Macromolecular Recognition and Macroscopic Interactions by Cyclodextrins

Herein macromolecular recognition by cyclodextrins (CDs) is summarized. Recognition of macromolecules by CDs is classified as main‐chain recognition or side‐chain recognition. We found that CDs form inclusion complexes with various polymers with high selectivity. Polyrotaxanes in which many CDs are entrapped in a polymer chain were prepared. Tubular polymers were prepared from the polyrotaxanes. CDs were found to recognize side‐chains of polymers selectively. CD host polymers were found to form gels with guest polymers in water. These gels showed self‐healing properties. When azobenzene was used as a guest, the gel showed sol‐gel transition by photoirradiation. When ferrocene was used, redox‐responsive gels were obtained. Macroscopic self‐assembly through molecular recognition has been discovered. Photoswitchable gel association and dissociation have been observed.     

Advances in Anion Supramolecular Chemistry: From Recognition to Chemical Applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under‐exploited anion–π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion‐templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.     

Responsive Inverse Opal Hydrogels for the Sensing of Macromolecules

Dual responsive inverse opal hydrogels were designed as autonomous sensor systems for (bio)macromolecules, exploiting the analyte‐induced modulation of the opal’s structural color. The systems that are based on oligo(ethylene glycol) macromonomers additionally incorporate comonomers with various recognition units. They combine a coil‐to‐globule collapse transition of the LCST type with sensitivity of the transition temperature toward molecular recognition processes. This enables the specific detection of macromolecular analytes, such as glycopolymers and proteins, by simple optical methods. While the inverse opal structure assists the effective diffusion even of large analytes into the photonic crystal, the stimulus responsiveness gives rise to strong shifts of the optical Bragg peak of more than 100 nm upon analyte binding at a given temperature. The systems’ design provides a versatile platform for the development of easy‐to‐use, fast, and low‐cost sensors for pathogens.     

Non‐Covalent Polyvalent Ligands by Self‐Assembly of Small Glycodendrimers: A Novel Concept for the Inhibition of Polyvalent Carbohydrate–Protein Interactions In Vitro and In Vivo

Polyvalent carbohydrate–protein interactions occur frequently in biology, particularly in recognition events on cellular membranes. Collectively, they can be much stronger than corresponding monovalent interactions, rendering it difficult to control them with individual small molecules. Artificial macromolecules have been used as polyvalent ligands to inhibit polyvalent processes; however, both reproducible synthesis and appropriate characterization of such complex entities is demanding. Herein, we present an alternative concept avoiding conventional macromolecules. Small glycodendrimers which fulfill single molecule entity criteria self‐assemble to form non‐covalent nanoparticles. These particles—not the individual molecules—function as polyvalent ligands, efficiently inhibiting polyvalent processes both in vitro and in vivo. The synthesis and characterization of these glycodendrimers is described in detail. Furthermore, we report on the characterization of the non‐covalent nanoparticles formed and on their biological evaluation.     

Dendritic macromers for hydrogel formation: Tailored materials for ophthalmic,orthopedic, and biotech applications

Dendritic macromolecules are well‐defined highly branched macromolecules synthesized via a divergent or convergent approach. A salient feature of the macromolecules described herein, and a goal of our research effort, is to prepare dendritic macromolecules suitable for in vitro and in vivo use by focusing on biocompatible building blocks and biodegradable linkages. These dendritic macromolecules can be subsequently crosslinked to form hydrogels using a photochemical acrylate‐based or a chemical ligation strategy. The properties—mechanical, swelling, degradation, and so forth—of the hydrogels can be tuned by altering the composition, crosslinking chemistry, wt %, generation number and so forth. The utility and diverse applicability is demonstrated through successful use of these hydrogels in three unique applications: hydrogel adhesives for repairing corneal wounds, hydrogel scaffolds for cartilage tissue engineering, and hydrogel reaction chambers for high throughput screening of molecular recognition events. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 383–400, 2008.     

