses to flavin mononucleotide, folic acid, and nicotinamide adenine dinucleotide, which contain smaller π-scaffolds.

Methodology of deconvolution of total solute retention on chemically modified stationary phases to structure specific contributions of bound compounds

The total solute retention by a chemically modified stationary phase (CMSP) has been shown several times to be a potential tool for studying the binding abilities of the bound compound. In this article, we present a methodology for the deconvolution of the total retention into structure-specific contributions. Three complementary silica-based CMSPs were prepared: 1) non-modified silica, 2) silica modified by syn-bis-Tröger's base (a molecular tweezer) and 3) silica modified by anti-bis-Tröger's base (a non-tweezer molecule). These were characterized by elemental analysis and Raman spectroscopy, and used to assemble liquid chromatography (LC) columns. The total retention factors were estimated for electron-deficient nitro- and cyano-derivatives of benzene in both normal and reverse elution modes. The total retention factor was considered to be the sum of structure-specific retention factors, each related to the affinity (the binding constant) of a specific structure (the binding site), and its content in the modified silica, as defined for weak-affinity chromatography (WAC). The obtained structure-specific contributions are in line with the binding studies of ligands in solution. They reveal details of the retention mechanism, suggesting a more suitable attachment of ligands, and expose the shortcomings of evaluations based solely on the total retentions.

An original self-assembly using a tetrathiafulvalene-based molecular clip for the recognition of fullerene C60

A glycoluril-based molecular clip incorporating tetrathiafulvalene sidewalls self-assembles with fullerene C₆₀ in a 2 : 1 stoichiometry in solution.

Dual switchable molecular tweezers incorporating anisotropic MnIII-salphen complexes

An alternative strategy for the synthesis of terpyridine based switchable molecular tweezers has been developed to incorporate anisotropic Mn(iii)-salphen complexes. The free ligand was synthesized using a building block strategy based on Sonogashira coupling reactions and was then selectively metalated with manganese in a last step. The conformation of the tweezers was switched from an open 'W' shaped form to a closed 'U' form by Zn(ii) coordination to the terpyridine unit bringing the two Mn-salphen moieties in close spatial proximity as confirmed by X-ray crystallography. An alternate switching mechanism was observed by the intercalation of a bridging cyanide ligand between the two Mn-salphen moieties that resulted in the closing of the tweezers. These dual stimuli are attractive for achieving multiple controls of the mechanical motion of the tweezers. A crystallographic structure of unexpected partially oxidized closed tweezers was also obtained. One of the two Mn-salphen moieties underwent a ligand-centered oxidation of an imino to an amido group allowing an intramolecular Mn-Oamide-Mn linkage. The magnetic properties of the manganese(iii) dimers were investigated to evaluate the magnetic exchange interaction and analyze the single molecule magnet behavior.

Substrate-dependent allosteric regulation by switchable catalytic molecular tweezers

Allosteric regulation is exploited by biological systems to regulate the activity and/or selectivity of enzymatic reactions but remains a challenge for artificial catalysts. Here we report switchable terpy(Zn-salphen)2 molecular tweezers and their metal-dependent allosteric regulation of the acetylation of pyridinemethanol isomers. Zinc-salphen moieties can both act as a Lewis acid to activate the anhydride reagents and provide a binding site for pyridinemethanol substrates. The tweezers’ conformation can be reversibly switched between an open and a closed form by a metal ion stimulus. Both states offer distinct catalytic profiles, with closed tweezers showing superior catalytic activity towards ortho substrates, while open tweezers presenting higher rate for the acetylation of meta and para substrates. This notable substrate dependent allosteric response is rationalized by a combination of experimental results and calculations supporting a bimetallic reaction in the closed form for ortho substrate and an inhibition of the cavity for meta and para substrates.

