Self-assembly and proton conductance of organic nanotubes from pillar[5]arenes

Transmembrane Ion Channels: From Natural to Artificial Systems

Natural channel proteins allow the selective permeation of ions, water or other nutritious entities across bilayer membranes, facilitating various essential physiological functions in living systems. Inspired by nature, chemists endeavor to simulate the structural features and transport behaviors of channel proteins through biomimetic strategies. In this review, we start from introducing the inherent traits of channel proteins such as their crystal structures, functions and mechanisms. Subsequently, different kind of synthetic ion channels including their design principles, dynamic regulations and therapeutic applications were carefully reviewed. Finally, the potential challenges and opportunities in this research field were also carefully discussed. It is anticipated that this review could provide some inspiring ideas and future directions towards the construction of novel bionic ion channels with higher-level structures, properties, functions and practical applications.

Combinatorial Screening of Water/Proton Permeation of Self-Assembled Pillar[5]arene Artificial Water Channel Libraries

Artificial water channels (AWCs) that selectively transport water and reject ions through bilayer membranes have potential to act as synthetic Aquaporins (AQPs). AWCs can have a similar osmotic permeability, better stability, with simpler manufacture on a larger-scale and have higher functional density and surface permeability when inserted into the membrane. Here, we report the screening of combinatorial libraries of symmetrical and unsymmetrical rim-functionalized PAs A-D that are able to transport ca. 107 -108 water molecules/s/channel, which is within 1 order of magnitude of AQPs' and show total ion and proton rejection. Among the four channels, C and D are 3-4 times more water permeable than A and B when inserted in bilayer membranes. The binary combinations of A-D with different molar ratios could be expressed as an independent (linear ABA), a recessive (inhibition AB, AC, DB, ACA), or a dominant (amplification, DBD) behavior of the water net permeation events.

Smart organic materials based on macrocycle hosts

Innovative design of smart organic materials is of great importance for the advancement of modern technology. Macrocycle hosts, possessing cyclic skeletons, intrinsic cavities, and specific guest binding properties, have demonstrated pronounced potential for the elaborate fabrication of a variety of functional organic materials with smart stimuli-responsive characteristics. In this tutorial review, we outline the current development of smart organic materials based on macrocycle hosts as key building blocks, focusing on the design principles and functional mechanisms of the tailored systems. Three main types of macrocycle-based smart organic materials are exemplified as follows according to the distinct forms of construction patterns: (1) supramolecular polymeric materials and nanoassemblies; (2) adaptive molecular crystals; (3) smart porous organic materials. The responsive performances of macrocycle-containing smart materials in versatile aspects, including mechanically adaptive polymers, soft optoelectronic devices, data encryption, drug delivery systems, artificial transmembrane channels, crystalline-state gas adsorption/separation, and fluorescence sensing, are illustrated by discussing the representative studies as paradigms, where the roles of macrocycles in these systems are highlighted. We also provide in the conclusion part the perspectives and remaining challenges in this burgeoning field.

Biomimetic Artificial Proton Channels

One of the most common biochemical processes is the proton transfer through the cell membranes, having significant physiological functions in living organisms. The proton translocation mechanism has been extensively studied; however, mechanistic details of this transport are still needed. During the last decades, the field of artificial proton channels has been in continuous growth, and understanding the phenomena of how confined water and channel components mediate proton dynamics is very important. Thus, proton transfer continues to be an active area of experimental and theoretical investigations, and acquiring insights into the proton transfer mechanism is important as this enlightenment will provide direct applications in several fields. In this review, we present an overview of the development of various artificial proton channels, focusing mostly on their design, self-assembly behavior, proton transport activity performed on bilayer membranes, and comparison with protein proton channels. In the end, we discuss their potential applications as well as future development and perspectives.

High-Performance Biomimetic Water Channel: The Constructive Interplay of Interaction Parameters and Hydrophilic Doping Levels

In this study, we introduce a superfast biomimetic water channel mimicking the hydrophobicity scales of the Aquaporin (AQP) pore lining. Molecular dynamics simulation is used to scrutinize the impact of hydrophilic doping level in the nanotube and the water-wall interaction strength on water permeability. In the designed biomimetic channel, the constructive interplay of Lennard-Jones (LJ) ε parameters and hydrophilic doping levels increased the possibility of ultrafast water transport. Moreover, a unique set of LJ parameters is discovered for each biomimetic channel with different hydrophilic doping levels, enhancing water permeation. Inside high-performance biomimetic channels, water distribution surprisingly implies a varying pore geometry that narrows down in the middle, mimicking the pattern obtained from GplF pore analysis, evoking the narrow pore induced by the aromatic/arginine selectivity filter. This exciting accordance occurred as a result of tailoring specific hydrophilic arrays within the hydrophobic channel backbone by mimicking the AQP pore interior. The main takeaway of hydrophilic doping arrays implanted within the hydrophobic nanotube is to break the large barrier in the water-wall vdW energy profile into multiple reduced ones to increase water conduction. Consequently, the "water jumping" phenomenon in the middle of the biomimetic channel occurs under specific circumstances. The biomimetic channel with the highest value of water permeability of about 13.67 ± 0.66 × 10^-13 cm³·s^-1 exhibits the best mechanism for artificial water channels (AWCs), serving superfast water transport considering the low entrance barrier and weak water-wall interaction.

