Biocatalytic Self‐Assembly Cascades

The properties of supramolecular materials are dictated by both kinetic and thermodynamic aspects, providing opportunities to dynamically regulate morphology and function. Herein, we demonstrate time‐dependent regulation of supramolecular self‐assembly by connected, kinetically competing enzymatic reactions. Starting from Fmoc‐tyrosine phosphate and phenylalanine amide in the presence of an amidase and phosphatase, four distinct self‐assembling molecules may be formed which each give rise to distinct morphologies (spheres, fibers, tubes/tapes and sheets). By varying the sequence or ratio in which the enzymes are added to mixtures of precursors, these structures can be (transiently) accessed and interconverted. The approach provides insights into dynamic self‐assembly using competing pathways that may aid the design of soft nanostructures with tunable dynamic properties and life times.     

Thioamides: Versatile Bonds To Induce Directional and Cooperative Hydrogen Bonding in Supramolecular Polymers

The amide bond is a versatile functional group and its directional hydrogen‐bonding capabilities are widely applied in, for example, supramolecular chemistry. The potential of the thioamide bond, in contrast, is virtually unexplored as a structuring moiety in hydrogen‐bonding‐based self‐assembling systems. We report herein the synthesis and characterisation of a new self‐assembling motif comprising thioamides to induce directional hydrogen bonding. N,N′,N′′‐Trialkylbenzene‐1,3,5‐tris(carbothioamide)s (thioBTAs) with either achiral or chiral side‐chains have been readily obtained by treating their amide‐based precursors with P₂S₅. The thioBTAs showed thermotropic liquid crystalline behaviour and a columnar mesophase was assigned. IR spectroscopy revealed that strong, three‐fold, intermolecular hydrogen‐bonding interactions stabilise the columnar structures. In apolar alkane solutions, thioBTAs self‐assemble into one‐dimensional, helical supramolecular polymers stabilised by three‐fold hydrogen bonding. Concentration‐ and temperature‐dependent self‐assembly studies performed by using a combination of UV and CD spectroscopy demonstrated a cooperative supramolecular polymerisation mechanism and a strong amplification of supramolecular chirality. The high dipole moment of the thioamide bond in combination with the anisotropic shape of the resulting cylindrical aggregate gives rise to sufficiently strong depolarised light scattering to enable depolarised dynamic light scattering (DDLS) experiments in dilute alkane solution. The rotational and translational diffusion coefficients, D_trans and D_rot, were obtained from the DDLS measurements, and the average length, L, and diameter, d, of the thioBTA aggregates were derived (L=490 nm and d=3.6 nm). These measured values are in good agreement with the value L_w=755 nm obtained from fitting the temperature‐dependent CD data by using a recently developed equilibrium model. This experimental verification validates our common practice for determining the length of BTA‐based supramolecular polymers from model fits to experimental CD data. The ability of thioamides to induce cooperative supramolecular polymerisation makes them effective and broadly applicable in supramolecular chemistry.     

The Origin of Hierarchical Structure Formation in Highly Grafted Symmetric Supramolecular Double‐Comb Diblock Copolymers

Involving supramolecular chemistry in self‐assembling block copolymer systems enables design of complex macromolecular architectures that, in turn, could lead to complex phase behavior. It is an elegant route, as complicated and sensitive synthesis techniques can be avoided. Highly grafted double‐comb diblock copolymers based on symmetric double hydrogen bond accepting poly(4‐vinylpyridine)‐block‐poly(N‐acryloylpiperidine) diblock copolymers and donating 3‐nonadecylphenol amphiphiles are realized and studied systematically by changing the molecular weight of the copolymer. Double perpendicular lamellae‐in‐lamellae are formed in all complexes, independent of the copolymer molecular weight. Temperature‐resolved measurements demonstrate that the supramolecular nature and ability to crystallize are responsible for the formation of such multiblock‐like structures. Because of these driving forces and severe plasticization of the complexes in the liquid crystalline state, this supramolecular approach can be useful for steering self‐assembly of both low‐ and high‐molecular‐weight block copolymer systems.     