Surface-Modified Chitosan: An Adsorption Study of a “Tweezer-Like” Biopolymer with Fluorescein

Tweezer-like adsorbents with enhanced surface area were synthesized by grafting aniline onto the amine sites of a chitosan biopolymer scaffold. The chemical structure and textural properties of the adsorbents were characterized by thermogravimetric analysis (TGA) and spectral methods, including Fourier transform infrared (FT-IR), nuclear magnetic resonance (1H- and, 13C-NMR) and scanning electron microscopy (SEM). Equilibrium solvent swelling results for the adsorbent materials provided evidence of a more apolar biopolymer surface upon grafting. Equilibrium uptake studies with fluorescein at ambient pH in aqueous media reveal a high monolayer adsorption capacity (Qm) of 61.8 mg·g−1, according to the Langmuir isotherm model. The kinetic adsorption profiles are described by the pseudo-first order kinetic model. 1D NMR and 2D-NOESY NMR spectra were used to confirm the role of π-π interactions between the adsorbent and adsorbate. Surface modification of the adsorbent using monomeric and dimeric cationic surfactants with long hydrocarbon chains altered the hydrophile-lipophile balance (HLB) of the adsorbent surface, which resulted in attenuated uptake of fluorescein by the chitosan molecular tweezers. This research contributes to a first example of the uptake properties for a tweezer-like chitosan adsorbent and the key role of weak cooperative interactions in controlled adsorption of a model anionic dye.

Design and synthesis of a 4-aminoquinoline-based molecular tweezer that recognizes protoporphyrin IX and iron(iii) protoporphyrin IX and its application as a supramolecular photosensitizer

We report on the design and synthesis of a new type of 4-aminoquinoline-based molecular tweezer 1 which forms a stable host-guest complex with protoporphyrin IX (PPIX) via multiple interactions in a DMSO and HEPES buffer (pH 7.4) mixed solvent system. The binding constant for the 1 : 1 complex (K 11) between 1 and PPIX is determined to be 4 × 106 M-1. Furthermore, 1 also forms a more stable complex with iron(iii) protoporphyrin IX (Fe(iii)PPIX), the K 11 value for which is one order of magnitude greater than that for PPIX, indicating that 1 could be used as a recognition unit of a synthetic heme sensor. On the other hand, the formation of the stable PPIX·1 complex (supramolecular photosensitizer) prompted us to apply it to photodynamic therapy (PDT). Cell staining experiments using the supramolecular photosensitizer and evaluations of its photocytotoxicity indicate that the PDT activity of PPIX is improved as the result of the formation of a complex with 1.

Synthesis of Doubly Annulated m-Terphenyl-Based Molecular Tweezers and Their Charge-Transfer Complexes with DDQ as a Guest

The synthesis of a doubly-annulated m-terphenyl-based tweezer platform has been developed, which affords ready incorporation of various pincer units from monobenzenoid to polybenzenoid electron donors. The complexation study with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as guest has been carried out, and the crystal structure of T-Py∩DDQ reveals the sandwich-type binding mode in the solid state.

Donor-acceptor-type supramolecular polymers on the basis of preorganized molecular tweezers/guest complexation

The bottom-up self-assembly of donor-acceptor (D-A) units has received tremendous attention in recent years. Charge-transfer interactions, which are inherently embedded in D-A pairs, have suffered from some disadvantages such as erratic arrangements and weak binding affinity, thus hampering the precise arrangement of D-A units into long-range-ordered supramolecular polymers. To address this issue, a feasible protocol is to incorporate D-A units into molecular tweezers/guest recognition motifs, which concurrently feature high complexation directionality, strong binding affinity and stimuli-responsiveness. In this tutorial review, we have summarized the recent advances on the tweezering directed formation of D-A-type supramolecular polymers, with particular emphasis on the design principles of monomers and macroscopic behaviors of supramolecular polymers, together with future challenges in this research field.

