Ryou Kubota

In situ real-time imaging of self-sorted supramolecular nanofibres

Self-sorted supramolecular nanofibres-a multicomponent system that consists of several types of fibre, each composed of distinct building units-play a crucial role in complex, well-organized systems with sophisticated functions, such as living cells. Designing and controlling self-sorting events in synthetic materials and understanding their structures and dynamics in detail are important elements in developing functional artificial systems. Here, we describe the in situ real-time imaging of self-sorted supramolecular nanofibre hydrogels consisting of a peptide gelator and an amphiphilic phosphate. The use of appropriate fluorescent probes enabled the visualization of self-sorted fibres entangled in two and three dimensions through confocal laser scanning microscopy and super-resolution imaging, with 80 nm resolution. In situ time-lapse imaging showed that the two types of fibre have different formation rates and that their respective physicochemical properties remain intact in the gel. Moreover, we directly visualized stochastic non-synchronous fibre formation and observed a cooperative mechanism.

In situ X-ray snapshot analysis of transient molecular adsorption in a crystalline channel

Molecular adsorption is a fundamental phenomenon in porous materials and is usually characterized by the efficiency and selectivity of molecular separations and reactions. However, for functional porous materials, analysis of the dynamic behaviour of molecular adsorbents is a major challenge. Here, we use in situ single-crystal X-ray diffraction to analyse multi-step molecular adsorption in a crystalline nanochannel of a metal-macrocycle framework. The pore surface of the metal-macrocycle framework crystal contains five different enantiomerically paired binding pockets, to which the adsorption of a (1R)-1-(3-chlorophenyl)ethanol solution was monitored with time. The resulting X-ray snapshot analyses suggest that the guest adsorption process takes a two-step pathway before equilibrium, in which the guest molecule is temporarily trapped by a neighbouring binding site. This demonstrates the potential for using X-ray analyses to visualize a transient state during a non-covalent self-assembly process.

Simultaneous arrangement of up to three different molecules on the pore surface of a metal-macrocycle framework: cooperation and competition.

Porous crystals are excellent materials with potential spatial functions through molecular encapsulation within the pores. Co-encapsulation of multiple different molecules further expands their usability and designability. Herein we report the simultaneous arrangement of up to three different guest molecules, TTF (tetrathiafulvalene), ferrocene, and fluorene, on the pore surfaces of a porous crystalline metal-macrocycle framework (MMF). The position and orientation of adsorbed molecules arranged in the pore were determined by single-crystal X-ray diffraction analysis. The anchoring effect of hydrogen bonds between the hydroxy groups of the guest molecules and inter-guest cooperation and competition are significant factors in the adsorption behaviors of the guest molecules. This finding would serve as a design basis of multicomponent functionalized nanospaces for elaborate reactions that are realized in enzymes.

Non-covalent surface modification of metal-macrocycle framework with mono-substituted benzenes

Recently, we have reported a metal-macrocycle framework (MMF) with five enantiomerically paired molecular binding pockets that exhibit site-selective guest arrangement on the nano-channel surface in soaking experiments using a variety of guest molecules. The guest inclusion is based largely on molecular exchange between solvent molecules such as CH3CN and guest molecules on the surface. Herein, we report that the molecular arrangement on the nano-channel surface varies with size, shape and/or chemical properties of functional groups of guests, mono-substituted benzene derivatives, such as benzonitrile, acetophenone and nitrobenzene. In their inclusion complexes, polar nitrile, acetyl and nitro groups serve as molecular anchors to a macrocyclic cavity through hydrogen bonding. Notably, benzonitrile and benzenesulphonic acid bind only to one pair of enantiomeric binding pockets. Such a highly site-selective binding would enable further multi-component surface modifications in the MMF.

