Yuichiro Kobayashi

Photoswitchable gel assembly based on molecular recognition

The formation of effective and precise linkages in bottom-up or top-down processes is important for the development of self-assembled materials. Self-assembly through molecular recognition events is a powerful tool for producing functionalized materials. Photoresponsive molecular recognition systems can permit the creation of photoregulated self-assembled macroscopic objects. Here we demonstrate that macroscopic gel assembly can be highly regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules. A photoregulated gel assembly system is developed using polyacrylamide-based hydrogels functionalized with azobenzene (guest) or cyclodextrin (host) moieties. Reversible adhesion and dissociation of the host gel from the guest gel may be controlled by photoirradiation. The differential affinities of α-cyclodextrin or β-cyclodextrin for the trans-azobenzene and cis-azobenzene are employed in the construction of a photoswitchable gel assembly system.

Reversible self-assembly of gels through metal-ligand interactions

Metal-ligand interactions with various proteins form in vivo metal assemblies. In recent years, metallosupramolecular approaches have been utilized to forge an assortment of fascinating two- and three-dimensional nano-architectures, and macroscopic materials, such as metal-ligand coordination polymeric materials, have promise in artificial systems. However to the best of our knowledge, the self-assembly of macroscopic materials through metal-ligand interactions has yet to be reported. Herein we demonstrate a gel assembly formed via metal-ligand interactions using polyacrylamide modified with Fe-porphyrin and L-histidine moieties. The stress values for the assembly increase as the concentration of Fe-porphyrin or L-histidine in the gels increases. Moreover, agitation of Fe-porphyrin gel, Zn-porphyrin gel, and L-histidine gel in an 80 mM Tris-acetate buffer (pH 9.0) results in selective adhesion of the Fe-porphyrin gel to the L-histidine gel based on the affinities of Fe-porphyrin and Zn-porphyrin with L-histidine.

Supramolecular Adhesives to Hard Surfaces: Adhesion Between Host Hydrogels and Guest Glass Substrates Through Molecular Recognition

Supramolecular materials based on host-guest interactions should exhibit high selectivity and external stimuli-responsiveness. Among various stimuli, redox and photo stimuli are useful for its wide application. An external stimuli-responsive adhesive system between CD host-gels (CD gels) and guest molecules modified glass substrates (guest Sub) is focused. Here, the selective adhesion between host gels and guest substrates where adhesion depends on molecular complementarity is reported. Initially, it is thought that adhesion of a gel material onto a hard material might be difficult unless many guest molecules modified linear polymers immobilize on the surface of hard materials. However, reversible adhesion of the CD gels is observed by dissociating and re-forming inclusion complex in response to redox and photo stimuli.

Preparation of Porous Polysaccharides Templated by Coordination Polymer with Three-Dimensional Nanochannels

Polymerization of monosaccharide monomers usually suffers from the production of polysaccharides with ill-defined structures because of the uncontrolled random reactions among many reactive hydroxyl groups on saccharide monomers. In particular, rational synthesis of polysaccharides with porosity approximating molecular dimensions is still in its infancy, despite their usefulness as drug carriers. Here, we disclose an efficient synthetic methodology for the preparation of polysaccharides with controllable mesoporosity in the structure, utilizing [Cu3(benzene-1,3,5-tricarboxylate)]n (HKUST-1; 1) as templates. Cationic ring-opening polymerization of 1,6-anhydro glucose was performed in nanochannels of 1, followed by removal of the host frameworks, giving polysaccharide particles as replicas of the original molds. Nitrogen adsorption measurement revealed that the obtained polysaccharide particles contained high mesoporosity in the structure, which could be controlled systematically depending on the polymerization conditions. Because of the large specific surface area, tunable porosity and particle size, we could also demonstrate the capabilities of our polysaccharides for loading and releasing of a drug molecule and protein.