Cooperative Supramolecular Polymerization: Comparison of Different Models Applied on the Self‐Assembly of Bis(merocyanine) Dyes

Three new molecular building blocks 1 a – c for supramolecular polymerization are described that feature two dipolar merocyanine dyes tethered by p‐xylylene spacers. Concentration‐ and temperature‐dependent UV/Vis spectroscopy in chloroform combined with dynamic light scattering, capillary viscosimetry and atomic force microscopy investigations were applied to elucidate the mechanistic features of the self‐assembly of these strongly dipolar dyes. Our detailed studies reveal that the self‐assembly is very pronounced for bis(merocyanines) 1 a , b bearing linear alkyl chains, but completely absent for bis(merocyanine) 1 c bearing sterically more bulky ethylhexyl substituents. Both temperature‐ and concentration‐dependent UV/Vis data provide unambiguous evidence for a cooperative self‐assembly process for bis(merocyanines) 1 a , b , which was analyzed in detail by the Meijer–Schenning–Van‐der‐Schoot model (applicable to temperature‐dependent data) and by the Goldstein–Stryer model (applicable to concentration‐dependent data). By combining both methods all parameters of interest to understand the self‐assembly process could be derived, including in particular the nucleus size (8–10 monomeric units), the cooperativity factor (ca. 0.006), and the nucleation and elongation constants of about 10³ and 10⁶ M ^?1 in chloroform at room temperature, respectively.     

Construction of Three‐Dimensional DNA Hydrogels from Linear Building Blocks

A three‐dimensional DNA hydrogel was generated by self‐assembly of short linear double‐stranded DNA (dsDNA) building blocks equipped with sticky ends. The resulting DNA hydrogel is thermoresponsive and the length of the supramolecular dsDNA structures varies with temperature. The average diffusion coefficients of the supramolecular dsDNA structures formed by self‐assembly were determined by diffusion‐ordered NMR spectroscopy (DOSY NMR) for temperatures higher than 60 °C. Temperature‐dependent rheological measurements revealed a gel point of 42±1 °C. Below this temperature, the resulting material behaved as a true gel of high viscosity with values for the storage modulus G′ being significantly larger than that for the loss modulus G′′. Frequency‐dependent rheological measurements at 20 °C revealed a mesh size (ξ) of 15 nm. AFM analysis of the diluted hydrogel in the dry state showed densely packed structures of entangled chains, which are also expected to contain multiple interlocked rings and catenanes.     

Cooperative Effect of a Classical and a Weak Hydrogen Bond for the Metal‐Induced Construction of a Self‐Assembled β‐Turn Mimic

A novel metal‐induced template for the self‐assembly of two independent phosphane ligands by means of unprecedented multiple noncovalent interactions (classical hydrogen bond, weak hydrogen bond, metal coordination, π‐stacking interaction) was developed and investigated. Our results address the importance and capability of weak hydrogen bonds (WHBs) as important attractive interactions in self‐assembling processes based on molecular recognition. Together with a classical hydrogen bond, WHBs may serve as promoters for the specific self‐assembly of complementary monomeric phosphane ligands into supramolecular hybrid structures. The formation of an intermolecular C? H???N hydrogen bond and its persistence in the solid state and in solution was studied by X‐ray crystal analysis, mass spectrometry and NMR spectroscopy analysis. Further evidence was demonstrated by DFT calculations, which gave specific geometric parameters for the proposed conformations and allowed us to estimate the energy involved in the hydrogen bonds that are responsible for the molecular recognition process. The presented template can be regarded as a new type of self‐assembled β‐turn mimic or supramolecular pseudo amino acid for the nucleation of β‐sheet structures when attached to oligopeptides.     