An adaptive supramolecular hydrogel comprising self-sorting double nanofibre networks

Novel soft materials should comprise multiple supramolecular nanostructures whose responses (for example, assembly and disassembly) to external stimuli can be controlled independently. Such multicomponent systems are present in living cells and control the formation and break-up of a variety of supramolecular assemblies made of proteins, lipids, DNA and RNA in response to external stimuli; however, artificial counterparts are challenging to make. Here, we present a hybrid hydrogel consisting of a self-sorting double network of nanofibres in which each network responds to an applied external stimulus independent of the other. The hydrogel can be made to change its mechanical properties and rates of release of encapsulated proteins by adding Na2S2O4 or bacterial alkaline phosphatase. Notably, the properties of the gel depend on the order in which the external stimuli are applied. Multicomponent hydrogels comprising orthogonal stimulus-responsive supramolecular assemblies would be suitable for designing novel adaptive materials.Two hydrogelators form a dual network of orthogonal nanofibres that can be externally addressed independently to activate two different functions.

Publisher Correction: An adaptive supramolecular hydrogel comprising self-sorting double nanofibre networks

In the version of this Article originally published online, in Fig. 4b, in the lower-right image, the value of r was incorrect; it should have read ‘r = 0.72’. This has now been corrected in all versions of the Article.

Rational synthesis of benzimidazole[3]arenes by CuII-catalyzed post-macrocyclization transformation

A new series of calix[n]arene analogues, benzimidazole[3]arenes, was rationally synthesized by CuII-catalyzed post-macrocyclization transformation of a tris(o-phenylenediamine) macrocycle, and fully characterized by NMR, MS, and single-crystal X-ray diffraction (XRD) analyses. The resulting syn- and anti-benzimidazole[3]arenes have a bowl-shaped and a warped structure, respectively, in their crystalline states, and both display a dynamic inversion behavior in solution. This modification resulted in strong fluorescence due to the generated benzimidazole moieties. The mechanistic study of the post-macrocyclization transformation demonstrated that the formation of both benzimidazole[3]arenes was catalyzed, via triimine intermediates, by CuII ions in air through oxidation and cyclization of the tris(o-phenylenediamine) macrocycle.

Imaging-Based Study on Control Factors over Self-Sorting of Supramolecular Nanofibers Formed from Peptide- and Lipid-type Hydrogelators

Multicomponent self-assembly is a fascinating strategy for the construction of smart soft materials. Among them, supramolecular hydrogels comprising self-sorting nanofibers have recently attracted significant attention owing to their rationally incorporated stimulus responsiveness. However, there have been limited investigations of the crucial factors that control the self-sorting phenomena. Here, we describe an imaging-based approach to evaluate the factors that control the formation of self-sorting nanofibers from peptide- and lipid-type hydrogelators. We screened a small library of hydrogelators with distinct chemical properties by direct visualization of their self-assembly behavior by using confocal laser scanning microscopy. Our systematic research identified two important factors that influence the self-sorting behavior of nanofibers: (i) the surface charge of the hydrogelators; and (ii) the hydrophobicity of the side chain on the peptide-type hydrogelators. We determined that the same net/surface charge on the hydrogelators and side chains with a lower hydrophobicity on the peptide-type hydrogelators were preferred. These findings, in combination with the previously reported kinetic factors, were used to design and successfully prepare a three-component orthogonal self-assembly composed of supramolecular nanofibers from peptide- and lipid-type hydrogelators and a cationic organorhodium complex. Our findings would be beneficial for the design of intelligent soft materials based on self-sorting phenomena.

Chemogenetic Approach Using Ni(II) Complex–Agonist Conjugates Allows Selective Activation of Class A G-Protein-Coupled Receptors

Investigating individual G-protein-coupled receptors (GPCRs) involved in various signaling cascades can unlock a myriad of invaluable physiological findings. One of the promising strategies for addressing the activity of each subtype of receptor is to design chemical turn-on switches on the target receptors. However, valid methods to selectively control class A GPCRs, the largest receptor family encoded in the human genome, remain limited. Here, we describe a novel approach to chemogenetically manipulate activity of engineered class A GPCRs carrying a His4 tag, using metal complex–agonist conjugates (MACs). This manipulation is termed coordination tethering. With the assistance of coordination bonds, MACs showed 10–100-fold lower EC50 values in the engineered receptors, compared with wild-type receptors. Such coordination tethering enabled selective activation of β2-adrenoceptors and muscarinic acetylcholine receptors, without loss of natural receptor responses, in living mammalian cells, including primary cultured astrocytes. Our generalized, modular chemogenetic approach should facilitate more precise control and deeper understanding of individual GPCR signaling pathways in living systems.