Bilayer versus Polymeric Artificial Water Channel Membranes: Structural Determinants for Enhanced Filtration Performances

Artificial water channels (AWCs) and their natural aquaporin counterparts selectively transport water. They represent a tremendous source of inspiration to devise biomimetic membranes for several applications, including desalination. They contain variable water-channel constructs with adaptative architectures and morphologies. Herein, we critically discuss the structural details that can impact the performances of biomimetic I quartets, obtained via adaptive self-assembly of alkylureido-ethylimidazoles HC4-HC18 in bilayer or polyamide (PA) membranes. We first explore the performances in bilayer membranes, identifying that hydrophobicity is an essential key parameter to increase water permeability. We compare various I quartets with different hydrophobic tails (from HC4 to HC18), and we reveal that a huge increase in single-channel water permeability, from 10⁴ to 10⁷ water molecules/s/channel, is obtained by increasing the size of the alkyl tail. Quantitative assessment of AWC-PA membranes shows that water permeability increases roughly from 2.09 to 3.85 L m^-2 h^-1 bar^-1, for HC4 and HC6 reverse osmosis membranes, respectively, while maintaining excellent NaCl rejection (99.25-99.51%). Meanwhile, comparable HC8 loading induces a drop of performance reminiscent of a defective membrane formation. We show that the production of nanoscale sponge-like water channels can be obtained with insoluble, low soluble, and low dispersed AWCs, explaining the observed subpar performance. We conclude that optimal solubility enabling breakthrough performance must be considered to not only maximize the inclusion and the stability in the bilayer membranes but also achieve an effective homogeneous distribution of percolated particles that minimizes the defects in hybrid polyamide membranes.

Shape induced sorting via rim-to-rim complementarity in the formation of pillar[5, 6]arene-based supramolecular organogels

The first two-component rim-to-rim pillar[6]arene-based supramolecular organogels were prepared. Shape complementarity was found to be an important determinant in the formation of such gels which also show shape-induced sorting in their formation.

The concept of strongly interacting groups in self-assembly of soft matter

Amphiphilic molecules in solution typically produce structures coming from cooperative interactions of many synergetically acting functional units. If all essential interactions are weak, the structure can be treated theoretically based on a free energy expansion for small interaction parameters. However, most self-assembling soft matter systems involve strong specific interactions of functional units leading to qualitatively new structures of highly soluble micellar or fibrillar aggregates. Here we focus on the systems with the so-called strongly interacting groups (SIGs) incorporated into unimer molecules and discuss the effects of packing frustrations and unimer chirality as well as the origins of spontaneous morphological chirality in the case of achiral unimers. We describe several theoretical approaches (overcoming the limitations of weak interaction models) including the concepts of super-strong segregation, geometrical mismatch and orientational frustration. We also review some recently developed phenomenological theories of surfactant membranes and multiscale hierarchical approaches based on all-atomic modeling of packing structures of amphiphilic molecules with SIGs. In particular, we discuss self-assembling structures in systems possessing simultaneously several distinct types of SIGs: solutions of beta-sheet oligopeptides (showing different fibrillar morphologies), aromatic diamide-ester molecules (forming membranes, helical ribbons and tubules), and triarylamine amide derivatives (producing light-controlled supramolecular nanowires).

Supramolecular gel from self-assembly of a C3-symmetrical discotic molecular bearing pillar[5]arene

A novel C₃-symmetric benzene-1,3,5-tricarboxamide (BTAs) decorated with three identical pillar[5]arene tails was designed, synthesized and characterized. The compound can gelate acetonitrile at low concentration (0.2 wt%) upon sonication at room temperature, but a precipitate was obtained by a conventional heating-cooling process. Scanning electron microscopy revealed that the gel and precipitate were constructed by entangled, high-aspect-ratio flexible bundles of nanofibrils. UV-vis spectroscopy, circular dichroism, Fourier transform infrared microscopy and powder X-ray diffraction showed that the compound formed chiral, elongated, columnar aggregates with nanofiber morphology by a combination of intermolecular hydrogen bonding between the N-H and C[double bond, length as m-dash]O of amides, π-π stacking (H-aggregates) and hydrophobic interactions of peripheral groups.