Synthesis and Guest Recognition of Switchable Pt-Salphen Based Molecular Tweezers

Molecular tweezers are artificial receptors that have an open cavity generated by two recognition units pre-organized by a spacer. Switchable molecular tweezers, using a stimuli-responsive spacer, are particularly appealing as prototypes of the molecular machines that combine mechanical motion and allosteric recognition properties. In this present study, the synthesis of switchable molecular tweezers composed of a central terpyridine unit substituted in 4,4″ positions by two Pt(II)-salphen complexes is reported. The terpyridine ligand can be reversibly converted upon Zn(II) coordination from a free ‘U’-shaped closed form to a coordinated ‘W’ open form. This new substitution pattern enables a reverse control of the mechanical motion compared to the previously reported 6,6″ substituted terpyridine-based tweezers. Guest binding studies with aromatic guests showed an intercalation of coronene in the cavity created by the Pt-salphen moieties in the closed conformation. The formation of 1:1 host-guest complex was investigated by a combination of NMR studies and DFT calculations.

Acyclic Cucurbit[n]uril-type Receptors: Preparation, Molecular Recognition Properties and Biological Applications

This article traces the development of acyclic cucurbit[n]uril-type receptors with a focus on work from the Isaacs group. First, we describe the synthesis of methylene bridged glycoluril dimers capped with aromatic sidewalls which allowed us to probe the interconversion of the S- and C-shaped dimers which is a fundamental step in CB[n] formation. The C-shaped compounds were found to undergo discrete self-assembly (dimerization) in both water and organic solvents which lead us to investigate multicomponent self-sorting systems. We supressed the self-association of 8 by electrostatic repulsion in the putative dimer which allowed expression of its innate molecular recognition properties toward methylene blue and related planar cationic dyes. Longer glycoluril oligomers (trimer - hexamer, acyclic decamer) were prepared by starving the CB[n]-forming reaction of formaldehyde. The longer oligomers (e.g. 15 and 16) bind to alkylammonium ions in water ≈ 100-fold weaker than macrocyclic CB[n] highlighting the high preorganization of the acyclic but polycyclic framework. We prepared a wide variety of acyclic CB[n] compounds (wall variants, solubilizing group variants, linker variants) based on glycoluril trimer and tetramer. In particular, 26 and 27 have been shown to possess a wide variety of chemically and biologically interesting functions. For example, 26 was used to formulate the insoluble drug Albendazole and treat mice bearing SK-OV-3 xenograft tumors. Compound 27 binds tightly to the neuromuscular blocking agents rocuronium, vecuronium, and cisatracurium and acts as an in vivo reversal agent for these compounds in anesthetized rats. Container 27 was also found to modulate the hyperlocomotive effect of rats that had been treated with methamphetamine. Finally, 38 has been used as a cross reactive component of sensor arrays that are capable of classifying and quantifying cancer related nitroamines and a range of over the counter drugs. Overall, the work demonstrates that acyclic CB[n]-type compounds are nicely pre-organized and therefore retain the essential aspects of the recognition properties of macrocyclic CB[n] but allow for more straightforward tailoring of structure and solubility that enables a variety of chemically and biologically important applications.

The Molecular Tweezer CLR01 Stabilizes a Disordered Protein-Protein Interface

Protein regions that are involved in protein-protein interactions (PPIs) very often display a high degree of intrinsic disorder, which is reduced during the recognition process. A prime example is binding of the rigid 14-3-3 adapter proteins to their numerous partner proteins, whose recognition motifs undergo an extensive disorder-to-order transition. In this context, it is highly desirable to control this entropy-costly process using tailored stabilizing agents. This study reveals how the molecular tweezer CLR01 tunes the 14-3-3/Cdc25CpS216 protein-protein interaction. Protein crystallography, biophysical affinity determination and biomolecular simulations unanimously deliver a remarkable finding: a supramolecular "Janus" ligand can bind simultaneously to a flexible peptidic PPI recognition motif and to a well-structured adapter protein. This binding fills a gap in the protein-protein interface, "freezes" one of the conformational states of the intrinsically disordered Cdc25C protein partner and enhances the apparent affinity of the interaction. This is the first structural and functional proof of a supramolecular ligand targeting a PPI interface and stabilizing the binding of an intrinsically disordered recognition motif to a rigid partner protein.

