Dual-Cavity Basket Promotes Encapsulation in Water in an Allosteric Fashion

A Cages-on-Cluster Structure Constructed by Post-Clustering Covalent Modifications and Guest-Enabled Stimuli-Responsive Luminescence

A gold(I)-cluster-based twin-cage has been constructed by post-clustering covalent modification of a hexa-aldehyde cluster precursor with triaminotriethylamines. The cages-on-cluster structure has double cavities and four binding sites, which show site-discriminative binding for silver(I) and copper(I) guests. The guests in the tripodal hats affect the luminescence of the cluster: the tetra-silver(I) host-guest complex is weakly red-emissive, while the bis-copper(I)-bis-silver(I) one is non-emissive but is a stimuli-responsive supramolecule. The copper(I) ion inside the tri-imine cavity is oxidation sensitive, which enables the release of the bright emissive precursor cluster triggered by H2O2 solution. The hybridization of a cluster with cavities to construct a cluster-based cage presents an innovative concept for functional cluster design, and the post-clustering covalent modification opens up new avenues for finely tuning the properties of clusters.

Supramolecular Detection of a Sub-ppm Nerve Agent Simulant by a Smartphone Tool

The widespread use of smartphones and related tools is extending their applications in several fields. Herein, we report a reusable smartphone coupled portable detection system for the sensing of sub-ppm level of a nerve agent mimic (dimethylmethylphosphonate) in the gas phase. The detection system is based on multiple hydrogen-bond interactions of the vapor analyte with an ad-hoc functionalized Bodipy chromophore scaffold. The multitopic approach used for the molecular recognition of DMMP leads to the highest binding constant values, high selectivity, and low limits of detection.

Dissipative Formation of Covalent Basket Cages

Living systems use chemical fuels to transiently assemble functional structures. As a step toward constructing abiotic mimics of such structures, we herein describe dissipative formation of covalent basket cage CBC 5 by reversible imine condensation of cup-shaped aldehyde 2 (i.e., basket) with trivalent aromatic amine 4. This nanosized [4+4] cage (V=5 nm3 , Mw =6150 Da) has shape of a truncated tetrahedron with four baskets at its vertices and four aromatic amines forming the faces. Importantly, tris-aldehyde basket 2 and aliphatic tris-amine 7 undergo condensation to give small [1+1] cage 6. The imine metathesis of 6 and aromatic tris-amine 4 into CBC 5 was optimized to bias the equilibrium favouring 6. Addition of tribromoacetic acid (TBA) as a chemical fuel perturbs this equilibrium to result in the transient formation of CBC 5, with subsequent consumption of TBA via decarboxylation driving the system back to the starting state.

Recent Advances in the Applications of Water-soluble Resorcinarene-based Deep Cavitands

We review here the use of container molecules known as cavitands for performing organic reactions in water. Central to these endeavors are binding forces found in water, and among the strongest of these is the hydrophobic effect. We describe how the hydrophobic effect can be used to drive organic molecule guests into the confined space of cavitand hosts. Other forces participating in guest binding include cation-π interactions, chalcogen bonding and even hydrogen bonding to water involved in the host structure. The reactions of guests take advantage of their contortions in the limited space of the cavitands which enhance macrocyclic and site-selective processes. The cavitands are applied to the removal of organic pollutants from water and to the separation of isomeric guests. Progress is described on maneuvering the containers from stoichiometric participation to roles as catalysts.

A double-decker cage for allosteric encapsulation of ATP

In this work, we describe the preparation of double-decker cage [1-H₆]⁶⁺ comprising two binding pockets, each with three ammonium and three amide hydrogen bonding sites. This novel host possesses a high affinity for trapping two molecules of ATP in an allosteric fashion, with both experiments and theory suggesting the synergistic action of charged hydrogen bonds and π-π stacking in the encapsulation.

Rigidified Cavitand Hosts in Water: Bent Guests, Shape Selectivity, and Encapsulation

We report the synthesis and characterization of two water-soluble container compounds (cavitand hosts) with rigidified open ends. One cavitand uses four (CH₂)₄'s as spacers to bridge the adjacent walls, while another cavitand uses four CH₂CH₂OCH₂CH₂'s bridges and features a wider open end. The spacers preorganize the deep cavitands into vase-like, receptive shapes and prevent their unfolding to the unreceptive kite-like conformation. Cycloalkane guests (C6-C8) and small n-alkanes (C5-C7) form 1:1 complexes with the cavitands and move freely in the cavitands' spaces. Hydrophilic compounds 1,4-dioxane, tetrahydrofuran, tetrahydropyran, pyridine, and 1-methylimidazole also showed good binding affinity to the new cavitands. Longer alkanes (C11-C14) and n-alcohols (C11-C16) are taken up with a -CH₃ group fixed at the bottom of the cavity and the groups near the rim in compressed conformations. The methylene bridges appear to divide the cavitand into a narrow hydrophobic compartment and a broader space with exposure to the aqueous medium. Longer alkane guests (C15-C18), N,N-dimethyldioctylammonium, and dioctylamine induce the formation of capsules (2:1 host:guest complexes). The new cavitands showed selectivity for p/m-cresol isomers and xylene isomers. The cavitand with CH₂CH₂OCH₂CH₂ bridges bound long-chain α,ω-diols (C13-C15) and diamines in folded, U-shaped conformations with polar functions exposed to the aqueous medium. It was used to separate o-xylene from its isomers by using simple extraction procedures.

