A self-assembled nanotube for the direct aldol reaction in water

Chiral Supramolecular Self-Assembly Catalysts with Enhanced Metal Ion Interaction for Higher Enantioselectivity

Understanding the interaction between metal ions as catalytic centers and supramolecular scaffolds as chiral substrates plays an important role in developing chiral supramolecular catalysts with high enantioselectivity. Herein, we found that compared with l-norleucine chiral amphiphile (l-NorC16), l-methionine chiral amphiphile (l-MetC16) with the only heteroatom of S site difference in the hydrophilic group can form a similar supramolecular chiral nanoribbon (NR) with the bilayer structure through the self-assembly approach; yet, the interaction between the Cu(II) ion catalytic centers and supramolecular scaffolds is reinforced, favoring the chirality transfer and therefore enhancing their catalytic enantioselectivity of Diels-Alder reaction from 23% [l-NorC16-NR-Cu(II)] to 78% [l-MetC16-NR-Cu(II)]. Our work demonstrates a new strategy from the perspective of strengthening the metal ion-supramolecular scaffold interaction for the preparation of chiral supramolecular catalysts with good catalytic enantioselectivity.

Porous Organic Nanotubes: Chemistry of One-Dimensional Space

ConspectusOne-dimensional organic nanotubes feature unique properties, such as confined chemical environments and transport channels, which are highly desirable for many applications. Advances in synthetic methods have enabled the creation of different types of organic nanotubes, including supramolecular, hydrogen-bonded, and carbon nanotube analogues. However, challenges associated with chemical and mechanical stability along with difficulties in controlling aspect ratios remain a significant bottleneck. The fascination with structured porous materials has paved the way for the emergence of reticular solids such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and organic cages. Reticular materials with tubular morphology promise architectural stability with the additional benefit of permeant porosity. Despite this, the current synthetic approaches to these reticular nanotubes focus more on structural design resulting in less reliable morphological uniformity. This Account, highlights the design motivation behind various classes of organic nanotubes, emphasizing their porous interior space. We explore the strategic assembly of organic nanotubes based on their bonding characteristics, from weak supramolecular to robust covalent interactions. Special attention is given to reticular nanotubes, which have gained prominence over the past two decades due to their distinctive micro and mesoporous structures. We examine the synergy of covalent and noncovalent interactions in constructing assembly of these nanotube structures.This Account furnishes a comprehensive overview of our efforts and advancements in developing porous covalent organic nanotubes (CONTs). We describe a general synthetic approach for creating robust imine-linked nanotubes based on the reticular chemistry principles. The use of spatially oriented tetratopic triptycene-based amine and linear ditopic aldehyde building blocks facilitates one-dimensional nanotube growth. The interplay between directional covalent bonds and solvophobic interactions is crucial for forming uniform, well-defined, and high aspect ratio nanotubes. The nanotubes derive their permeant porosity and thermal and chemical stability from their covalent architecture. We also highlight the adaptability of our synthetic methodology to guide the transformation of one-dimensional nanotubes to toroidal superstructures and two-dimensional thin fabrics. Such morphological transformation can be directed by tuning the reaction time or incorporating additional intermolecular interactions to control the intertwining behavior of individual nanotubes. The cohesion of covalent and noncovalent interactions in the tubular nanostructures manifests superior viscoelastic mechanical properties in the assembled CONT fabrics. We establish a strong correlation between structural framework design and nanostructures by translating reticular synthesis to morphological space and gaining insights into the assembly processes. We anticipate that the present Account will lay the foundation for exploring new designs and chemistry of organic nanotubes for many application platforms.