Multicavity Metallosupramolecular Architectures

Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

Six States Switching of Redox-Active Molecular Tweezers by Three Orthogonal Stimuli

A six level molecular switch based on terpyridine(Ni-salphen)₂ tweezers and addressable by three orthogonal stimuli (metal coordination, redox reaction, and guest binding) is reported. By a metal coordination stimulus, the tweezers can be mechanically switched from an open "W"-shaped conformation to a closed "U"-shaped form. Theses two states can each be reversibly oxidized by the redox stimulus and bind to a pyrazine guest resulting in four additional states. All six states are stable and accessible by the right combination of stimuli and were studied by NMR, XRD, EPR spectroscopy, and DFT calculations. The combination of the supramolecular concepts of mechanical motion and guest binding with the redox noninnocent and valence tautomerism properties of Ni-salphen complexes added two new dimensions to a mechanical switch.

Overview of the SAMPL5 host-guest challenge: Are we doing better?

The ability to computationally predict protein-small molecule binding affinities with high accuracy would accelerate drug discovery and reduce its cost by eliminating rounds of trial-and-error synthesis and experimental evaluation of candidate ligands. As academic and industrial groups work toward this capability, there is an ongoing need for datasets that can be used to rigorously test new computational methods. Although protein-ligand data are clearly important for this purpose, their size and complexity make it difficult to obtain well-converged results and to troubleshoot computational methods. Host-guest systems offer a valuable alternative class of test cases, as they exemplify noncovalent molecular recognition but are far smaller and simpler. As a consequence, host-guest systems have been part of the prior two rounds of SAMPL prediction exercises, and they also figure in the present SAMPL5 round. In addition to being blinded, and thus avoiding biases that may arise in retrospective studies, the SAMPL challenges have the merit of focusing multiple researchers on a common set of molecular systems, so that methods may be compared and ideas exchanged. The present paper provides an overview of the host-guest component of SAMPL5, which centers on three different hosts, two octa-acids and a glycoluril-based molecular clip, and two different sets of guest molecules, in aqueous solution. A range of methods were applied, including electronic structure calculations with implicit solvent models; methods that combine empirical force fields with implicit solvent models; and explicit solvent free energy simulations. The most reliable methods tend to fall in the latter class, consistent with results in prior SAMPL rounds, but the level of accuracy is still below that sought for reliable computer-aided drug design. Advances in force field accuracy, modeling of protonation equilibria, electronic structure methods, and solvent models, hold promise for future improvements.

Glycoluril-Derived Molecular Clips are Potent and Selective Receptors for Cationic Dyes in Water

Molecular clip 1 remains monomeric in water and engages in host-guest recognition processes with suitable guests. We report the K_a values for 32 1⋅guest complexes measured by ¹ H NMR, UV/Vis, and fluorescence titrations. The cavity of 1 is shaped by aromatic surfaces of negative electrostatic potential and therefore displays high affinity and selectivity for planar and cationic aromatic guests that distinguishes it from CB[n] receptors that prefer aliphatic over aromatic guests. Electrostatic effects play a dominant role in the recognition process whereby ion-dipole interactions may occur between ammonium ions and the C=O groups of 1, between the SO₃^- groups of 1 and pendant cationic groups on the guest, and within the cavity of 1 by cation-π interactions. Host 1 displays a high affinity toward dicationic guests with large planar aromatic surfaces (e.g. naphthalene diimide NDI+ and perylene diimide PDI+) and cationic dyes derived from acridine (e.g. methylene blue and azure A). The critical importance of cation-π interactions was ascertained by a comparison of analogous neutral and cationic guests (e.g. methylene violet vs. methylene blue; quinoline vs. N-methylquinolinium; acridine vs. N-methylacridinium; neutral red vs. neutral red H⁺ ) the affinities of which differ by up to 380-fold. We demonstrate that the high affinity of 1 toward methylene blue (K_a =3.92×10⁷  m^-1 ; K_d =25 nm) allows for the selective sequestration and destaining of U87 cells stained with methylene blue.

