Cyclodextrin-Based Side-Chain Polyrotaxane with Unidirectional Inclusion in Aqueous Media

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

Reversible Photoresponsive Modulation of Osmotic Pressure via Macromolecular Host-Guest Interaction

The high spatiotemporal resolution of light as an external stimulus allows the control of shape, mechanical properties, and even forces generated by photoresponsive soft materials. For this purpose, supramolecular systems that respond readily and reversibly to photoirradiation and convert microscopic changes into macroscopic effects are needed. This work demonstrates the reversible light-responsive modulation of the osmotic pressure of an aqueous solution of an azobenzene-containing polymer (azopolymer) and α-cyclodextrin. Osmometry shows that this multivalent and photoresponsive host-guest complex can be used to modulate the concentration of solutes in the solution. Upon alternating irradiation with UV and blue light, the osmolality is reversibly switched by 28 mOsm kg^-1. The switching amplitude increases linearly with the concentration of azopolymer and cyclodextrin. This drastic change in osmotic pressure is achieved by carefully designing an azopolymer that provides multivalent interactions as well as high water solubility. In this way, our study demonstrates a tunable control of colligative properties by photoinduced modulation of supramolecular interactions.

α-Cyclodextrin-Catalyzed Symmetry Breaking and Precise Regulation of Supramolecular Self-Assembly Handedness with Harata-Kodaka's Rule

Harata-Kodaka's rule predicting the induced chirality of the guest molecules by cyclodextrins has been discovered in the 1970-1990s, yet its ability to control the supramolecular handedness of self-assembled structures has not been sufficiently recognized. Here we show that in a coordinating self-assembly system that is able to form racemic cone shells symmetry breaking occurs if the ligand is prethreaded into α-cyclodextrin prior to metal ion addition, and the handedness of cone shells can be rationally manipulated by creating the two scenarios of the Harata-Kadaka rule through controlling the host-guest dynamics. Since the coordination complexes have strong self-assembling ability, the coordinating ligand would dethread from the cavity of α-cyclodextrin but leaving the induced chirality to the coordinating self-assembly, thus catalyzing symmetry breaking. This work reveals that the dynamic factors such as concentration and molar ratio may play important roles in symmetry breaking at the supramolecular level. The current strategy provides a promising method for the symmetry breaking and manipulation of the handedness of self-assembled materials formed by achiral molecules.

The macroscopic shape of assemblies formed from microparticles based on host-guest interaction dependent on the guest content

Biological macroscopic assemblies have inspired researchers to utilize molecular recognition to develop smart materials in these decades. Recently, macroscopic self-assemblies based on molecular recognition have been realized using millimeter-scale hydrogel pieces possessing molecular recognition moieties. During the study on macroscopic self-assembly based on molecular recognition, we noticed that the shape of assemblies might be dependent on the host-guest pair. In this study, we were thus motivated to study the macroscopic shape of assemblies formed through host-guest interaction. We modified crosslinked poly(sodium acrylate) microparticles, i.e., superabsorbent polymer (SAP) microparticles, with β-cyclodextrin (βCD) and adamantyl (Ad) residues (βCD(x)-SAP and Ad(y)-SAP microparticles, respectively, where x and y denote the mol% contents of βCD and Ad residues). Then, we studied the self-assembly behavior of βCD(x)-SAP and Ad(y)-SAP microparticles through the complexation of βCD with Ad residues. There was a threshold of the βCD content in βCD(x)-SAP microparticles for assembly formation between x = 22.3 and 26.7. On the other hand, the shape of assemblies was dependent on the Ad content, y; More elongated assemblies were formed at a higher y. This may be because, at a higher y, small clusters formed in an early stage can stick together even upon collisions at a single contact point to form elongated aggregates, whereas, at a smaller y, small clusters stick together only upon collisions at multiple contact points to give rather circular assemblies. On the basis of these observations, the shape of assembly formed from microparticles can be controlled by varying y.