Enantioselective Construction of Modular and Asymmetric Baskets

The precise positioning of functional groups about the inner space of abiotic hosts is a challenging task and of interest for developing more effective receptors and catalysts akin to those found in nature. To address it, we herein report a synthetic methodology for preparing basket-like cavitands comprised of three different aromatics as side arms with orthogonal esters at the rim for further functionalization. First, enantioenriched A (borochloronorbornene), B (iodobromonorbornene), and C (boronorbornene) building blocks were obtained by stereoselective syntheses. Second, consecutive A-to-B and then AB-to-C Suzuki-Miyaura (SM) couplings were optimized to give enantioenriched ABC cavitand as the principal product. The robust synthetic protocol allowed us to prepare (a) an enantioenriched basket with three benzene sides and each holding either tBu, Et, or Me esters, (b) both enantiomers of a so-called "spiral staircase" basket with benzene, naphthalene, and anthracene groups surrounding the inner space, and (c) a photo-responsive basket bearing one anthracene and two benzene arms.

From Selection to Instruction and Back: Competing Conformational Selection and Induced Fit Pathways in Abiotic Hosts

Two limiting cases of molecular recognition, induced fit (IF) and conformational selection (CS), play a central role in allosteric regulation of natural systems. The IF paradigm states that a substrate "instructs" the host to change its shape after complexation, while CS asserts that a guest "selects" the optimal fit from an ensemble of preexisting host conformations. With no studies that quantitatively address the interplay of two limiting pathways in abiotic systems, we herein and for the first time describe the way by which twisted capsule M-1, encompassing two conformers M-1(+) and M-1(-), trap CX₄ (X=Cl, Br) to give CX₄ ⊂M-1(+) and CX₄ ⊂M-1(-), with all four states being in thermal equilibrium. With the assistance of 2D EXSY, we found that CBr₄ would, at its lower concentrations, bind M-1 via a M-1(+)→M-1(-)→CBr₄ ⊂M-1(-) pathway corresponding to conformational selection. For M-1 complexing CCl₄ though, data from 2D EXSY measurements and 1D NMR line-shape analysis suggested that lower CCl₄ concentrations would favor CS while the IF pathway prevailed at higher proportions of the guest. Since CS and IF are not mutually exclusive, we reason that our work sets the stage for characterizing the dynamics of a wide range of already existing hosts to broaden our fundamental understanding of their action. The objective is to master the way in which encapsulation takes place for designing novel and allosteric sequestering agents, catalysts and chemosensors akin to those found in nature.

Supramolecular Sensing of Chemical Warfare Agents

Chemical warfare agents are a class of organic molecules used as chemical weapons due to their high toxicity and lethal effects. For this reason, the fast detection of these compounds in the environment is crucial. Traditional detection methods are based on instrumental techniques, such as mass spectrometry or HPLC, however the use of molecular sensors able to change a detectable property (e. g., luminescence, color, electrical resistance) can be cheaper and faster. Today, molecular sensing of chemical warfare agents is mainly based on the "covalent approach", in which the sensor reacts with the analyte, or on the "supramolecular approach", which involves the formation of non-covalent interactions between the sensor and the analyte. This Review is focused on the recent developments of supramolecular sensors of organophosphorus chemical warfare agents (from 2013). In particular, supramolecular sensors are classified by function of the sensing mechanism: i) Lewis Acids, ii) hydrogen bonds, iii) macrocyclic hosts, iv) multi-topic sensors, v) nanosensors. It is shown how the supramolecular non-covalent approach leads to a reversible sensing and higher selectivity towards the selected analyte respect to other interfering molecules.

Tuning the allosteric sequestration of anticancer drugs for developing cooperative nano-antidotes

Dual-cavity baskets, carrying six γ-aminobutyric acids sequester anticancer anthracyclines in a cooperative manner to be of interest for creating nano-antidotes.

A Dynamic Hydrogen-Bonded Azo-Macrocycle for Precisely Photo-Controlled Molecular Encapsulation and Release

A light-responsive system constructed from hydrogen-bonded azo-macrocycles demonstrates precisely controlled propensity in molecular encapsulation and release process. A significant decrease in the size of the cavity is observed in the course of the E→Z photoisomerization based on the results from DFT calculations and traveling wave ion mobility mass spectrometry. These macrocyclic hosts exhibit a rare 2:1 host-guest stoichiometry and guest-dependent slow or fast exchange on the NMR timescale. With the slow host-guest exchange and switchable shape change of the cavity, quantitative release and capture of bipyridinium guests is achieved with the maximum release of 68 %. This work underscores the importance of slow host-guest exchange on realizing accurate release of organic cations in a stepwise manner under light irradiation. The light-responsive system established here could advance further design of novel photoresponsive molecular switches and mechanically interlocked molecules.