Exploration of the Chiral Recognition of Sugar‐Based Diindolylmethane Receptors: Anion and Receptor Structures

In this study, we have conducted a systematic investigation of the chiral recognition of carboxylic anions by D ‐glucuronic acid/diindolylmethane receptors. We investigate the influence of the anion structure on chiral recognition in the diindolylmethane/glucuronic acid‐based receptor 1 a . We found that presence of an additional hydrogen‐bond donor at the α position to the carboxylic function is essential for effective chiral differentiation in these systems. Furthermore, we present a synthetic procedure that allows for the synthesis of sugar‐decorated receptors that possess a modified substituent at the anomeric position. Four new receptors 1 b – e have been synthesized, and their chiral‐discrimination ability toward model carboxylates is studied. The obtained results show that the chiral recognition of these receptors can be fine‐tuned by incorporation of a proper substituent into the receptor structure.     

Ion‐Responsive Behavior of Ionic‐Liquid Surfactant Aggregates with Applications in Controlled Release and Emulsification

We propose a simple but efficient, rapid, and quantitative ion‐responsive micelle system based on counter‐anion exchange of a surfactant with an imidazolium unit. The ion‐exchange reaction results in the amphiphilic‐to‐hydrophobic transition of the imidazolium salt, leading to the destruction of the micelles, which has been successfully applied to controlled release and emulsification. The proposed design offers a novel alternative stimulus to control these smart physical aggregates besides pH, temperature and light—with extra advantages. Our finding greatly benefits both fundamental research and industry.     

Artificial Allosteric Receptors

Cooperative effects in the binding of two or more substrates to different binding sites of a receptor that are a result of a conformational change caused by the binding of the first substrate—also referred to as the effector—are called allosteric effects. In biological systems, allosteric regulation is a widely used mechanism to control the function of proteins and enzymes in cellular metabolism. Inspired by this a lot of efforts have been made in supramolecular chemistry to implement this concept into artificial systems to control functions as molecular recognition, signal amplification, or even reactivity and catalysis. This review gives an up‐to‐date overview over the different approaches that have been reported ever since the first examples from the late 1970s/early 1980s. It covers both homo‐ and heterotropic examples and is divided according to the nature of the effector—cationic, anionic, or neutral—effectors and systems that use combinations of those.     

Stimulus‐Triggered Formation of an Anion–Cation Exciplex in Copper(I) Complexes as a Mechanism for Mechanochromic Phosphorescence

The investigation of the mechanisms of mechanochromic luminescence is of fundamental importance for the development of materials for photonic sensors, data storage, and luminescence switches. The structural origin of this phenomenon in phosphorescent molecular systems is rarely known and thus the formulation of structure–property relationships remains challenging. Changes in the M–M interactions have been proposed as the main mechanism with d¹⁰ coinage metal compounds. Herein, we describe a new mechanism—a mechanically induced reversible formation of a cation–anion exciplex based on Cu–F interactions—that leads to highly efficient mechanochromic phosphorescence and unusual large emission shifts from UV‐blue to yellow for Cu^I complexes. The low‐energy luminescence is thermo‐ and vaporesponsive, thus allowing the generation of white light as well as for recovering the original UV‐blue emission.     

Protection‐Group‐Free Synthesis of Sequence‐Defined Macromolecules via Precision λ‐Orthogonal Photochemistry

An advanced light‐induced avenue to monodisperse sequence‐defined linear macromolecules via a unique photochemical protocol is presented that does not require any protection‐group chemistry. Starting from a symmetrical core unit, precision macromolecules with molecular weights up to 6257.10 g mol^?1 are obtained via a two‐monomer system: a monomer unit carrying a pyrene functionalized visible light responsive tetrazole and a photo‐caged UV responsive diene, enabling an iterative approach for chain growth; and a monomer unit equipped with a carboxylic acid and a fumarate. Both light‐induced chain growth reactions are carried out in a λ‐orthogonal fashion, exciting the respective photosensitive group selectively and thus avoiding protecting chemistry. Characterization of each sequence‐defined chain (size‐exclusion chromatography (SEC), high‐resolution electrospray ionization mass spectrometry (ESI‐MS), and NMR spectroscopy), confirms the precision nature of the macromolecules.     

