Supramolecular Chemistry in Water

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.

Self-assembly of cucurbiturils and cyclodextrins to supramolecular millstones with naphthalene derivatives capable of translocations in the host cavities

Naphthylpyridine derivatives and cavitands form inclusive complexes of different structure and stoichiometry; the guests are capable of translocations within cucurbit[7,8]uril cavities.

Catalytic antioxidants for therapeutic medicine

In this Review, we focus on catalytic antioxidant study based on transition metal complexes, organoselenium compounds, supramolecules and protein scaffolds.

Stackable molecular chairs

Molecular chairs, carrying three amino acids or peptides, stack in an antiparallel fashion to give hexavalent assemblies for bottom-up construction of novel soft materials and therapeutics.

Selective binding of (thio)sulfate and phosphate in water by quaternary ammonium functionalized oligo-ureas

Functionalization of oligo-ureas with quaternary ammonium groups leads to water soluble receptors for selective binding of adenosine phosphates in water.

Unveiling the formation 1 : 2 supramolecular complexes between cucurbit[7]uril and a cationic calix[4]arene derivative

The formation of host–guest complexes between cucurbit[7]uril (CB7) and a tetracationic calix[4]arene derivative in the so-called cone conformation was investigated by ¹H NMR, DOSY NMR, isothermal titration calorimetry and ESI-MS.

Binding orientation and reactivity of alkyl α,ω-dibromides in water-soluble cavitands

Host–guest complexation of α,ω-dibromides showed rabid tumbling conformation on NMR timescales and afforded mono hydroxyl bromides after hydrolysis in D₂O.

A new water-soluble cavitand with deeper guest binding properties

A new water-soluble cavitand captured hydrophobic or amphiphilic molecules in an aqueous solution owing to its deep aromatic pocket.

Supramolecular Packing and Macroscopic Alignment Controls Actuation Speed in Macroscopic Strings of Molecular Motor Amphiphiles

Three-dimensional organized unidirectionally aligned and responsive supramolecular structures have much potential in adaptive materials ranging from biomedical components to soft actuator systems. However, to control the supramolecular structure of these stimuli responsive, for example photoactive, materials and control their actuation remains a major challenge. Toward the design of “artificial muscles”, herein, we demonstrate an approach that allows hierarchical control of the supramolecular structure, and as a consequence its photoactuation function, by electrostatic interaction between motor amphiphiles (MA) and counterions. Detailed insight into the effect of various ions on structural parameters for self-assembly from nano- to micrometer scale in water including nanofiber formation and nanofiber aggregation as well as the packing structure, degree of alignment, and actuation speed of the macroscopic MA strings prepared from various metal chlorides solution, as determined by electronic microscopy, X-ray diffraction, and actuation speed measurements, is presented. Macroscopic MA strings prepared from calcium and magnesium ions provide a high degree of alignment and fast response photoactuation. By the selection of metal ions and chain length of MAs, the macroscopic MA string structure and function can be controlled, demonstrating the potential of generating multiple photoresponsive supramolecular systems from an identical molecular structure.

Native Quercetin as a Chloride Receptor in an Organic Solvent

The binding properties of quercetin toward chloride anions were investigated by means of 1H-NMR, 13C-NMR, and electrospray ionization mass spectrometry (ESI-MS) measurements, as well as computational calculations. The results indicate that quercetin behaves primarily as a ditopic receptor with the binding site of the B ring that exhibits stronger chloride affinity compared to the A ring. However, these sites are stronger receptors than those of catechol and resorcinol because of their conjugation with the carbonyl group located on the C ring. The 1:1 and 1:2 complexation of this flavonoid with Cl− was also supported by ESI mass spectrometry.

Tuning surface-cross-linking of molecularly imprinted cross-linked micelles for molecular recognition in water

Molecular recognition in water is an important challenge in supramolecular chemistry. Surface-core double cross-linking of template-containing surfactant micelles by the click reaction and free radical polymerization yields molecularly imprinted nanoparticles (MINPs) with guest-complementary binding sites. An important property of MINP-based receptors is the surface-cross-linking between the propargyl groups of the surfactants and a diazide cross-linker. Decreasing the number of carbons in between the two azides enhanced the binding affinity of the MINPs, possibly by keeping the imprinted binding site more open prior to the guest binding. The depth of the binding pocket can be controlled by the distribution of the hydrophilic/hydrophobic groups of the template and was found to influence the binding in addition to electrostatic interactions between oppositely charged MINPs and guests. Cross-linkers with an alkoxyamine group enabled two-stage double surface-cross-linking that strengthened the binding constants by an order of magnitude, possibly by expanding the binding pocket of the MINP into the polar region. The binding selectivity among very similar isomeric structures also improved.

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.