On the encapsulation and assembly of anticancer drugs in a cooperative fashion

In this study, we report the remarkable recognition and assembly characteristics of D 3h symmetric basket 1 6- containing two adjoining and nonpolar cavities with six biocompatible GABA residues at their northern and southern termini. From the results of experimental (1H NMR, fluorescence and UV-Vis spectroscopies) and computational (MM-MC/OPLS3e) investigations, we deduced that hexaanionic 1 6- captured two molecules of anticancer drug doxorubicin 2 + in water and accommodated them in its two deep cavities. The formation of stable 1 6-⊂2 2 2+ (K a = 3 × 1012 M-2) was accompanied by the exceptional homotopic cooperativity (α = 4K 2/K 1 = 112) in which K 1 = 3.2 ± 0.8 × 105 M-1 and K 2 = 9 ± 1 × 106 M-1. Furthermore, bolaamphiphilic 1 6-⊂2 2 2+ assembled into spherical nanoparticles (DLS, cryo-TEM and TEM) possessing 41% drug loading. The preorganization of abiotic receptor 1 6- and its complementarity to 2 + have been proposed to play a part in the positive cooperativity in which ten favorable noncovalent contacts (i.e. hydrogen bonds, salt bridges, C-H···π and π-π contacts) are formed between doxorubicin and the dual-cavity host. In the case of topotecan 3 +, however, the absence of multiple and favorable basket⊂drug interactions resulted in the predominant formation of a binary 1 6- ⊂ 3 + complex (K 1 = 2.12 ± 0.01 × 104 M-1) and the negative homotopic allostery (α ≪ 1). To summarize, our study lays out a roadmap for creating a family of novel, accessible and multivalent hosts capable of complexing anticancer agents in a cooperative manner. As basket⊂drug complexes organize into highly loaded nanoparticles, the reported soft material is amenable to the bottom-up construction of stimuli-responsive nanomedicine capable of effective scavenging and/or delivery of drugs.

Multivalent and Photoresponsive Assembly of Dual-Cavity Baskets in Water

Large unilamellar vesicles [1]_n , composed of bolaamphiphilic baskets 1, were found to complex photoresponsive guest 3 and divalent 4 to, respectively, give stable vesicular assemblies [1₂ -3]_n and [1₂ -4₃ ]_n . With the assistance of ¹ H NMR spectroscopy, electron microscopy, and dynamic light scattering, it was deduced that [1₂ -3]_n vesicles comprise ternary [1₂ -3] organized into a curved membrane in which a pair of baskets entraps a laterally positioned dicationic 3. In the case of [1₂ -4₃ ]_n vesicles, however, the spectroscopic results suggest that three guest molecules 4 insert vertically between four baskets 1 to give pentanary [1₂ -4₃ ] packed into the membrane of [1₂ -4₃ ]_n . Importantly, nanostructured [1₂ -3]_n and [1₂ -4₃ ]_n retain rhodamine B (RhB) in their reservoir (fluorescence microscopy) and can be switched from one into another using UV light, with a disproportionate release of RhB dye. The reported complexes, organized into photoresponsive capsular materials, are rather unprecedented, demonstrating the potential of multivalency for creating functional structures of great interest in the areas of catalysis and delivery.

Assembly and Folding of Twisted Baskets in Organic Solvents

A synthetic method for obtaining enantiopure and twisted baskets of type (P)-3 is described. These chiral cavitands were found to fold quinoline gates, at the rim of their twisted platform, in acetonitrile and give molecular capsules that assemble into large unilamellar vesicles. In a less polar dichloromethane, however, cup-shaped (P)-3 packed into vesicles but with the quinoline gates in an unfolded orientation. The ability of twisted baskets to form functional nanostructured materials could be of interest for building stereoselective sensors and catalysts.

Applications of isothermal titration calorimetry - the research and technical developments from 2011 to 2015

Isothermal titration calorimetry is a widely used biophysical technique for studying the formation or dissociation of molecular complexes. Over the last 5 years, much work has been published on the interpretation of isothermal titration calorimetry (ITC) data for single binding and multiple binding sites. As over 80% of ITC papers are on macromolecules of biological origin, this interpretation is challenging. Some researchers have attempted to link the thermodynamics constants to events at the molecular level. This review highlights work carried out using binding sites characterized using x-ray crystallography techniques that allow speculation about individual bond formation and the displacement of individual water molecules during ligand binding and link these events to the thermodynamic constants for binding. The review also considers research conducted with synthetic binding partners where specific binding events like anion-π and π-π interactions were studied. The revival of assays that enable both thermodynamic and kinetic information to be collected from ITC data is highlighted. Lastly, published criticism of ITC research from a physical chemistry perspective is appraised and practical advice provided for researchers unfamiliar with thermodynamics and its interpretation. Copyright © 2016 John Wiley & Sons, Ltd.

Efficiency parameters in artificial allosteric systems

It is shown that the conformational energy ΔG_C required for the formation of a suitable cavity or cleft for ligand binding in absence of an effector E is the essential factor for the efficiency of synthetic allosteric systems with positive cooperativity.