Water-Templated Transmembrane Nanopores from Shape-Persistent Oligocholate Macrocycles

Synthetic Receptors Based on Abiotic Cyclo(pseudo)peptides

Work on the use of cyclic peptides or pseudopeptides as synthetic receptors started even before the field of supramolecular chemistry was firmly established. Research initially focused on the development of synthetic ionophores and involved the use of macrocycles with a repeating sequence of subunits along the ring to facilitate the correlation between structure, conformation, and binding properties. Later, nonnatural amino acids as building blocks were also considered. With growing research in this area, cyclopeptides and related macrocycles developed into an important and structurally diverse receptor family. This review provides an overview of these developments, starting from the early years. The presented systems are classified according to characteristic structural elements present along the ring. Wherever possible, structural aspects are correlated with binding properties to illustrate how natural or nonnatural amino acids affect binding properties.

Protonation-induced self-assembly of bis-phenanthroline macrocycles into nanofibers arrayed with tetrachloroaurate, hexachloroplatinate or phosphomolybdate ions

One-dimensional self-assembly of macrocycles is one of the important strategies for constructing fibrous nanomaterials with anisotropic functions such as one-dimensional transport and accumulation of molecules and ions. Herein we report on the synthesis and properties of self-assembled nanofibers using macrocycles to develop a multipurpose template for one-dimensional array of noble metal ions. The nanofibers were prepared by protonation-induced self-assembly of bis-phenanthroline macrocycles, which have enabled the accumulation of some metal-containing anions, such as tetrachloroaurate, hexachloroplatinate and phosphomolybdate. Microscopic observations have demonstrated that the supramolecular nanofibers were reproducibly formed in a similar way, regardless of the structures and charge numbers of the anions. Moreover, the resulting nanofibers, arrayed with several metal ions, were chemically reduced, producing dispersible gold nanoparticles and mixed-valence nanofibers.

Light-controlled switchable complexation by a non-photoresponsive hydrogen-bonded amide macrocycle

A light controlled switchable host–guest system based on a non-photoresponsive H-bonded macrocycle and pyridinium salts was developed using a photoacid.

A synthetic channel that efficiently inserts into mammalian cell membranes and destroys cancer cells

A tubular molecule with terminal positively charged amino groups that displays a strong ability to insert into the membrane of mammalian cells.

Self-organizing p-quinquephenyl building blocks incorporating lateral hydroxyl and methoxyl groups into supramolecular nano-assemblies

The self-assembling behavior of coil-rod-coil molecules 1a, 1b, and 2a, 2b was investigated using DSC, POM, SAXS, and AFM in bulk and aqueous solutions. These molecules contain p-quinquephenyl groups as rod segments incorporating lateral hydroxyl or methoxyl groups in the center positions and oligo(ethylene oxide)s as the coil segments. Molecules 1a and 1b, with lateral methoxyl groups in the rod segments, self-assemble into oblique columnar structures in the crystalline phase and transform into nematic phases. On the other hand, molecules 2a and 2b, with hydroxyl groups in the center of their rod segments, self-organize into hexagonal perforated lamellar and oblique columnar nano-structures in the crystalline and liquid crystalline phase, respectively. In aqueous solutions, these molecules aggregate into nano-ribbons and vesicles, depending on their lateral groups and oligo(ethylene oxide) chain lengths. These results imply that the lateral methoxyl or hydroxyl groups, present in the center of the rod segments, significantly influence the formation of various supramolecular nano-structures in the bulk state and in aqueous solution. This is achieved via tuning of the non-covalent interactions of the rod building blocks.

Conformationally switchable water-soluble fluorescent bischolate foldamers as membrane-curvature sensors

Membrane curvature is an important parameter in biological processes such as cellular movement, division, and vesicle fusion and budding. Traditionally, only proteins and protein-derived peptides have been used as sensors for membrane curvature. Three water-soluble bischolate foldamers were synthesized, all labeled with an environmentally sensitive fluorophore to report their binding with lipid membranes. The orientation and ionic nature of the fluorescent label were found to be particularly important in their performance as membrane-curvature sensors. The bischolate with an NBD group in the hydrophilic α-face of the cholate outperformed the other two analogues as a membrane-curvature sensor and responded additionally to the lipid composition including the amounts of cholesterol and anionic lipids in the membranes.

Cholate-based synthesis of size-tunable cage compounds

We describe cholate-based cage amphiphiles with a unique architecture that combines elements of structural rigidity and flexibility. The cage compounds are built by extending and bridging three polar chains underneath the concave steroid rings of cholate and capping with another rigid, symmetrically trifunctionalized cyanuric acid moiety. The connecting chains are varied to include, for instance, oligo(ethylene glycol) or chains containing 1,2,3-triazole units to present flexibility in the chemical and structural space and potentially deliver functional molecules for molecular recognition applications.

Investigation of hydrogen bonding patterns in a series of multi-component molecular solids formed by tetrabromoterephthalic acid with selected N-heterocycles

Hydrogen bonding patterns in a series of multi-component molecules constructed by tetrabromoterephthalic acid with N-heterocycles are discussed in this study.