Bis-15-crown-5-ether-pillar[5]arene K+-Responsive Channels

An artificial selective K⁺ channel is formed from the supramolecular organization on bis(benzo-15-crown-5- ether-ureido)-pillar[5]arene compound. This channel achieves a selectivity of S_K⁺/Na⁺ = 5 for an initial transport rate of k_K⁺ = 3.2 × 10^-3 s^-1. The cation-file diffusion occurs via selective macrocyclic-filters anchored on inactive supporting pillar[5]arene relays. The sandwich-type binding geometry of the K⁺ cation by two 15-crown-5 moieties sites is a key feature influencing channel efficiency.

Solvent-controlled reversible switching between adsorbed self-assembled nanoribbons and nanotubes

We have demonstrated that solutions of 3,5-bis-(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10) can form metastable nanostructures on solid substrates and in the bulk. BHPB-10 is an achiral molecule involving several distinct, strongly interacting groups (SIGs), one aromatic-ester ring and two amide groups per molecule. Specific solvents affect the interactions between particular SIGs, thus promoting various nano-structures: lamellae, nanoribbons, helical ribbons, or nanotubes. In cyclohexane, a solvent allowing for both inter-amide hydrogen bonds and mutual attraction of rings, the formation of nanotubes with a diameter of 28 ± 5 nm was observed in the bulk and on surfaces. By contrast, in cyclohexanone, which suppresses inter-amide hydrogen bonds, flat nanoribbons with a specific width of 12 ± 4 nm were formed on solid substrates after drying. By annealing in cyclohexane vapor, we followed the process of switching surface structures from nanoribbons to nanotubes and observed helical ribbons as the precursor of nanotubes. We also turned nanotubes back into nanoribbons by adding cyclohexanone, thus demonstrating reversible switching along the route: tubes → lamellae → flat ribbons → helical ribbons → tubes. We propose models explaining the observed nanostructures and their transformations, including the origin of spontaneous chirality of the helical ribbons. Our findings on the self-assembly in the achiral BHPB-10 solutions provide insight into the influence of complementary inter-molecular specific SIG-based interactions and demonstrate an effective route for tailoring the shape and size of nanostructures derived from the same building unit.

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as "pillar[n]arenes". Today, pillar[n]arenes are recognized as key players in supramolecular chemistry because of their facile synthesis, unique pillar shape, versatile functionality, interesting host-guest properties, and original supramolecular assembly characteristics, which have resulted in numerous electrochemical and biomedical material applications. In this Review, we have provided historical background to macrocyclic chemistry, followed by a detailed discussion of the fundamental properties of pillar[n]arenes, including their synthesis, structure, and host-guest properties. Furthermore, we have discussed the applications of pillar[n]arenes to materials science, as well as their applications in supramolecular chemistry, in terms of their fundamental properties. Finally, we have described the future perspectives of pillar[n]arene chemistry. We hope that this Review will provide a useful reference for researchers working in the field and inspire discoveries concerning pillar[n]arene chemistry.

Piling Up Pillar[5]arenes To Self-Assemble Nanotubes

New liquid-crystalline pillar[5]arene derivatives have been prepared by grafting first-generation Percec-type poly(benzylether) dendrons onto the macrocyclic scaffold. The molecules adopt a disc-shaped structure perfectly suited for self-organization into a columnar liquid-crystalline phase. In this way, the pillar[5]arene cores are piled up, thus forming a nanotubular wire encased within a shell of peripheral dendrons. The capability of pillar[5]arenes to form inclusion complexes has been also exploited. Specifically, detailed binding studies have been carried out in solution with 1,6-dicyanohexane as the guest. Inclusion complexes have also been prepared in the solid state. Supramolecular organization into the Colh mesophase has been deduced from X-ray diffraction data and found to be similar to that observed within the crystal lattice of a model inclusion complex prepared from 1,4-dimethoxypillar[5]arene and 1,6-dicyanohexane.

A nano-sized container for specific encapsulation of isolated water molecules

This calix[4]arene-based molecular box is able to encapsulate specifically two isolated water molecules in both non-protic and protic solvents.