Chiral supramolecular polymers

As an active branch within the field of supramolecular polymers, chiral supramolecular polymers (SPs) are an excellent benchmark to generate helical structures that can clarify the origin of homochirality in Nature or help determine new exciting functionalities of organic materials. Herein, we highlight the most utilized strategies to build up chiral SPs by using chiral monomeric units or external stimuli. Selected examples of transfer of asymmetry, in which the point or axial chirality contained by the monomeric units is efficiently transferred to the supramolecular scaffold yielding enantioenriched helical structures, will be presented. The importance of the thermodynamics and kinetics associated with those processes is stressed, especially the influence that parameters such as the helix reversal and mismatch penalties exert on the achievement of amplification of asymmetry in co-assembled systems will also be considered. Remarkable examples of breaking symmetry, in which chiral supramolecular polymers can be attained from achiral self-assembling units by applying external stimuli like stirring, solvent or light, are highlighted. Finally, the specific and promising applications of chiral supramolecular polymers are presented with recent relevant examples.

Anion-(π)n -π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)n -π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)n -π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)n -π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.

Dissipative self-assembly of a proline catalyst for temporal regulation of the aldol reaction

The spatiotemporal regulation of chemical reactivity in biological systems permits a network of metabolic reactions to take place within the same cellular environment. The exquisite control of reactivity is often mediated by out-of-equilibrium structures that remain functional only as long as fuel is present to maintain the higher energy, active state. An important goal in supramolecular chemistry aims to develop functional, energy dissipating systems that approach the sophistication of biological machinery. The challenge is to create strategies that couple the energy consumption needed to promote a molecule to a higher energy, assembled state to a functional property such as catalytic activity. In this work, we demonstrated that the assembly of a spiropyran (SP) dipeptide (1) transiently promoted the proline-catalyzed aldol reaction in water when visible light was present as fuel. The transient catalytic activity emerged from 1 under light illumination due to the photoisomerization of the monomeric, O-protonated (1-MCH⁺) merocyanine form to the spiropyran (1-SP) state, which rapidly assembled into nanosheets capable of catalyzing the aldol reaction in water. When the light source was removed, thermal isomerization to the more stable MCH⁺ form caused the nanosheets to dissociate into a catalytically inactive, monomeric state. Under these conditions, the aldol reaction could be repeatedly activated and deactivated by switching the light source on and off.

Construction of Zn-heptapeptide bionanozymes with intrinsic hydrolase-like activity for degradation of di(2-ethylhexyl) phthalate

Nanozyme with intrinsic enzyme-like activity has emerged as favorite artificial catalyst during recent years. However, current nanozymes are mainly limited to inorganic-derived nanomaterials, while biomolecule-sourced nanozyme (bionanozyme) are rarely reported. Herein, inspired by the basic structure of natural hydrolase family, we constructed 3 oligopeptide-based bionanozymes with intrinsic hydrolase-like activity by implementing zinc induced self-assembly of histidine-rich heptapeptides. Under mild condition, divalent zinc (Zn²⁺) impelled the spontaneous assembly of short peptides (i.e. Ac-IHIHIQI-CONH₂, Ac-IHIHIYI-CONH₂, and Ac-IHVHLQI-CONH₂), forming hydrolase-mimicking bionanozymes with β-sheet secondary conformation and nanofibrous architecture. As expected, the resultant bionanozymes were able to hydrolyze a serious of p-nitrophenyl esters, including not only the simple substrate with short side-chain (p-NPA), but also more complicated ones (p-NPB, p-NPH, p-NPO, and p-NPS). Moreover, the self-assembled Zn-heptapeptide bionanozymes were also proven to be capable of degrading di(2-ethylhexyl) phthalate (DEHP), a typical plasticizer, showing great potential for environmental remediation. Based on this study, we aim to provide theoretical references and exemplify a specific case for directing the construction and application of bionanozyme.

Guanosine Borate Hydrogel and Self-Assembled Nanostructures Capable of Enantioselective Aldol Reaction in Water

A guanosine-based hydrogel formed by the self-assembly of guanosine and 4-((l-prolinamide)methyl)phenylboronic acid was constructed. The G quartets were selectively stabilized by K⁺ ions to form a self-supporting transparent hydrogel. These guanosine-derived assemblies were used to catalyze the aldol reaction in water without any additives, affording desirable conversion and enantioselectivity of the product. The controlled assays of small-molecule components indicated that the stable assemblies were the definite species that achieved high enantioselective catalysis. The current catalytic system can be readily recovered by simple extraction and still acquired good performance of the reaction after four cycles.