Cyclodextrins based delivery systems for macro biomolecules

Macro biomolecules are of vital importance in regulating the biofunctions in organisms, in which proteins (including peptides when mentioned below) and nucleic acids (NAs) are the most important. Therefore, these proteins and NAs can be applied as "drugs" to regulate the biofunctions from abnormal to normal. Either for proteins and NAs, the most challenging thing is to avoid the biodegradation or physicochemical degradation before they reach the targeted location, and then functions as complete functional structures. Hence, appropriate delivery systems are very important which can protect them from these degradations. Cyclodextrins (CDs) based delivery systems achieved mega successes due to their outstanding pharmaceutical properties and there have been several reviews on CDs based small molecule drug delivery systems recently. But for biomolecules, which are getting more and more important for modern therapies, however, there are very few reviews to systematically summarize and analyze the CDs-based macro biomolecules delivery systems, especially for proteins. In this review, there were some of the notable examples were summarized for the macro biomolecules (proteins and NAs) delivery based on CDs. For proteins, this review included insulin, lysozyme, bovine serum albumin (BSA), green fluorescent protein (GFP) and IgG's, etc. deliveries in slow release, stimulating responsive release or targeting release manners. For NAs, this review summarized cationic CD-polymers and CD-cluster monomers as NAs carriers, notably, including the multicomponents targeting CD-based carriers and the virus-like RNA assembly method siRNA carriers.

Light-Switchable Azobenzene-Containing Macromolecules: From UV to Near Infrared

Azobenzene-containing macromolecules (azo-macromolecules) such as azobenzene-containing polymers (azopolymers) and azobenzene-functionalized biomacromolecules are photoswitchable macromolecules. Trans-to-cis photoisomerization in conventional azo-macromolecules is induced by ultraviolet (UV) light. However, UV light cannot penetrate deeply into issue and has a very small fraction in sunlight. Therefore, conventional azo-macromolecules are problematic for biomedical and solar-energy-related applications. In this Feature Article, the strategies for constructing visible and near-infrared (NIR) light-responsive azo-macromolecules are reviewed, and the potential applications of visible- and NIR-light-responsive azo-macromolecules in biomedicine and solar energy conversion are highlighted. The remaining challenges in the field of photoswitchable azo-macromolecules are discussed.

A Photoresponsive Orthogonal Supramolecular Complex Based on Host-Guest Interactions

We synthesized a novel green-light-responsive tetra-ortho-isopropoxy-substituted azobenzene (ipAzo). Cis-ipAzo forms a strong host-guest complex with γ-cyclo dextrin (γ-CD) whereas trans-ipAzo binds weakly. This new photoresponsive host-guest interaction is reverse to the well-known azobenzene (Azo)/α-cyclodextrin (α-CD) complex, which is strong only between trans-Azo and α-CD. By combining the UV-light-responsive Azo/α-CD and green-light-responsive ipAzo/γ-CD host-guest complexes, a photoresponsive orthogonal supramolecular system is developed.

Supramolecular Graft Copolymerization of a Polyester by Guest-Selective Encapsulation of a Self-Assembled Capsule

Repeating guest units of polyesters poly-(R)-2 were selectively encapsulated by capsule 1(BF₄ )₄ to produce supramolecular graft polymers. The encapsulation of the guest units was confirmed by ¹ H NMR spectroscopy. The graft polymer structures were confirmed by the increase in the hydrodynamic radii and the solution viscosities of the polyesters upon complexation of the capsule. After the capsule was formed, atomic force microscopy showed extension of the polyester chains. The introduction of the graft chains onto poly-(R)-2 resulted in the main chain of the polymer having an M-helical morphology. The complexation of copolymers poly-[(R)-2-co-(S)-2] by the capsule gave rise to the unique chiral amplification known as the majority-rules effect.