Biocatalytic Pathway Selection in Transient Tripeptide Nanostructures

Structural adaption in living systems is achieved by competing catalytic pathways that drive assembly and disassembly of molecular components under the influence of chemical fuels. We report on a simple mimic of such a system that displays transient, sequence‐dependent formation of supramolecular nanostructures based on biocatalytic formation and hydrolysis of self‐assembling tripeptides. The systems are catalyzed by α‐chymotrypsin and driven by hydrolysis of dipeptide aspartyl‐phenylalanine‐methyl ester (the sweetener aspartame, DF‐OMe). We observed switch‐like pathway selection, with the kinetics and consequent lifetime of transient nanostructures controlled by the peptide sequence. In direct competition, kinetic (rather than thermodynamic) component selection is observed.     

Steric Constraints Induced Frustrated Growth of Supramolecular Nanorods in Water

A unique example of supramolecular polymerisation in water based on monomers with nanomolar affinities, which yield rod‐like materials with extraordinarily high thermodynamic stability, yet of finite length, is reported. A small library of charge‐neutral dendritic peptide amphiphiles was prepared, with a branched nonaphenylalanine‐based core that was conjugated to hydrophilic dendrons of variable steric demand. Below a critical size of the dendron, the monomers assemble into nanorod‐like polymers, whereas for larger dendritic side chains frustrated growth into near isotropic particles is observed. The supramolecular morphologies observed by electron microscopy, X‐ray scattering and diffusion NMR spectroscopy studies are in agreement with the mechanistic insights obtained from fitting polymerisation profiles: non‐cooperative isodesmic growth leads to degrees of polymerisation that match the experimentally determined nanorod contour lengths of close to 70 nm. The reported designs for aqueous self‐assembly into well‐defined anisotropic particles has promising potential for biomedical applications and the development of functional supramolecular biomaterials, with emerging evidence that anisotropic shapes in carrier design outperform conventional isotropic materials for targeted imaging and therapy.     

Dynamic Equilibrium between a Supramolecular Capsule and Bowl Generated by Inter‐ and Intramolecular Metal Clipping

The metal‐induced self‐assembly of a resorcin[4]arene derivative 1 that has four pyridine units as pendent groups and two equivalents of [M(dppp)(OTf)₂] (M=Pd, Pt) results in a dynamic equilibrium between an interclipped supramolecular capsule 3 and an intraclipped bowl 4 in nitromethane, although the interclipped capsule 3 is formed as a sole adduct in chloroform/methanol and the intraclipped bowl 4 is formed exclusively in an aqueous phase. This demonstrates how metal‐induced self‐assembly can be tuned by subtle changes in the solvent system. The coexistence of the two structures in nitromethane was characterized by NMR spectroscopy and coldspray ionization mass spectrometry (CSI‐MS). The crystal structure of the interclipped capsule 3 b , which is composed of two units of ligand 1 and four Pt^II ions, reveals the capsule cavity to have nanoscale dimensions of 15×20 Å. NMR spectra show that the dynamic equilibrium between 3 and 4 is dependent on concentration and temperature. Temperature‐dependent ¹H NMR spectroscopy was carried out from 273 to 343 K to verify the thermodynamic parameters that control the dynamic equilibrium process; the conversion from the interclipped supramolecular capsule 3 a to the intraclipped bowl 4 a is entropically favored and enthalpically disfavored. The rotational barrier of the restricted rotation of pyridine units in the intraclipped bowl 4 was determined by line‐shape analysis.     

Access to Metastable Gel States Using Seeded Self‐Assembly of Low‐Molecular‐Weight Gelators

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties.     