Sequence-Selective Recognition of Peptides in Aqueous Solution: A Supramolecular Approach through Micellar Imprinting

Sequence-selective recognition of peptides in water has been one of the most important and yet unsolved problems in bioorganic and supramolecular chemistry. The motivation comes from not only the importance of these molecules in biology but also the fundamental challenges involved in the research. Molecular imprinting in doubly cross-linked surfactant micelles offers a unique solution to this problem by creating a "supramolecular code" on the micelle surface that matches the structural features of the peptide chain. Hydrophobic "dimples" are constructed on imprinted micelles that match the hydrophobic side chains of the peptide precisely in size and shape. Polar binding functionalities are installed at correct positions to interact with specific acidic and basic groups on the peptide. Secondary hydrogen-bonding and electrostatic interactions are introduced through imprinting to enhance the binding affinity and specificity further. Binding affinities of tens of nanomolar are readily achieved in water for biological peptides with over a dozen residues. Excellent binding selectivity is observed even for subtly different peptides. The synthesis of these protein-sized nanoparticles involves a one-pot reaction complete within 2 days; purification requires nothing but precipitation and solvent washing. These features make the molecularly imprinted nanoparticles (MINPs) highly promising peptide-binding "artificial antibodies" for chemical and biological applications.

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.

Surface Ligands in the Imprinting and Binding of Molecularly Imprinted Cross-Linked Micelles

Molecular recognition in water is challenging but water-soluble molecularly imprinted nanoparticle (MINP) receptors were produced readily by double cross-linking of surfactant micelles in the presence of suitable template molecules. When the micellar surface was decorated with different polyhydroxylated ligands, significant interactions could be introduced between the surface ligands and the template. Flexible surface ligands worked better than rigid ones to interact with the polar moiety of the template, especially for those template molecules whose water-exposed surface is not properly solvated by water. The importance of these hydrophilic interactions was examined in the context of different substrates, density of the surface ligands, and surface-cross-linking density of the MINP. Together with the hydrophobic interactions in the core, the surface hydrophilic interactions can be used to enhance the binding of guest molecules in water.

Strategies for sensing neurotransmitters with responsive MRI contrast agents

A great deal of research involving multidisciplinary approaches is currently dedicated to the understanding of brain function. The complexity of physiological processes that underlie neural activity is the greatest hurdle to faster advances. Among imaging techniques, MRI has great potential to enable mapping of neural events with excellent specificity, spatiotemporal resolution and unlimited tissue penetration depth. To this end, molecular imaging approaches using neurotransmitter-sensitive MRI agents have appeared recently to study neuronal activity, along with the first successful in vivo MRI studies. Here, we review the pioneering steps in the development of molecular MRI methods that could allow functional imaging of the brain by sensing the neurotransmitter activity directly. We provide a brief overview of other imaging and analytical methods to detect neurotransmitter activity, and describe the approaches to sense neurotransmitters by means of molecular MRI agents. Based on these initial steps, further progress in probe chemistry and the emergence of innovative imaging methods to directly monitor neurotransmitters can be envisaged.

Supramolecular Pharmaceutical Sciences: A Novel Concept Combining Pharmaceutical Sciences and Supramolecular Chemistry with a Focus on Cyclodextrin-Based Supermolecules

Supramolecular chemistry is an extremely useful and important domain for understanding pharmaceutical sciences because various physiological reactions and drug activities are based on supramolecular chemistry. However, it is not a major domain in the pharmaceutical field. In this review, we propose a new concept in pharmaceutical sciences termed "supramolecular pharmaceutical sciences," which combines pharmaceutical sciences and supramolecular chemistry. This concept could be useful for developing new ideas, methods, hypotheses, strategies, materials, and mechanisms in pharmaceutical sciences. Herein, we focus on cyclodextrin (CyD)-based supermolecules, because CyDs have been used not only as pharmaceutical excipients or active pharmaceutical ingredients but also as components of supermolecules.

Selective Binding of Folic Acid and Derivatives by Imprinted Nanoparticle Receptors in Water

Folate receptors are overexpressed on cancer cells and frequently used for targeted delivery. Creation of synthetic receptors to bind folic acid and its analogues in water, however, is challenging because of its complex hydrogen-bonding patterns and competition for hydrogen bonds from the solvent. Micellar imprinting within cross-linkable surfactants circumvented these problems because the nonpolar micellar environment strengthened the hydrogen bonds between the amide group in the surfactant and the template molecule. Incorporation of polymerizable thiouronium functional monomers further enhanced the binding through hydrogen-bond-reinforced ion pairs with the glutamate moiety of the template. The resulting imprinted micelles were able to bind folate and their analogues with submicromolar affinity and distinguish small changes in the hydrogen-bonding patterns as well as the number/position of carboxylic acids. The binding constant obtained was 2-3 orders of magnitude higher than those reported for small-molecule synthetic receptors. Our binding study also revealed interesting details in the binding. For example, the relative contributions of different segments of the molecule to the binding followed the order of carboxylates > pyrimidine ring > pyrazine ring.