Supramolecular polymeric materials via cyclodextrin-guest interactions

CONSPECTUS: Cyclodextrins (CDs) have many attractive functions, including molecular recognition, hydrolysis, catalysis, and polymerization. One of the most important uses of CDs is for the molecular recognition of hydrophobic organic guest molecules in aqueous solutions. CDs are desirable host molecules because they are environmentally benign and offer diverse functions. This Account demonstrates some of the great advances in the development of supramolecular materials through host-guest interactions within the last 10 years. In 1990, we developed topological supramolecular complexes with CDs, polyrotaxane, and CD tubes, and these preparation methods take advantage of self-organization between the CDs and the polymers. The combination of polyrotaxane with αCD forms a hydrogel through the interaction of αCDs with the OH groups on poly(ethylene glycol). We categorized these polyrotaxane chemistries within main chain type complexes. At the same time, we studied the interactions of side chain type supramolecular complexes with CDs. In these systems the guest molecules modified the polymers and selectively formed inclusion complexes with CDs. The systems that used low molecular weight compounds did not show such selectivity with CDs. The multivalency available within the complex cooperatively enhances the selective binding of CD with guest molecules via the polymer side chains, a phenomenon that is analogous to binding patterns observed in antigen-antibody complexes. To incorporate the molecular recognition properties of CDs within the polymer side chains, we first prepared stimuli-responsive sol-gel switching materials through host-guest interactions. We chose azobenzene derivatives for their response to light and ferrocene derivatives for their response to redox conditions. The supramolecular materials were both redox-responsive and self-healing, and these properties resulted from host-guest interactions. These sol-gels with built in switches gave us insight for creating materials that were self-healing or could serve as artificial muscle. Furthermore, we developed another self-healing material with CD inclusion complexes that showed selective self-healing properties after its surface was cut. These CD self-healing materials do not include chemical cross-linkers; instead the inclusion complex of CDs with guest molecules stabilized the material's strength. However, by introducing chemical cross-linkers into the hydrogels, we produced materials that could expand and contract. The chemical cross-linked hydrogels with responsive groups bent in response to external stimuli, and the cross-linkers controlled the ratio of inclusion complexes. Furthermore, we used the molecular recognition of CDs to achieve macroscopic self-assemblies, and this chemistry can direct these macroscopic objects into even larger aggregated structures. As we have demonstrated, reversible host-guest interactions have tremendous potential for the creation of a wide variety of functional materials.

Synthesis of a poly(N-isopropylacrylamide) charm bracelet decorated with a photomobile α-cyclodextrin charm

Cyclic poly(N-isopropylacrylamide) (PNIPAM) with an interlocked α-cyclodextrin was prepared using supramolecular interactions with an azobenzene group inserted in the PNIPAM ring.

Unraveling unidirectional threading of α-cyclodextrin in a [2]rotaxane through spin labeling approach

We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on-off by changing the pH of the solution in the presence of a competing macrocyclic host.

The design, synthesis and photochemical study of a biomimetic cyclodextrin model of photoactive yellow protein (PYP)

The design, synthesis and study of the photophysical and photochemical properties of the first biomimetic cyclodextrin (CD) model of photoactive yellow protein (PYP) are described. This model bears a deprotonated trans-p-coumaric acid chromophore, covalently linked via a cysteine moiety to a permethylated 6-monoamino β-CD. NMR and UV/Visible spectroscopy studies showed the formation of strong self-inclusion complexes in water at basic pH. Steady-state photolysis demonstrated that, unlike the free chromophore in solution, excitation of the model molecule leads to the formation of a photoproduct identified as the cis isomer by NMR spectroscopy. These observations provide evidence that the restricted CD cavity offers a promising framework for the design of biomimetic models of the PYP hydrophobic pocket.