Hydrogen Bonding Directed Supramolecular Polymerisation of Oligo(Phenylene‐Ethynylene)s: Cooperative Mechanism,Core Symmetry Effect and Chiral Amplification

The design of supramolecular motifs with tuneable stability and adjustable supramolecular polymerisation mechanisms is of crucial importance to precisely control the properties of supramolecular assemblies. This report focuses on constructing π‐conjugated oligo(phenylene ethynylene) (OPE)‐based one‐dimensional helical supramolecular polymers that show a cooperative growth mechanism. Thus, a novel set of discotic molecules comprising a rigid OPE core, three amide groups, and peripheral solubilising wedge groups featuring C₃ and C₂ core symmetry was designed and synthesised. All of the discotic molecules are crystalline compounds and lack a columnar mesophase in the solid state. In dilute methylcyclohexane solution, one‐dimensional supramolecular polymers are formed stabilised by threefold intermolecular hydrogen bonding and π–π interactions, as evidenced by ¹H NMR measurements. Small‐angle X‐ray and light scattering measurements reveal significant size differences between the columnar aggregates of C₃‐ and C₂‐symmetrical discotics, that is, the core symmetry strongly influences the nature of the supramolecular polymerisation process. Temperature‐dependent CD measurements show a highly cooperative polymerisation process for the C₃‐symmetrical discotics. In contrast, the self‐assembly of C₂‐symmetrical discotics shows a smaller enthalpy release upon aggregation and decreased cooperativity. In all cases, the peripheral stereogenic centres induce a preferred handedness in the columnar helical aggregates. Moreover, one stereogenic centre suffices to fully bias the helicity in the C₂‐symmetrical discotics. Finally, chiral amplification studies with the C₃‐symmetrical discotics were performed by mixing chiral and achiral discotics (sergeants‐and‐soldiers experiment) and discotics of opposite chirality (majority‐rules experiment). The results demonstrate a very strong sergeants‐and‐soldiers effect and a rather weak majority‐rules effect.     

Controlling the Structure and Length of Self‐Synthesizing Supramolecular Polymers through Nucleated Growth and Disassembly

Directing self‐assembly processes out‐of‐equilibrium to yield kinetically trapped materials with well‐defined dimensions remains a considerable challenge. Kinetically controlled assembly of self‐synthesizing peptide‐functionalized macrocycles through a nucleation–growth mechanism is reported. Spontaneous fiber formation in this system is effectively shut down as most of the material is diverted into metastable non‐assembling trimeric and tetrameric macrocycles. However, upon adding seeds to this mixture, well‐defined fibers with controllable lengths and narrow polydispersities are obtained. This seeded growth strategy also allows access to supramolecular triblock copolymers. The resulting noncovalent assemblies can be further stabilized through covalent capture. Taken together, these results show that self‐synthesizing materials, through their interplay between dynamic covalent bonds and noncovalent interactions, are uniquely suited for out‐of‐equilibrium self‐assembly.     

Dendronized Triangular Oligo(phenylene ethynylene) Amphiphiles: Nanofibrillar Self‐Assembly and Dye Encapsulation

Triangular‐shaped oligo(phenylene ethynylene) amphiphiles 1 a and 1 b decorated in their periphery with two‐ and four‐branched hydrophilic triethyleneglycol dendron wedges, have been synthesized and their self‐assembling properties in solution and onto surfaces investigated. The steric demand produced by the dendritic substituents induces a face‐to‐face rotated π stacking of the aromatic moieties. Studies on the concentration and temperature dependence confirm this mechanism and provide binding constants of 1.2×10⁵ and 1.7×10⁵ M ^?1 in acetonitrile for 1 a and 1 b , respectively. Dynamic and static light scattering measurements complement the study of the self‐assembly in solution and demonstrate the formation of rod‐like supramolecular structures in aqueous solution. The nanofibers formed in solution can be efficiently transferred onto surfaces. Thus, TEM images reveal the presence of strands of various thickness, with the most common being several micrometers long and with diameters of around 70 nm. Some of these nanofibers present folded edges that are indicative of their ribbon‐like nature. Interestingly, compound 1 b can also form thick filaments with a rope‐like appearance, which points to a chiral arrangement of the fibers. AFM images under highly diluted conditions also reveal long fibers with height profiles that fit well with the molecular dimensions calculated for both amphiphiles. Finally, we have demonstrated the intercalation of the hydrophobic dye Disperse Orange 3 within the filaments and its subsequent release upon increasing the temperature.     