Biomimetic Polymers Responsive to a Biological Signaling Molecule: Nitric Oxide Triggered Reversible Self‐assembly of Single Macromolecular Chains into Nanoparticles

Novel nitric oxide (NO) responsive monomers (NAPMA and APUEMA) containing o‐phenylenediamine functional groups have been polymerized to form NO‐responsive macromolecular chains as truly biomimetic polymers. Upon exposure to NO—a ubiquitous cellular signaling molecule—the NAPMA‐ and APUEMA‐labeled thermoresponsive copolymers exhibited substantial changes in solubility, clearly characterized by tuneable LCST behavior, thereby inducing self‐assembly into nanoparticulate structures. Moreover, the NO‐triggered self‐assembly process in combination with environmentally sensitive fluorescence dyes could be employed to detect and image endogenous NO.     

Investigating the imidazolium–anion interaction through the anion-templated construction of interpenetrated and interlocked assemblies

The interaction between imidazolium cations and coordinating anions is investigated through the anion‐templated assembly of interpenetrated and interlocked structures. The orientation of the imidazolium motif with respect to anion binding, and hence the hydrogen bond donor arrangement, was varied in acyclic receptors, interpenetrated assemblies, and the first mono‐imidazolium interlocked systems. Their anion recognition properties and co‐conformations were studied by solution‐phase ¹H NMR investigations, solid‐state structures, molecular dynamics simulations, and density functional theory calculations. Our findings suggest that the imidazolium‐anion binding interaction is dominated by electrostatics with hydrogen‐bonding contributions having weak orientational dependence.     

Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective

The recognition of anions in water remains a key challenge in modern supramolecular chemistry, and is essential if proposed applications in biological, medical, and environmental arenas that typically require aqueous conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water has been achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C?H hydrogen bonding and halogen bonding. We also look beyond the field of small‐molecule recognition into the macromolecular domain, covering recent advances in anion recognition based on biomolecules, polymers, and nanoparticles.     

Synthesis and characterization of dual‐responsive micrometer‐sized core‐shell composite polymer particles

Dual‐responsive micrometer‐sized core‐shell composite polymer particles were prepared by dispersion polymerization followed by seeded copolymerization. Polystyrene (PS) particles prepared by dispersion polymerization were used as core particles. N‐isopropyl acrylamide (NIPAM) and methacrylic acid (MAA) were used to induce dual‐responsive that is thermo‐ and pH‐responsive properties in the shell layer of composite polymer particles, prepared by seeded copolymerization with PS core particles. Temperature‐ and pH‐dependent adsorption behaviors of some macromolecules on composite polymer particles indicate that produced composite polymer particles exhibit dual‐responsive surface properties. Copyright © 2007 John Wiley & Sons, Ltd.     

A Versatile 3D and 4D Printing System through Photocontrolled RAFT Polymerization

Reversible addition‐fragmentation chain‐transfer (RAFT) polymerization is a valuable tool for synthesizing macromolecules with controlled topologies and diverse chemical functionalities. However, the application of RAFT polymerization to additive‐manufacturing processes has been prevented due to the slow polymerization rates of typical systems. In this work, we developed and optimized a rapid visible (green) light mediated RAFT polymerization process and applied it to an open‐air 3D printing system. The reaction components are non‐toxic, metal free and environmentally friendly, which tailors these systems toward biomaterial fabrication. The inclusion of RAFT agent in the photosensitive resin provided control over the mechanical properties of 3D printed materials and allowed these materials to be post‐functionalized after 3D printing. Additionally, photoinduced spatiotemporal control of the network structure provided a one‐pass approach to 4D printed materials. This RAFT‐mediated 3D and 4D printing process should provide access to a range of new functional and stimuli‐responsive materials.     

Self‐Assembled Vesicles with Functionalized Membranes

Biological membranes play a key role for the function of living organisms. Thus, many artificial systems have been designed to mimic natural cell membranes and their functions. A useful concept for the preparation of functional membranes is the embedding of synthetic amphiphiles into vesicular bilayers. The dynamic nature of such noncovalent assemblies allows the rapid and simple development of bio‐inspired responsive nanomaterials, which find applications in molecular recognition, sensing or catalysis. However, the complexity that can be achieved in artificial functionalized membranes is still rather limited and the control of their dynamic properties and the analysis of membrane structures down to the molecular level remain challenging.     