Anisotropic nanomaterials for asymmetric synthesis

The production of enantiopure chemicals is an essential part of modern chemical industry. Hence, the emergence of asymmetric catalysis led to dramatic changes in the procedures of chemical synthesis, and now it provides the most advantageous and economically executable solution for large-scale production of chiral chemicals. In recent years, nanostructures have emerged as potential materials for asymmetric synthesis. Indeed, on the one hand, nanomaterials offer great opportunities as catalysts in asymmetric catalysis, due to their tunable absorption, chirality, and unique energy transfer properties; on the other hand, the advantages of the larger surface area, increased number of unsaturated coordination centres, and more accessible active sites open prospects for catalyst encapsulation, partial or complete, in a nanoscale cavity, pore, pocket, or channel leading to alteration of the chemical reactivity through spatial confinement. This review focuses on anisotropic nanomaterials and considers the state-of-the-art progress in asymmetric synthesis catalysed by 1D, 2D and 3D nanostructures. The discussion comprises three main sections according to the nanostructure dimensionality. We analyze recent advances in materials and structure development, discuss the functional role of the nanomaterials in asymmetric synthesis, chirality, confinement effects, and reported enantioselectivity. Finally, the new opportunities and challenges of anisotropic 1D, 2D, and 3D nanomaterials in asymmetric synthesis, as well as the future prospects and current trends of the design and applications of these materials are analyzed in the Conclusions and outlook section.

Biocatalysts Based on Peptide and Peptide Conjugate Nanostructures

Peptides and their conjugates (to lipids, bulky N-terminals, or other groups) can self-assemble into nanostructures such as fibrils, nanotubes, coiled coil bundles, and micelles, and these can be used as platforms to present functional residues in order to catalyze a diversity of reactions. Peptide structures can be used to template catalytic sites inspired by those present in natural enzymes as well as simpler constructs using individual catalytic amino acids, especially proline and histidine. The literature on the use of peptide (and peptide conjugate) α-helical and β-sheet structures as well as turn or disordered peptides in the biocatalysis of a range of organic reactions including hydrolysis and a variety of coupling reactions (e.g., aldol reactions) is reviewed. The simpler design rules for peptide structures compared to those of folded proteins permit ready ab initio design (minimalist approach) of effective catalytic structures that mimic the binding pockets of natural enzymes or which simply present catalytic motifs at high density on nanostructure scaffolds. Research on these topics is summarized, along with a discussion of metal nanoparticle catalysts templated by peptide nanostructures, especially fibrils. Research showing the high activities of different classes of peptides in catalyzing many reactions is highlighted. Advances in peptide design and synthesis methods mean they hold great potential for future developments of effective bioinspired and biocompatible catalysts.

Long-lived triplet charge-separated state in naphthalenediimide based donor-acceptor systems

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor-acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π-π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet-triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor-acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Controlling the length of self-assembled microtubes through mechanical stress-induced scission

We demonstrate that mechanical stress-induced scission is an effective strategy to control the length of self-assembled microtubes. By applying mechanical stress with variable magnitude and mode, the length of microtubes can be tightly regulated. We have succeeded in reducing the average length of microtubes ∼twenty-fold through stretching and compression. The mechanical stress-induced scission of self-assembled, long microtubes into smaller fragments has no adverse effect on the functionality of the microtubes. This work will foster the applications of length-controlled, self-assembled microtubes in various fields.