Tubular Unimolecular Transmembrane Channels: Construction Strategy and Transport Activities

Lipid bilayer membranes separate living cells from their environment. Membrane proteins are responsible for the processing of ion and molecular inputs and exports, sensing stimuli and signals across the bilayers, which may operate in a channel or carrier mechanism. Inspired by these wide-ranging functions of membrane proteins, chemists have made great efforts in constructing synthetic mimics in order to understand the transport mechanisms, create materials for separation, and develop therapeutic agents. Since the report of an alkylated cyclodextrin for transporting Cu(2+) and Co(2+) by Tabushi and co-workers in 1982, chemists have constructed a variety of artificial transmembrane channels by making use of either the multimolecular self-assembly or unimolecular strategy. In the context of the design of unimolecular channels, important advances have been made, including, among others, the tethering of natural gramicidin A or alamethicin and the modification of various macrocycles such as crown ethers, cyclodextrins, calixarenes, and cucurbiturils. Many of these unimolecular channels exhibit high transport ability for metal ions, particularly K(+) and Na(+). Concerning the development of artificial channels based on macrocyclic frameworks, one straightforward and efficient approach is to introduce discrete chains to reinforce their capability to insert into bilayers. Currently, this approach has found the widest applications in the systems of crown ethers and calixarenes. We envisioned that for macrocycle-based unimolecular channels, control of the arrangement of the appended chains in the upward and/or downward direction would favor the insertion of the molecular systems into bilayers, while the introduction of additional interactions among the chains would further stabilize a tubular conformation. Both factors should be helpful for the formation of new efficient channels. In this Account, we discuss our efforts in designing new unimolecular artificial channels from tubular pillar[n]arenes by extending their lengths with various ester, hydrazide, and short peptide chains. We have utilized well-defined pillar[5]arene and pillar[6]arene as rigid frameworks that allow the appended chains to afford extended tubular structures. We demonstrate that the hydrazide and peptide chains form intramolecular N-H···O═C hydrogen bonds that enhance the tubular conformation of the whole molecule. The new pillar[n]arene derivatives have been successfully applied as unimolecular channels for the selective transport of protons, water, and amino acids and the voltage-gated transport of K(+). We also show that aromatic hydrazide helices and macrocycles appended with peptide chains are able to mediate the selective transport of NH4(+).

Solid-state assembly of carboxylic acid substituted pillar[5]arene and its host–guest complex with tetracaine

The first examples of carboxylic acid substituted pillar[5]arene structures in the form of its inclusion complexes with ethanol and the anesthetic drug tetracaine are reported.

Pillar[5]- and pillar[6]arene-based supramolecular assemblies built by using their cavity-size-dependent host–guest interactions

We discuss various pillar[5]- and pillar[6]arene-based supramolecular assemblies built using their host–guest interactions.

Construction of muscle-like metallo-supramolecular polymers from a pillar[5]arene-based [c2]daisy chain

A muscle-like metallo-supramolecular polymer based on a solvent-driven [c2]daisy chain has been prepared from an amino-modified pillar[5]arene. The integration of terpyridine moieties on both ends of the [c2]daisy chain and the stiff architecture of pillar[5]arene units facilitated the efficient formation of the metallo-supramolecular polymer.

A low pH sensor from an esterified pillar[5]arene

An esterified pillar[5]arene, incorporated into a PVC membrane, displays a non-Nernstian response to changes in pH.

An enzyme-responsive supra-amphiphile constructed by pillar[5]arene/acetylcholine molecular recognition

A novel molecular recognition motif between a water-soluble pillar[5]arene (WP5) and acetylcholine is established with an association constant of (5.05 ± 0.13) × 10⁴ M⁻¹.

Pillar[n]arenes (n = 8-10) with two cavities: synthesis, structures and complexing properties

Three pillar[n]arenes (n = 8-10) were synthesized. X-ray analysis demonstrated that, different from early reported small pillar[n]arenes (n = 5, 6), these larger macrocycles gave rise to two cavities. (1)H NMR and MS experiments revealed that pillar[9]arene complexed one n-octyltrimethyl ammonium in chloroform, while pillar[10]arene could complex two.

Theoretical investigations on vibrational spectra of pillar[5]arene-bis(pyridinium) complexes

Electronic structure and vibrational spectra of pillar[5]arene (P5) complexes with bis(pyridinium) derivatives viz., 1,2-ethylenedipyridine (edpy), 1,2-propylenedipyridine (3-pdpy), 1,2-butylenedipyridine (bdpy), 1,2-pentamethylenedipyridine (pdpy) and 1,2-hexamethylenedipyridine (hdpy) are investigated employing density functional theory. B3LYP based density functional calculations predicted that interaction energies for complexation decreases steadily with increasing alkyl chain of bis(pyridinium) guest. The calculations have shown that O-H⋯O hydrogen bonded and non-bonded hydroxyls in the host led to distinct vibrations at the 3515 cm(-1) and 3681 cm(-1), respectively in the vibrational spectra. Complexation of bis(pyridinium) guest engenders frequency down-shift for aromatic C-H(α) vibrations owing to C-H⋯O interactions with P5 portals. Moreover, C-H⋯π interactions are inferred in edpy@P5 and 3-pdpy@P5 complexes which results in frequency up-shift (blue shift) of nearly 22-15 cm(-1) for the corresponding C-H(α) vibration.