Ion‐Free and Ion‐Pairing Assemblies of Anion‐Responsive π‐Electronic Systems Possessing Directly Linked Alkyl Chains

Alkyl‐substituted pyrrole‐based anion‐responsive π‐electronic systems formed supramolecular gels and liquid crystals through effective π–π stacking and van der Waals interactions. The addition of chloride as a planar cation salt afforded ion‐pairing assemblies as soft materials comprising planar receptor‐Cl^? complexes and the cation.     

Anion–π Slides for Transmembrane Transport

The recognition and transport of anions is usually accomplished by hydrogen bonding, ion pairing, metal coordination, and anion–dipole interactions. Here, we elaborate on the concept to use anion–π interactions for this purpose. Different to the popular cation–π interactions, applications of the complementary π‐acidic surfaces do not exist. This is understandable because the inversion of the aromatic quadrupole moment to produce π‐acidity is a rare phenomenon. Here, we suggest that π‐acidic aromatics can be linked together to produce an unbendable scaffold with multiple binding sites for anions to move along across a lipid bilayer membrane. The alignment of multiple anion–π sites is needed to introduce a cooperative multi‐ion hopping mechanism. Experimental support for the validity of the concept comes from preliminary results with oligonaphthalenediimide (O‐NDI) rods. Predicted by strongly positive facial quadrupole moments, the cooperativity and chloride selectivity found for anion transport by O‐NDI rods were consistent with the existence of anion–π slides. The proposed mechanism for anion transport is supported by DFT results for model systems, as well as MD simulations of rigid O‐NDI rods. Applicability of anion–π slides to achieve electroneutral photosynthesis is elaborated with the readily colorizable oligoperylenediimide (O‐PDI) rods. To clarify validity, scope and limitations of these concepts, a collaborative research effort will be needed to address by computer modeling and experimental observations the basic questions in simple model systems and to design advanced multifunctional anion–π architectures.     

π‐Electron Systems That Form Planar and Interlocked Anion Complexes and Their Ion‐Pairing Assemblies

Interactions between designed charged species are important for the ordered arrangements of π‐electron systems in assembled structures. As precursors of π‐electron anion units, new arylethynyl‐substituted dipyrrolyldiketone boron complexes, which showed anion‐responsive behavior, were synthesized. They formed a variety of receptor–anion complexes ([1+1] and [2+1] types) in solution, and the stabilities of these complexes were discussed in terms of their thermodynamic parameters. Solid‐state ion‐pairing assemblies of [1+1]‐ and [2+1]‐type complexes with countercations were also revealed by single‐crystal X‐ray analysis. In particular, a totally charge‐segregated assembly was constructed based on negatively and positively charged layers fabricated from [2+1]‐type receptor–anion complexes and tetrabutylammonium cations, respectively. Furthermore, the [1+1]‐type anion complex of the receptor possessing long alkyl chains exhibited mesophases based on columnar assembled structures with contributions from charge‐by‐charge and charge‐segregated arrangements, which exhibited charge‐carrier transporting properties.     

Recognition of Anions by Synthetic Receptors in Aqueous Solution

Natural anion binding systems achieve high substrate affinity and selectivity most often by arranging converging binding sites inside a cavity or cleft that is well shielded from surrounding solvent molecules by the folded peptide chain. Types of interactions employed for anion recognition are electrostatic interactions, hydrogen-bonding, and coordination to a Lewis-acidic metal center. In this review, successful strategies aimed at the development of synthetic receptors active in water or aqueous solvent mixtures are described. It is shown that considerable progress has been made during recent years in the development of potent anion receptors and that for every type of interaction used in nature for anion binding, corresponding synthetic models exist today. Representative examples of these systems are presented with a special emphasis on synthetic receptors whose characterization involved a detailed thermodynamic analysis of complex formation to demonstrate the important interplay between enthalpy and entropy for anion recognition in water.