Self-Assembled Bio-Organometallic Nanocatalysts for Highly Enantioselective Direct Aldol Reactions

Supramolecular nanocatalysts were designed for asymmetric reactions through the self-assembly process of a bio-organometallic molecule, ferrocene-l-prolinamide (Fc-CO-NH-P). Fc-CO-NH-P could self-assemble into versatile nanostructures in water, including nanospheres, nanosheets, nanoflowers, and pieces. In particular, the self-assembled nanoflowers exhibited a superior specific surface area, high stability, and delicate three-dimensional (3D) chiral catalytic active sites. The nanoflowers could serve as heterogeneous catalysts with an excellent catalytic performance toward direct aldol reactions in aqueous solution, achieving both high yield (>99%) and stereoselectivity (anti/syn = 97:3, ee% >99%). This study proposed a significant strategy to fabricate supramolecular chiral catalysts, serving as a favorable template for designing new asymmetric catalysts.

Ultrathin Two Dimensional (2D) Supramolecular Assembly and Anisotropic Conductivity of an Amphiphilic Naphthalene-Diimide

Two-dimensional (2D)-supramolecular assemblies of π-conjugated chromophores are relatively less common compared to a large number of recent examples on their low dimensional (0D or 1D) assemblies or 3D architectures. This article reports a rational design for the 2D supramolecular assembly of an amphiphilic core-substituted naphthalene-diimide derivative (cNDI-1). The building block contains a naphthalene-diimide (NDI) chromophore, symmetrically substituted with two dodecyl chains from the aromatic core while the imide positions are functionalized with two hydrophilic wedges containing oligo-oxyethylene chains. In water, it exhibits entropically favorable self-assembly with a critical aggregation concentration of 1.5 × 10^-5 M and a lower critical solution temperature of 55 °C. The UV/vis absorption spectrum in water shows bathochromically shifted absorption bands compared to that of the monomeric dye in THF, indicating offset π-stacking among the NDI chromophores. C-H symmetric and asymmetric stretching frequencies in the FT-IR spectrum support the presence of organized hydrocarbon chains in trans conformation in the self-assembled state, similar to that in the crystalline n-alkanes, which is further supported by studying the general polarization (GP) values of a noncovalently entrapped Laurdan dye. The atomic force microscopy (AFM) image shows the formation of ultrathin (height < 2.0 nm) ribbons for the spontaneously assembled sample which eventually produces a large-area 2D nanosheet by the lateral organization. The powder X-ray diffraction pattern of the drop-casted film, prepared from the preformed aggregates, reveals sharp peaks that indicate a crystalline lamellar packing along the direction of the 2D growth. Differential scanning calorimetry trace shows the melting of the crystalline alkyl chain domain at T > 75 °C, which destroys the 2D assembly. Local-scale photoconductivity of the ordered 2D assembly, studied by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique, reveals an anisotropic conductivity with ∼3 times larger conductivity along the parallel direction compared to that along the perpendicular one.

Molecularly imprinted peptide-based enzyme mimics with enhanced activity and specificity

We herein report the construction of peroxidase (POD)-mimicking catalysts based on the strategy of peptide assembly and molecular imprinting. Upon co-assembly of Fmoc-FFH and Hemin, we firstly fabricated CA-H/Hemin which displayed POD-like catalytic activity and showed a 21-fold rate acceleration in the oxidation of 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) compared to the uncatalyzed reaction. Then, upon combining CA-H/Hemin with the ABTS-imprinted polymer, the obtained imprinted catalyst (MIP-H/Hemin) showed 52-fold acceleration due to the enhanced re-binding toward ABTS. Moreover, by introducing cationic monomers, a 137-fold rate enhancement was further achieved for the positively charged imprinted catalyst (MIP+-H/Hemin), from the synergistic effect of molecular imprinting and electrostatic attraction. Remarkably, by comparing the catalytic activity of these POD mimics towards ABTS and 3,3',5,5'-tetramethylbenzidine (TMB), we also highlighted the substrate specificity of MIP-H/Hemin and MIP+-H/Hemin toward ABTS. This study provides a promising approach to improve the catalytic activity and specificity of peptide-based enzyme mimics.