The aqueous supramolecular chemistry of cucurbit[n]urils, pillar[n]arenes and deep-cavity cavitands

This tutorial review summarizes the continuing exploration of three prominent water-soluble hosts: cucurbiturils, pillar[n]arenes and deep-cavity cavitands. As we describe, these hosts are revealing how orchestrating the hydrophobic effect can lead to a broad range of properties and applications, from: nano-reactors, supramolecular polymers, stimuli-responsive biointerfaces, switches, and novel purification devices. We also describe how their study is also revealing more details about the properties of water and aqueous solutions.

Aerogen bonds formed between AeOF2 (Ae = Kr, Xe) and diazines: comparisons between σ-hole and π-hole complexes

The interaction between KrOF₂ or XeOF₂ and the 1,2, 1,3, and 1,4 diazines is characterized chiefly by a Kr/XeN aerogen bond, as deduced from ab initio calculations. The most stable dimers take advantage of the σ-hole on the aerogen atom, wherein the two molecules lie in the same plane. The interaction is quite strong, as much as 18 kcal mol^-1. A second class of dimer geometry utilizes the π-hole above the aerogen atom in an approximate perpendicular arrangement of the two monomers; these structures are not as strongly bound: 6-8 kcal mol^-1. Both sorts of dimers contain auxiliary CHF H-bonds which contribute to their stability, but even with their removal, the aerogen bond energy remains as high as 14 kcal mol^-1. The nature and strength of each specific interaction is confirmed and quantified by AIM, NCI, NBO, and electron density shift patterns. There is not a great deal of sensitivity to the identity of either the aerogen atom or the position of the two N atoms in the diazine.

Programmable self-assembly of water-soluble organo-heterometallic cages [M12M'4L12] using 3-(3,5-dimethyl-1H-pyrazol-4-yl)pentane-2,4-dione (H2L)

A bifunctional ligand H₂L featuring primary (pyrazole) and secondary (acetylacetone) coordination sites was preferentially reacted with dimetallic [M₂(NO₃)₂](NO₃)₂ linkers at the pyrazolyl end of H₂L, giving rise to dimetallic corners. Subsequently, the corners serve as the secondary site with M' to form water-soluble organo-heterometallic [M₁₂M'₄L₁₂] cages in a stepwise mode.

Catalytic signal amplification for the discrimination of ATP and ADP using functionalised gold nanoparticles

Diagnostic assays that incorporate a signal amplification mechanism permit the detection of analytes with enhanced selectivity. Herein, we report a gold nanoparticle-based chemical system able to differentiate ATP from ADP by means of catalytic signal amplification. The discrimination between ATP and ADP is of relevance for the development of universal assays for the detection of enzymes which consume ATP. For example, protein kinases are a class of enzymes critical for the regulation of cellular functions, and act to modulate the activity of other proteins by transphosphorylation, transferring a phosphate group from ATP to give ADP as a byproduct. The system described here exploits the ability of cooperative catalytic head groups on gold nanoparticles to very efficiently catalyze chromogenic reactions such as the transphosphorylation of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP). A series of chromogenic substrates have been synthesized and evaluated by means of Michaelis-Menten kinetics (compounds 2, 4-6). 2-Hydroxypropyl-(3-trifluoromethyl-4-nitro)phenyl phosphate (5) was found to display higher reactivity (kcat) and higher binding affinity (KM) when compared to HPNPP. This higher binding affinity allows phosphate 5 to compete with ATP and ADP to different extents for binding on the monolayer surface, thus enabling a catalytically amplified signal only when ATP is absent. Overall, this represents a viable new approach for monitoring the conversion of ATP into ADP with high sensitivity.

Water-Soluble Nanoparticle Receptors Supramolecularly Coded for Acidic Peptides

Sequence-specific recognition of peptides is of enormous importance to many chemical and biological applications, but has been difficult to achieve due to the minute differences in the side chains of amino acids. Acidic peptides are known to play important roles in cell growth and gene expression. In this work, we report molecularly imprinted micelles coded with molecular recognition information for the acidic and hydrophobic side chains of acidic peptides. The imprinted receptors could distinguish acidic amino acids from other polar and nonpolar amino acids, with dissociation constants of tens of nanomolar for biologically active peptides containing up to 18 amino acids.

Maltodextrin recognition by a macrocyclic synthetic lectin

This carbohydrate receptor achieves high affinities in water and shows an unusual preference for α-linked maltodextrins.

Ion-pair electrostatic attraction-enhanced donor–acceptor interactions between the prototypic 1,4-dialkoxybenzene-viologen binding mode in water

Intermolecular ion-pair electrostatic attraction was demonstrated to remarkably enhance the donor–acceptor interaction between prototypic 1,4-dialkoxybenzene and viologen in water.