A Macroscopic Reaction: Direct Covalent Bond Formation between Materials Using a Suzuki-Miyaura Cross-Coupling Reaction

Cross-coupling reactions are important to form C–C covalent bonds using metal catalysts. Although many different cross-coupling reactions have been developed and applied to synthesize complex molecules or polymers (macromolecules), if cross-coupling reactions are realized in the macroscopic real world, the scope of materials should be dramatically broadened. Here, Suzuki-Miyaura coupling reactions are realized between macroscopic objects. When acrylamide gel modified with an iodophenyl group (I-gel) reacts with a gel possessing a phenylboronic group (PB-gel) using a palladium catalyst, the gels bond to form a single object. This concept can also be adapted for bonding between soft and hard materials. I-gel or PB-gel selectively bonds to the glass substrates whose surfaces are modified with an electrophile or nucleophile, respectively.

Manual control of catalytic reactions: Reactions by an apoenzyme gel and a cofactor gel

Enzymes play a vital role in catalysing almost all chemical reactions that occur in biological systems. Some enzymes must form complexes with non-protein molecules called cofactors to express catalytic activities. Although the control of catalytic reactions via apoenzyme–cofactor complexes has attracted significant attention, the reports have been limited to the microscale. Here, we report a system to express catalytic activity by adhesion of an apoenzyme gel and a cofactor gel. The apoenzyme and cofactor gels act as catalysts when they form a gel assembly, but they lose catalytic ability upon manual dissociation. We successfully construct a system with switchable catalytic activity via adhesion and separation of the apoenzyme gel with the cofactor gel. We expect that this methodology can be applied to regulate the functional activities of enzymes that bear cofactors in their active sites, such as the oxygen transport of haemoglobin or myoglobin and the electron transport of cytochromes.

Visible chiral discrimination via macroscopic selective assembly

The transfer of chirality from individual molecules to macroscopic objects, and the recognition of chirality on the macroscopic scale have potential for many practical applications, but they are still key challenges for the chiral research community. Here we present a strategy for visible chiral recognition by macroscopic assembly using polyacrylamide-based gels modified with β-cyclodextrin (βCD-gel) and d- or l-tryptophan (homochiral d- or l-Trp-gel), which differs from most methods reported, e.g., colorimetric or chromogenic methods, fluorescence, gel formation and collapse. The circular dichroism spectra demonstrate that the chirality of Trp molecules is successfully transferred and amplified in the corresponding Trp-gels. The chirality of the d- and l-Trp-gels is macroscopically recognized by the βCD-gel selectivity in aqueous NaCl through the amplification of interfacial enantioselective host–guest interactions.Amplifying enantioselective interactions to the macroscopic scale remains a challenging goal. Here, visible gel pieces assemble enantioselectively as a result of chiral recognition between tryptophan and cyclodextrin monomers.

Functional Supramolecular Materials Formed by Non-covalent Bonds

Molecular recognition is essential for realizing functional supramolecular materials. Non-covalent host-guest interactions are effective tools to introduce various functions and properties into materials. This review focuses on the functions such as selective molecular adhesions, self-healing, toughness, and actuation properties of the supramolecular polymeric materials. These functions have been achieved by using reversible bond formations between cyclodextrins (CDs) and guest molecules. The host-guest interactions involving CDs can be used to achieve efficient stimuli-responsive behaviors and self-healing properties. Furthermore, the supramolecular materials have been found to exhibit macroscopic rapid expanding and contracting driven by external stimuli under semidry conditions. Supramolecular actuator using host-guest complexations can be prepared by two approaches. The first is the functionalization of a supramolecular gel, which changes the cross-linking density between the polymers. The second is the utilization of a topological gel to change length of the polymer chain between cross-linked points. Both types of the actuators exhibit bending behaviors by external stimuli. This review summarizes the advancements within the past 10 years in supramolecular materials that utilize the host-guest interactions and the sliding motion of ring molecules functionalized by chemical or physical processes.