Influences of Hydrogen Bonding-Based Stabilization of Bolaamphiphile Layers on Molecular Diffusion within Organic Nanotubes Having Inner Carboxyl Groups

This paper reports molecular diffusion behavior in two bolaamphiphile-based organic nanotubes having inner carboxyl groups with different inner dimeters (10 and 20 nm) and wall structures, COOH-ONT_10nm and COOH-ONT_20nm, using imaging fluorescence correlation spectroscopy (imaging FCS). The results were compared to those previously obtained in a similar nanotube with inner amine groups (NH₂-ONT_10nm). COOH-ONT_10nm, as with NH₂-ONT_10nm, were formed from a rolled bolaamphiphile layer incorporating triglycine moieties, whereas COOH-ONT_20nm consisted of four stacks of triglycine-free bolaamphiphile layers. Imaging FCS measurements were carried out for anionic sulforhodamine B (SRB), zwitterionic/cationic rhodamine B (RB), and cationic rhodamine-123 (R123) diffusing within ONTs (1-9 μm long) at different pH (3.4-8.4) and ionic strengths (1.6-500 mM). Diffusion coefficients (D) of these dyes in the ONTs were very small (0.01-0.1 μm²/s), reflecting the significant contributions of molecule-nanotube interactions to diffusion. The D of SRB was larger at higher pH and ionic strength, indicating the essential role of electrostatic repulsion that was enhanced by the deprotonation of the inner carboxyl groups. Importantly, the D of SRB was virtually independent of nanotube inner diameter and wall structure, indicating the diffusion of the hydrophilic molecule was controlled by short time scale adsorption/desorption processes onto the inner surface. In contrast, pH effects on D were less clear for relatively hydrophobic R123 and RB, suggesting the significant contributions of non-Coulombic interactions. Interestingly, the diffusion of these molecules in COOH-ONT_20nm was slower than in COOH-ONT_10nm. Slower diffusion in COOH-ONT_20nm was attributable to relatively efficient partitioning of the hydrophobic dyes into the bolaamphiphile layers, which was reduced in COOH-ONT_10nm due to the stabilization of its layer by polyglycine-II-type hydrogen bonding networks. These results show that, by tuning the bolaamphiphile structures and their intermolecular interactions, unique environments can be created within the nanospaces for enhanced molecular separations and reactions.

Peptide Stapling with Anion-π Catalysts

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core-bridged NDIs matches that of one turn of an α helix. NDI-stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino-acid sidechains above the π-acidic NDI surface affords operational anion-π catalysts. According to an enolate chemistry benchmark reaction, anion-π catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated α-helix turns, and acyclic peptide controls. Elongation of isolated turns into short α helices significantly increases activity. This increase is consistent with remote control of anion-π catalysis from the α-helix macrodipole.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

Threading carbon nanotubes through a self-assembled nanotube

Achieving the co-assembly of more than one component represents an important challenge in the drive to create functional self-assembled nanomaterials. Multicomponent nanomaterials comprised of several discrete, spatially sorted domains of components with high degrees of internal order are particularly important for applications such as optoelectronics. In this work, single-walled carbon nanotubes (SWNTs) were threaded through the inner channel of nanotubes formed by the bolaamphiphilic self-assembly of a naphthalenediimide-lysine (NDI-Bola) monomer. The self-assembly process was driven by electrostatic interactions, as indicated by ζ-potential measurements, and cation-π interactions between the surface of the SWNT and the positively charged, NDI-Bola nanotube interior. To increase the threading efficiency, the NDI-Bola nanotubes were fragmented into shortened segments with lengths of <100 nm via sonication-induced shear, prior to co-assembly with the SWNTs. The threading process created an initial composite nanostructure in which the SWNTs were threaded by multiple, shortened segments of the NDI-Bola nanotube that progressively re-elongated along the SWNT surface into a continuous radial coating around the SWNT. The resultant composite structure displayed NDI-Bola wall thicknesses twice that of the parent nanotube, reflecting a bilayer wall structure, as compared to the monolayer structure of the parent NDI-Bola nanotube. As a final, co-axial outer layer, poly(p-phenyleneethynylene) (PPE-SO3Na, M W = 5.76 × 104, PDI - 1.11) was wrapped around the SWNT/NDI-Bola composite resulting in a three-component (SWNT/NDI-Bola/PPE-SO3Na) composite nanostructure.