Studies on supramolecular gel formation using DOSY NMR

Herein, we present the results obtained from our studies on supramolecular self‐assembly and molecular mobility of low‐molecular‐weight gelators (LMWGs) in organic solvents using pulsed field gradient (PFG) diffusion ordered spectroscopy (DOSY) NMR. A series of concentration‐dependent DOSY NMR experiments were performed on selected LMWGs to determine the critical gelation concentration (CGC) as well as to understand the behaviour of the gelator molecules in the gel state. In addition, variable‐temperature DOSY NMR experiments were performed to determine the gel‐to‐sol transition. The PFG NMR experiments performed as a function of gradient strength were further analyzed using monoexponential DOSY processing, and the results were compared with the automated Bayesian DOSY transformation to obtain 2D plots. Our results provide useful information on the stepwise self‐assembly of small molecules leading to gelation. We believe that the results obtained from these experiments are applicable in determining the CGC and gel melting temperatures of supramolecular gels. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of the Solvent and the Enantiomeric Purity on the Transition between Different Supramolecular Polymers

The self‐assembly of two enantiomerically pure hexa(oligo (p‐phenylene vinylene))‐substituted benzenes having 24 stereocenters was studied in pure methylcyclohexane (MCH) and in a mixture of MCH/toluene (4:1). Irrespective of the solvent a cooperative supramolecular polymerization mechanism was determined for these star‐shaped molecules by using temperature‐dependent CD and UV/Vis spectroscopy. Quite remarkably, a transition from one helical supramolecular state (A) to a second more thermodynamically stable supramolecular helical assembly (B) was observed. The rate of the A→B transition was strongly dependent on the nature of the solvent; being faster in the solvent mixture than in pure MCH. By using size exclusion chromatography we could relate the increased rate to a decreased stability of the supramolecular A state in the solvent mixture. Next, we mixed the two enantiomerically pure hexa‐substituted benzene derivatives in a so‐called majority‐rules experiment, which lead to the anitcipated chiral amplification in the A state. More importantly it appeared that the A→B transition was significantly hampered in these mixed systems. Furthermore, the absence of chiral amplification in the B state revealed the formation of separated enantiomerically pure assemblies. Therefore, by using a wide variety of spectroscopic and chromatographic techniques we determined the influence of solvent and enantiomeric purity on the transition between different supramolecular states.     

Supra‐dendron Gelator Based on Azobenzene–Cyclodextrin Host–Guest Interactions: Photoswitched Optical and Chiroptical Reversibility

A novel amphiphilic dendron ( AZOC₈GAc ) with three l ‐glutamic acid units and an azobenzene moiety covalently linked by an alkyl spacer has been designed. The compound formed hydrogels with water at very low concentration and self‐assembled into chiral‐twist structures. The gel showed a reversible macroscopic volume phase transition in response to pH variations and photo‐irradiation. During the photo‐triggered changes, although the gel showed complete reversibility in its optical absorptions, only an incomplete chiroptical property change was achieved. On the other hand, the dendron could form a 1:1 inclusion complex through a host–guest interaction with α‐cyclodextrin (α‐CD), designated as supra‐dendron gelator AZOC₈GAc/α‐CD . The supra‐dendron showed similar gelation behavior to that of AZOC₈GAc , but with enhanced photoisomerization‐transition efficiency and chiroptical switching capacity, which was completely reversible in terms of both optical and chiroptical performances. The self‐assembly of the supra‐dendron is a hierarchical or multi‐supramolecular self‐assembling process. This work has clearly illustrated that the hierarchical and multi‐supramolecular self‐assembling system endows the supramolecular nanostructures or materials with superior reversible optical and chiroptical switching.     