A journey in bioinspired supramolecular chemistry: from molecular tweezers to small molecules that target myotonic dystrophy

This review summarizes part of the author’s research in the area of supramolecular chemistry, beginning with his early life influences and early career efforts in molecular recognition, especially molecular tweezers. Although designed to complex DNA, these hosts proved more applicable to the field of host–guest chemistry. This early experience and interest in intercalation ultimately led to the current efforts to develop small molecule therapeutic agents for myotonic dystrophy using a rational design approach that heavily relies on principles of supramolecular chemistry. How this work was influenced by that of others in the field and the evolution of each area of research is highlighted with selected examples.

Pyrene appended thymine derivative for selective turn-on fluorescence sensing of uric acid in live cells

A new ‘turn-ON’ fluorescent probe, pyrene appended thymine acetamide (PTA), with high sensitivity and selectivity for the detection of uric acid (UA) was developed and first time imaging of uric acid in living cells in water was achieved.

Toward Improved Force-Field Accuracy through Sensitivity Analysis of Host-Guest Binding Thermodynamics

Improving the capability of atomistic computer models to predict the thermodynamics of noncovalent binding is critical for successful structure-based drug design, and the accuracy of such calculations remains limited by nonoptimal force field parameters. Ideally, one would incorporate protein-ligand affinity data into force field parametrization, but this would be inefficient and costly. We now demonstrate that sensitivity analysis can be used to efficiently tune Lennard-Jones parameters of aqueous host-guest systems for increasingly accurate calculations of binding enthalpy. These results highlight the promise of a comprehensive use of calorimetric host-guest binding data, along with existing validation data sets, to improve force field parameters for the simulation of noncovalent binding, with the ultimate goal of making protein-ligand modeling more accurate and hence speeding drug discovery.

Molecular cleft or tweezer compounds derived from trioxabicyclo[3.3.1]nonadiene diisocyanate and diacid dichloride

The structures of two derivatives of the bisdioxine diisocyanate 1, the bisurea 4 and the biscarbamate 5, are established by X-ray crystallography and DFT calculations. These compounds possess endo,endo structures, in the case of the bisurea 4 with two nearly parallel pendant chains. The X-ray structures are reproduced very well by DFT calculations. Similar endo,endo conformations are calculated for the bisamide crown ether derivatives 7, where two proximate and nearly parallel crown ether units endow the molecules with a claw-like molecular cleft or tweezer structure as evidenced by an enhanced ability to extract some alkali, alkaline earth and rare earth metal ions.

Mechanical switching of magnetic interaction by tweezers-type complex

An original approach using a mechanical motion to control the magnetic interaction between the two spin centers of terpy(Cu–salphen)₂ complexes is presented.

Drug release by pH-responsive molecular tweezers: atomistic details from molecular modeling

pH-responsive molecular tweezers have been proposed as an approach for targeting drug-delivery to tumors, which tend to have a lower pH than normal cells. We performed a computational study of a pH-responsive molecular tweezer using ab initio quantum chemistry in the gas-phase and molecular dynamics (MD) simulations in solution. The binding free energy in solution was calculated using steered MD. We observe, in atomistic detail, the pH-induced conformational switch of the tweezer and the resulting release of the drug molecule. Even when the tweezer opens, the drug molecule remains near a hydrophobic arm of the molecular tweezer. Drug release cannot occur, it seems, unless the tweezer is in a hydrophobic environment with low pH.