Reorganization from Kinetically Stable Aggregation States to Thermodynamically Stable Nanotubes of BINOL-Derived Amphiphiles in Water

The synthesis and self-assembly behavior of newly designed BINOL-based amphiphiles is presented. With minor structural modifications, the aggregation of these amphiphiles could be successfully tuned to form different types of assemblies in water, ranging from vesicles to cubic structures. Simple sonication induced the rearrangement of different kinetically stable aggregates into thermodynamically stable self-assembled nanotubes, as observed by cryo-TEM.

Chiral Gating for Size- and Shape-Selective Asymmetric Catalysis

A poor or mediocre stereoselectivity is a key roadblock for a chiral catalyst to find practical adoptions. We report a facile method to create a tunable chiral space near a chiral catalyst to augment its selectivity. The space was created rationally through templated polymerization within cross-linked micelles, using readily available amino acid derivatives. It provided gated entrance of reactants to the catalyst, enabling a mediocre prolinamide to catalyze aldol condensation in water with excellent yields and ee, in a size- and shape-selective manner.

Hierarchy of Asymmetry in Chiral Supramolecular Polymers: Toward Functional, Helical Supramolecular Structures

The formation of helical structures through the supramolecular polymerization of a variety of self-assembling units is reviewed. These scaffolds are usually obtained by efficient transfer or amplification of chirality phenomena, in which the starting self-assembling molecules possess different elements of asymmetry, such as point or axial chirality. Relevant examples of helical supramolecular structures investigated under thermodynamic control are reviewed, and the helical outcome of remarkable examples of chiral entities obtained through kinetic control are also highlighted. Finally, selected examples of flexible macroscopic chirality and catalysis are described to illustrate the applicability of helical aggregates.

The Emergence of Anion-π Catalysis

The objective of this Account is to summarize the first five years of anion-π catalysis. The general idea of anion-π catalysis is to stabilize anionic transition states on aromatic surfaces. This is complementary to the stabilization of cationic transition states on aromatic surfaces, a mode of action that occurs in nature and is increasingly used in chemistry. Anion-π catalysis, however, rarely occurs in nature and has been unexplored in chemistry. Probably because the attraction of anions to π surfaces as such is counterintuitive, anion-π interactions in general are much younger than cation-π interactions and remain under-recognized until today. Anion-π catalysis has emerged from early findings that anion-π interactions can mediate the transport of anions across lipid bilayer membranes. With this evidence for stabilization in the ground state secured, there was no reason to believe that anion-π interactions could not also stabilize anionic transition states. As an attractive reaction to develop anion-π catalysis, the addition of malonic acid half thioesters to enolate acceptors was selected. This choice was also made because without enzymes decarboxylation is preferred and anion-π interactions promised to catalyze selectively the disfavored but relevant enolate addition. Concerning anion-π catalysts, we started with naphthalene diimides (NDIs) because their intrinsic quadrupole moment is highly positive. The NDI scaffold was used to address questions such as the positioning of substrates on the catalytic π surface or the dependence of activity on the π acidity of this π surface. With the basics in place, the next milestone was the creation of anion-π enzymes, that is, enzymes that operate with an interaction rarely used in biology, at least on intrinsically π-acidic or highly polarizable aromatic amino-acid side chains. Electric-field-assisted anion-π catalysis addresses topics such as heterogeneous catalysis on electrodes and remote control of activity by voltage. On π-stacked foldamers, anion-(π) _n-π catalysis was discovered. Fullerenes emerged as the scaffold of choice to explore contributions from polarizability. On fullerenes, anionic transition states are stabilized by large macrodipoles that appear only in response to their presence. With this growing collection of anion-π catalysts, several reactions beyond enolate addition have been explored so far. Initial efforts focused on asymmetric anion-π catalysis. Increasing enantioselectivity with increasing π acidity of the active π surface has been exemplified for enamine and iminium chemistry and for anion-π transaminase mimics. However, the delocalized nature of anion-π interactions calls for the stabilization of charge displacements over longer distances. The first step in this direction was the formation of cyclohexane rings with five stereogenic centers from achiral acyclic substrates on π-acidic surfaces. Moreover, the intrinsically disfavored exo transition state of anionic Diels-Alder reactions is stabilized selectively on π-acidic surfaces; endo products and otherwise preferred Michael addition products are completely suppressed. Taken together, we hope that these results on catalyst design and reaction scope will establish anion-π catalysis as a general principle in catalysis in the broadest sense.