Supramolecular Chemistry in Microflow Fields: Toward a New Material World of Precise Kinetic Control

Constructing new and versatile self‐assembling systems in supramolecular chemistry is much like the development of new reactions or new catalysts in synthetic organic chemistry. As one such new technology, conventional supramolecular assembly systems have been combined with microflow techniques to control intermolecular or interpolymer interactions through precise regulation of a flowing self‐assembly field. The potential of the microflow system has been explored by using various simple model compounds. Uniform solvent diffusion in the microflow leads to rapid activation of molecules in a nonequilibrium state and, thereby, enhanced interactions. All of these self‐assembly processes begin from a temporally activated state and proceed in a uniform chemical environment, forming a synchronized cluster and resulting in effective conversion to supramolecules, with precise tuning of molecular (or polymer) interactions. This approach allows the synthesis of a variety of discrete microstructures (e.g., fibers, sheets) and unique supramolecules (e.g., hierarchical assemblies, capped fibers, polymer networks, supramolecules with time‐delayed action) that have previously been inaccessible.     

Spontaneous Self‐Assembly of γ‐Cyclodextrins in Dilute Solutions with Tunable Sizes and Thermodynamic Stability

The behavior in dilute solution of phosphate‐functionalized γ‐cyclodextrin macroanions with eight charges on the rim was explored. The hydrophilic macroions in mixed solvents show strong attraction between each other, mediated by the counterions, and consequently self‐assemble into blackberry‐type hollow spherical structures. Time‐resolved laser light scattering (LLS) measurements at high temperature ruled out the possibility of hydrogen bonding as the main driving force in the self‐assembly and indicated the good thermodynamic stability of assemblies regulated by the charge. The transition from single macroions to blackberries can be tuned by adjusting the content of organic solvent. The sizes of blackberries vary with the charge density of γ‐cyclodextrin by adjusting pH. It is the first report that pure cyclodextrins can generate supramolecular structures by themselves in dilute solution. The unique solution behavior of macroions provides a new opportunity to assemble cyclodextrin into functional materials and devices.     

Ultrasound Stimulated Nucleation and Growth of a Dye Assembly into Extended Gel Nanostructures

A squaraine dye functionalized with a bulky trialkoxy phenyl moiety through a flexible diamide linkage ( GA‐SQ ) capable of undergoing self‐assembly has been synthesized and fully characterized. Rapid cooling of a hot solution of GA‐SQ to 0 °C results in self‐assembled precipitates consisting of two types of nanostructures, rings and ill‐defined short fibers. The application of ultrasound modifies the conditions for the supersaturation‐mediated nucleation, generating only one kind of nuclei and prompting the formation of crystalline fibrous structures, inducing gelation of solvent molecules. The unique self‐assembling behavior of GA‐SQ under ultrasound stimulus has been investigated in detail by using absorption, emission, FT‐IR, XRD, SEM, AFM and TEM techniques. These studies reveal a nucleation growth mechanism of the self‐assembled material, an aspect rarely scrutinized in the area of sonication‐induced gelation. Furthermore, in order to probe the effects of nanoscale substrates on the sonication‐induced self‐assembly, a minuscule amount of single‐walled carbon nanotubes was added, which leads to acceleration of the self‐assembly through a heterogeneous nucleation process that ultimately affords a supramolecular gel with nanotape‐like morphology. This study demonstrates that self‐assembly of functional dyes can be judiciously manipulated by an external stimulus and can be further controlled by the addition of carbon nanotubes.     

分子纳米技术与金属有机分子纳米体系

基于超分子自组装的分子纳米技术是一种新兴的高新技术。本文从金属矢量操纵的自组装分子纳米体系、自组装的纳米微反应器与超分子催化和自组装金属超分子高分子纳米材料等三个方面评述了分子纳米技术及其在构筑金属有机分子纳米体系中的发展现状 ,进一步阐述了“金属矢量”的概念 ,并首次提出建立“自组装子工具箱” ,探讨了分子纳米技术的发展方向。