Aromatic amide and hydrazide foldamer-based responsive host-guest systems

CONSPECTUS: In host-guest chemistry, a larger host molecule selectively and noncovalently binds to a smaller guest molecule or ion. Early studies of host-guest chemistry focused on the recognition of spherical metal or ammonium ions by macrocyclic hosts, such as cyclic crown ethers. In these systems, preorganization enables their binding sites to cooperatively contact and attract a guest. Although some open-chain crown ether analogues possess similar, but generally lower, binding affinities, the design of acyclic molecular recognition hosts has remained challenging. One of the most successful examples was rigid molecular tweezers, acyclic covalently bonded preorganized host molecules with open cavities that bind tightly as they stiffen. Depending on the length of the atomic backbones, hydrogen bonding-driven aromatic amide foldamers can form open or closed cavities. Through rational design of the backbones and the introduction of added functional groups, researchers can regulate the shape and size of the cavity. The directionality of hydrogen bonding and the inherent rigidity of aromatic amide units allow researchers to predict both the shape and size of the cavity of an aromatic amide foldamer. Therefore, researchers can then design guest molecules with structure that matches the cavity shape, size, and binding sites of the foldamer host. In addition, because hydrogen bonds are dynamic, researchers can design structures that can adapt to outside stimuli to produce responsive supramolecular architectures. In this Account, we discuss how aromatic amide and hydrazide foldamers induced by hydrogen bonding can produce responsive host-guest systems, based on research by our group and others. First we highlight the helical chirality induced as binding occurs in solution, which includes the induction of helicity by chiral guests in oligomeric and polymeric foldamers, the formation of diastereomeric complexes between chiral foldamer hosts and guests, and the induction of helical chirality by chiral guests into inherently flexible backbones. In addition, molecular or ion-pair guests can produce supramolecular helical chirality in the organogel state. Such structures exhibit remarkable time-dependence and a "Sergeants and Soldiers" effect that are not observed for other two-component organogels that have been reported. We further illustrate that the reversible folding behavior of an aromatic amide foldamer segment can modulate the switching behavior of donor-acceptor interaction-based [2]rotaxanes. Finally we show that a folded oligomer can induce folding in one or two attached intrinsically flexible oligomers, an example of a solvent-responsive intramolecular host-guest system.

Dynamic properties of molecular tweezers with a bis(2-hydroxyphenyl)pyrimidine backbone

4,6-Bis(2-hydroxyphenyl)-2-alkylpyrimidines with two anthryl or 9-ethylnylanthryl substituents at the positions para to the OH groups prefer a U-shaped conformation supported by two intramolecular OH⋅⋅⋅N hydrogen bonds in the solid state and in CDCl3 solution. The compound with a hexyl substituent on the pyrimidine group and two 9-ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7-trinitrofluorenone. Its association constant K(a) was estimated to be 2100 M(-1) at 298 K, which is larger than those of other molecular tweezers (K(a) < 1000 M(-1)). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π-π interaction between the anthryl groups and 2,4,7-trinitrofluorenone. Addition of nBu4NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH⋅⋅⋅N hydrogen bonds, formation of new O⋅⋅⋅HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6-bis(2-hydroxyphenyl)-2-methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ϕ<0.01), owing to photoinduced electron transfer of the molecule with a U-shaped structure. However, the O-hexylated compound exhibits emission from the anthryl groups with ϕ=0.39.

Interactions in supramolecular complexes involving arenes: experimental studies

The process of learning by doing has fueled supramolecular chemistry and, more specifically, the understanding of noncovalent aromatic interactions in synthetic and natural systems. The preparation of new host molecules and the investigation of their complexations have produced many insights into significant noncovalent binding mechanisms. In this Account, we attempt to discuss significant binding contributions involving aromatic units and their practical applications. We use typical examples from our group and the literature, but this Account is not a comprehensive view of the field. Other than systems with saturated frameworks, host compounds based on arenes offer better controlled conformations and active interactions with many guest molecules. Because of their fluorescent properties, larger aryl systems are particularly suitable for sensors. The noncovalent interactions observed with different supramolecular complexes can be compared and exploited for interactions with biopolymers such as nucleic acids. Complexes formed with cyclophanes have been a constant source of inspiration for understanding noncovalent forces and their use for the design of functional supramolecular systems. Other than cyclodextrins or ionophores, which occur in nature, arene-based macrocycles are synthetic and provide more opportunities for structural variations than other macrocycles. These derivatives allow researchers to study and to exploit an unusually broad variety of binding mechanisms in both aqueous and organic media. Systematic analyses of complexes with different substituents and structures in solution, based also on flat aromatic systems such as porphyrins, can lead to a consistent picture of the noncovalent forces that dominate in these systems. These studies have elucidated attractive interactions between many heteroatoms and π systems including cyclopropanes . Through systematic analysis of the equilibrium measurements one can derive binding free energy increments for different interactions. The increments are usually additive and provide predictive tools for the design of new supramolecular systems, benchmarks for computational approaches, and an aid for drug design. In aqueous media, the major noncovalent forces between different aryl systems or between arenes and heteroatoms of larger polarizibility are dispersive, and hydrophobic forces play a minor role. In several examples, we show that electrostatic forces also contribute significantly if donor and acceptor groups show complimentarity. In early investigations, researchers found cation-π and, to a lesser degree, anion-π interactions with several cyclophanes in systems where the host or the guest molecules bear charges in an orientation that facilitates contact between charged and aryl portions of the molecules. In supramolecular complexes, hydrogen bonding effects are usually only visible in apolar media, but very strong acceptors such as phenolate anions can also work in water. To facilitate potential applications, researchers have primarily developed water-soluble, arene-containing receptors through the implementation of permanent charges. Supramolecular complexes that mimic enzymes can also rely on aryl interactions. Examples in this Account illustrate that the conformation of host-guest complexes may differ significantly between the solid and solution state, and suitable spectroscopic methods are needed to observe and control these conformations.