Self-Assembly of Amphiphilic Dipeptide with Homo- and Heterochiral Centers and Their Application in Asymmetric Aldol Reaction

Chiral self-assembly has drawn increasing interest in supramolecular chemistry. Here, we have designed amphiphilic l-Pro-l-Glu and l-Pro-d-Glu dipeptides and investigated their chiral self-assembly as well as asymmetric catalytic performance to disclose the synergistic effect of two stereogenic centers in the self-assembly and catalysis. It was found that both of the diastereomeric dipeptides can easily self-assemble into organogels with nanofibers. When these nanofibers were used as a catalyst for the asymmetric aldol reactions, enhanced enantioselectivity was obtained compared with their molecular state. Moreover, the L-L isomer assemblies showed higher enantioselectivity than the L-D isomer. It was revealed that both the supramolecular chirality of the nanofiber and the chiral catalytic site of l-proline played important roles in the asymmetric catalysis. In addition, the synergistic effect of two homochiral centers led to more efficient supramolecular catalysis that the L-L assemblies showed high yields (up to 97%), anti-diastereoselectivity (up to 99%), and excellent enantioselectivity (up to >99%).

Catalytic peptide assemblies

Self-assembly of molecules often results in new emerging properties. Even very short peptides can self-assemble into structures with a variety of physical and structural characteristics. Remarkably, many peptide assemblies show high catalytic activity in model reactions reaching efficiencies comparable to those found in natural enzymes by weight. In this review, we discuss different strategies used to rationally develop self-assembled peptide catalysts with natural and unnatural backbones as well as with metal-containing cofactors.

Thermoresponsive PEG-Coated Nanotubes as Chiral Selectors of Amino Acids and Peptides

A series of nanotubes with a dense layer of short poly(ethylene glycol) (PEG) chains on the inner surface are prepared by means of a coassembly process using glycolipids and PEG derivatives. Dehydration of the PEG chains by heating increases the hydrophobicity of the nanotube channel and fluorescent-dye-labeled amino acids are extracted from bulk solution. Rehydration of the PEG chains by cooling results in back-extraction of the amino acids into the bulk solution. Because of the supramolecular chirality of the nanotubes, amino acid enantiomers can be separated in the back-extraction procedure, which is detectable with the naked eye as a change in fluorescence as the amino acids are released from the nanotubes. The efficiency and selectivity of the chiral separation are enhanced by tuning the chemical features and inner diameter of the nanotube channels. For example, compared with wide nanotube channels (8 nm), narrow nanotube channels (4 nm) provide more effective electrostatic attraction and hydrogen bond interaction environments for the transporting amino acids. Introduction of branched alkyl chains to the inner surface of the nanotubes enables chiral separation of peptides containing hydrophobic amino acids. The system described here provides a simple, quick, and on-site chiral separation in biological and medical fields.