Conformationally controlled oligocholate membrane transporters: learning through water play

Controlled translocation of molecules and ions across lipid membranes is the basis of numerous biological functions. Because synthetic systems can help researchers understand the more complex biological ones, many chemists have developed synthetic mimics of biological transporters. Both systems need to deal with similar fundamental challenges. In addition to providing mechanistic insights into transport mechanisms, synthetic transporters are useful in a number of applications including separation, sensing, drug delivery, and catalysis. In this Account, we present several classes of membrane transporters constructed in our laboratory from a facially amphiphilic building block, cholic acid. Our "molecular baskets" can selectively shuttle glucose across lipid membranes without transporting smaller sodium ions. We have also built oligocholate foldamers that transiently fold into helices with internal hydrophilic binding pockets to transport polar guests. Lastly, we describe amphiphilic macrocycles, which form transmembrane nanopores in lipid bilayers through the strong associative interactions of encapsulated water molecules. In addition to presenting the different transport properties of these oligocholate transporters, we illustrate how fundamental studies of molecular behavior in solution facilitate the creation of new and useful membrane transporters, despite the large difference between the two environments. We highlight the strong conformational effect of transporters. Because the conformation of a molecule often alters its size and shape, and the distribution of functional groups, conformational control can be used rationally to tune the property of a transporter. Finally, we emphasize that, whenever water is the solvent, its unique properties--small size, strong solvation for ionic functionalities, and an extraordinary cohesive energy density (i.e., total intermolecular interactions per unit volume)--tend to become critical factors to be considered. Purposeful exploitation of these solvent properties may be essential to the success of the supramolecular process involved--this is also the reason for the "learning through water play" in the title of this Account.

Supramolecular self-assemblies as functional nanomaterials

In this review, we survey the diversity of structures and functions which are encountered in advanced self-assembled nanomaterials. We highlight their flourishing implementations in three active domains of applications: biomedical sciences, information technologies, and environmental sciences. Our main objective is to provide the reader with a concise and straightforward entry to this broad field by selecting the most recent and important research articles, supported by some more comprehensive reviews to introduce each topic. Overall, this compilation illustrates how, based on the rules of supramolecular chemistry, the bottom-up approach to design functional objects at the nanoscale is currently producing highly sophisticated materials oriented towards a growing number of applications with high societal impact.

Aggregation and dynamics of oligocholate transporters in phospholipid bilayers revealed by solid-state NMR spectroscopy

Macrocycles made of cholate building blocks were previously found to transport glucose readily across lipid bilayers. In this study, an (15)N, (13)Cα-labeled glycine was inserted into a cyclic cholate trimer and attached at the end of a linear trimer, respectively. The isotopic labeling allowed us to use solid-state NMR spectroscopy to study the dynamics, aggregation, and depth of insertion of these compounds in lipid membranes. The cyclic compound was found to be mostly immobilized in DLPC, POPC/POPG, and POPC/POPG/cholesterol membranes, whereas the linear trimer displayed large-amplitude motion that depended on the membrane thickness and viscosity. (13)C-detected (1)H spin diffusion experiments revealed the depth of insertion of the compounds in the membranes, as well as their contact with water molecules. The data support a consistent stacking model for the cholate macrocycles in lipid membranes, driven by the hydrophobic interactions of the water molecules in the interior of the macrocycles. The study also shows a strong preference of the linear trimer for the membrane surface, consistent with its lack of transport activity in earlier liposome leakage assays.

Ion permeation by a folded multiblock amphiphilic oligomer achieved by hierarchical construction of self-assembled nanopores

A multiblock amphiphilic molecule 1, with a tetrameric alternating sequence of hydrophilic and hydrophobic units, adopts a folded structure in a liposomal membrane like a multipass transmembrane protein, and is able to transport alkali metal cations through the membrane. Hill's analysis and conductance measurements, analyzed by the Hille equation, revealed that the tetrameric assembly of 1 forms a 0.53 nm channel allowing for permeation of cations. Since neither 3, bearing flexible hydrophobic units and forming no stacked structures in the membrane, nor 2, a monomeric version of 1, is able to transport cations, the folded conformation of 1 in the membrane is likely essential for realizing its function. Thus, function and hierarchically formed higher-order structures of 1, is strongly correlated with each other like proteins and other biological macromolecules.

Effects of amphiphile topology on the aggregation of oligocholates in lipid membranes: macrocyclic versus linear amphiphiles

A macrocyclic and a linear trimer of a facially amphiphilic cholate building block were labeled with a fluorescent dansyl group. The environmentally sensitive fluorophore enabled the aggregation of the two oligocholates in lipid membranes to be studied by fluorescence spectroscopy. Concentration-dependent emission wavelength and intensity revealed a higher concentration of water for the cyclic compound. Both compounds were shown by the red-edge excitation shift (REES) to be located near the membrane/water interface at low concentrations, but the cyclic trimer was better able to migrate into the hydrophobic core of the membrane than the linear trimer. Fluorescent quenching by a water-soluble (NaI) and a lipid-soluble (TEMPO) quencher indicated that the cyclic trimer penetrated into the hydrophobic region of the membrane more readily than the linear trimer, which preferred to stay close to the membrane surface. The fluorescent data corroborated with the previous leakage assays that suggested the stacking of the macrocyclic cholate trimer into transmembrane nanopores, driven by the strong associative interactions of water molecules inside the macrocycles in a nonpolar environment.

Translocation of hydrophilic molecules across lipid bilayers by salt-bridged oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141-147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol% cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into "noncovalent macrocycles" by the guanidinium-carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.