β-cyclodextrin as the vehicle for forming ratiometric mercury ion sensor usable in aqueous media, biological fluids, and live cells

The selective and sensitive detection methods for toxic transition-metal ions, which are rapid, facile, and applicable to the environmental and biological milieus, are of great importance. In this study, we designed a β-CD-based ratiometric sensor for detecting mercury ions in aqueous media, some biological fluids, and live cells. In this sensing platform, the thiocarbamido-containing probe dye was covalently linked onto the hydrophilic β-CD rim, which is conducive to complexing with metal ion, while the donor dye was anchored inside hydrophobic β-CD cavity via the adamantyl moiety, which is good for avoiding self-aggregation and enhancing the quantum yield of the donor dye. Upon associating with mercury ion, the probe dye undergoes ring-opening process and serves as the energy acceptor and constitutes the FRET system with the donor dye; by this way ratiometric detection of mercury ion in water can be realized with the detection limit of 10 nM. The cyclodextrin plays a crucial role for the sensing system; it not only accommodates both the donor dye and the probe dye which can form FRET system upon addition of Hg(2+) but also makes the sensor water-soluble and cell membrane permeable. This nontoxic sensing platform can be used for mercury ion detection in aqueous medium, biological fluids, and live cells (L929 and Hela). We also found that, upon being taken up by L929 cells, the sensor exhibited no cytotoxicity, and the cell proliferation was not affected.

Cyclodextrin supramolecular complex as a water-soluble ratiometric sensor for ferric ion sensing

Heightened concern for human health and environmental protection has stimulated active research on the potential impact of transition-metal ions and their toxic effects, thus it is very demanding to design transition-metal ion detection methods that are cost-effective, rapid, facile, and applicable to the environmental and biological milieus. In this study, we demonstrated an alternative strategy for constructing a water-soluble FRET-based ratiometric sensor for ferric ion detection by forming a supramolecular beta-cyclodextrin/dye complex. This water-soluble FRET system consists of a dansyl-linked beta-cyclodextrin (beta-CD-DNS) and a spirolactam rhodamine-linked adamantane (AD-SRhB). The dansyl moiety serves as the donor, and the spirolactam-rhodamine B derivative (SRhB) was chosen as a sensitive, selective chemosensor for Fe(III) ions and a very efficient ring-opening reaction induced by Fe(III) generates the long-wavelength rhodamine B fluorophore that can act as the energy acceptor. Moreover, the adamantyl (AD) group, which is known for its capability to form stable host-guest inclusion complexes with beta-CD derivatives, was covalently linked to the spirolactam rhodamine, thus the adamantyl moiety of the ion-recognition element can be anchored inside the CD cavity. In this way, the donor-acceptor separation can be kept within the critical Forster distance; accordingly, energy transfer can take place from the donor (dansyl) to the acceptor (rhodamine derivative/Fe(III) complex), and thus ratiometric detection for Fe(III) in an aqueous medium can be fulfilled. This FRET-based supramolecular sensor can be readily formed via an inclusion process using the donor part and the acceptor part, hence this strategy could afford a robust approach for constructing a wide range of FRET-based water-soluble sensing systems simply by assembling a specifically predesigned donor-linked CD and acceptor-linked adamantane.

Supramolecular Polymers Based on Cyclodextrins and Their Derivatives

Since Lehn introduced the concept in the 1990s, supramolecular polymers have been developed rapidly by several research groups. Supramolecular polymers should be key materials for establishment of a sustainable society. This short review describes briefly hydrogen-bonded and coordination supramolecular polymers, and then reviews our research on cyclodextrin-based supramolecular polymers.

Reversible 2D Pseudopolyrotaxanes Based On Cyclodextrins and Cucurbit[6]uril

In this paper, a pseudorotaxane (2) was synthesized by reaction of cucurbit [6]uril with 6-[(6-aminohexyl)amino]-6-deoxy-beta-cyclodextrin chloride. Subsequently, pseudorotaxanes 2 were further assembled to form a 2D pseudopolyrotaxane (3) through an alpha,omega-PPG2000 diamino polymer threading the cavities of cyclodextrins in 2, and the resulting pseudopolyrotaxane was comprehensively characterized by FT-IR, NMR, TG-DTA, elemental analysis, and transmission electron microscopy. Significantly, the 2D pseudopolyrotaxane can turn into a main-chain pseudopolyrotaxane in the presence of base, and then the addition of alpha-cyclodextrins may result in a reversible switch between two different 2D pseudopolyrotaxanes.