Aromatic interactions by molecular tweezers and clips in chemical and biological systems

Noncovalent interactions involving aromatic rings, such as π-stacking and CH-π, occur throughout a range of fundamental processes including self-assembly and (bio)catalysis. Molecular clips and tweezers possess a central parallel or torus-shaped cavity with a surrounding belt of convergent aromatic rings; hence these structures exploit multiple aromatic interactions in a positively cooperative manner. Both clips and tweezers demonstrate selective binding of cationic or neutral guests that bear acceptor groups. The electrostatic surface potentials (ESP) explain this unexpected behavior: calculated ESPs were highly negative inside the tweezer or clip cavity, providing complementary profiles to the positive ESP plots of their preferred guest molecules. This Account presents more complex systems that use aromatic clips and tweezers to alter the reactivities of included guest species, to distinguish between guest enantiomers, and to interfere with biological processes such as enzymatic activity and protein aggregation. Napthalene tweezers show potential applications in organocatalysis. When pyridinium moieties are bound within the spacious cavity of naphthyl-spaced tweezers, the resulting complex significantly influences the first step of single-electron reductions of (bi)pyridinium salts. In addition, the environment within the tweezer cavity strongly accelerates the Menshutkin reaction (the alkylation of pyridine derivatives). Introduction of phosphonate, phosphate, or sulfate anions into the central aromatic bridge renders clips and tweezers water-soluble. Larger systems form extremely tight intertwined dimers that rely on the nonclassical hydrophobic effect for their stability. Smaller clips and tweezers with a simple benzene bridge remain monomeric in buffered aqueous solution and display a complementary binding profile. While the clips with parallel sidewalls prefer flat aromatic cations such as pyridinium salts, the torus-shaped tweezers bind to basic amino acids lysine and arginine via a threading process. These mutually exclusive binding modes make water-soluble clips and tweezers valuable tools for probing critical biological interactions with positively charged amino acid side chains and cofactors. Molecular clips and tweezers can be employed for the complete inhibition of dehydrogenases. The clip extracts NAD(+) from its Rossman fold, while the tweezer complexes access strategic lysine residues around the active site. Our new enzyme inhibitors recognize the protein surface and thus offer additional targets for medicinal chemistry. Finally, the ability of molecular tweezers to cap critical lysine residues can be used to interfere with the pathology of protein misfolding diseases such as Alzheimer's disease, because many of them involve noncovalent interactions with these critical residues during their early stages. When the key protein produces a β-sheet-rich nucleus, this structure undergoes spontaneous polymerization into highly toxic oligomers, ultimately leading to mature fibrils. The benzene-spaced phosphate tweezer forms a specific complex with lysine residues 16 and 28 in Aβ42 and thus prevents the formation of misfolded oligomers rich in β-sheets. This entirely new process-specific mechanism that prevents pathologic protein aggregation also operates in many other related amyloidogenic proteins.