A self-assembled peroxidase from 5'-GMP and heme

Guanosine 5'-monophosphate (5'-GMP) and Fe(iii)-heme form a supramolecular catalyst with peroxidase activity. Catalysis, which depends on self-assembly of 5'-GMP into a G-quadruplex that binds hemin, can be modulated by nucleotide concentration, temperature and the identity of the nucleotide's sugar.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Anion-π catalysis: bicyclic products with four contiguous stereogenic centers from otherwise elusive diastereospecific domino reactions on π-acidic surfaces

Anion-π interactions have been introduced recently to catalysis. The idea of stabilizing anionic intermediates and transition states on π-acidic surfaces is a new fundamental concept. By now, examples exist for asymmetric enolate, enamine, iminium and transamination chemistry, and the first anion-π enzyme has been created. Delocalized over large aromatic planes, anion-π interactions appear particularly attractive to stabilize extensive long-distance charge displacements during domino processes. Moving on from the formation of cyclohexane rings with five stereogenic centers in one step on a π-acidic surface, we here focus on asymmetric anion-π catalysis of domino reactions that afford bicyclic products with quaternary stereogenic centers. Catalyst screening includes a newly synthesized, better performing anion-π version of classical organocatalysts from cinchona alkaloids, and anion-π enzymes. We find stereoselectivities that are clearly better than the best ones reported with conventional catalysts, culminating in unprecedented diastereospecificity. Moreover, we describe achiral salts as supramolecular chirality enhancers and report the first artificial enzyme that operates in neutral water with anion-π interactions, i.e., interactions that are essentially new to enzymes. Evidence in support of contributions of anion-π interactions to asymmetric catalysis include increasing diastereo- and enantioselectivity with increasing rates, i.e., asymmetric transition-state stabilization in the presence of π-acidic surfaces and inhibition with the anion selectivity sequence NO3- > Br- > BF4- > PF6-.

Controlling the length of self-assembled nanotubes by sonication followed by polymer wrapping

In this work, we report that sonication, followed by polymer-wrapping, is an effective strategy to reduce the length of self-assembled nanotubes and suspend their propensity to self-heal into their elongated precursors.

Non-zeolitic properties of the dipeptide l-leucyl–l-leucine as a result of the specific nanostructure formation

Non-zeolitic sorption properties of l-leucyl–l-leucine which results from a specific self-organization of the dipeptide into different micro- and nanostructures may be used for the separation of mixtures of organic compounds.

Tandem reactions in self-sorted catalytic molecular hydrogels

By equipping mutually incompatible carboxylic acid and proline catalytic groups with different self-assembling motives we have achieved self-sorting of the resulting catalytic gelators, namely SucVal8 and ProValDoc, into different supramolecular fibers, thus preventing the acidic and basic catalytic groups from interfering with each other. The resulting spatial separation of the incompatible catalytic functions is found to be essential to achieve one-pot deacetalization-aldol tandem reactions with up to 85% efficiency and 90% enantioselectivity. On the contrary, when SucVal8 was co-assembled with a structurally similar catalytically active hydrogelator (ProVal8), self-sorting was precluded and no tandem catalysis was observed.

Water in asymmetric organocatalytic systems: a global perspective

Asymmetric organocatalysis often operates under near ambient conditions, which means it is air and moisture compatible. However, in many examples water is indeed necessary for achieving excellent catalytic results. Ranging from the addition of small amounts of water to a reaction, to complex catalytic systems in the presence of water as the only reaction medium, this review offers an illustrative classification of the uses of water in asymmetric organocatalysis.

Emergent Catalytic Behavior of Self-Assembled Low Molecular Weight Peptide-Based Aggregates and Hydrogels

We report a series of short peptides possessing the sequence (FE)n or (EF)n and bearing l-proline at their N-terminus that self-assemble into high aspect ratio aggregates and hydrogels. We show that these aggregates are able to catalyze the aldol reaction, whereas non-aggregated analogues are catalytically inactive. We have undertaken an analysis of the results, considering the accessibility of catalytic sites, pKa value shifts, and the presence of hydrophobic pockets. We conclude that the presence of hydrophobic regions is indeed relevant for substrate solubilization, but that the active site accessibility is the key factor for the observed differences in reaction rates. The results presented here provide an example of the emergence of a new chemical property caused by self-assembly, and support the relevant role played by self-assembled peptides in prebiotic scenarios. In this sense, the reported systems can be seen as primitive aldolase I mimics, and have been successfully tested for the synthesis of simple carbohydrate precursors.