Artificial Processive Catalytic Systems.

Processive catalysts remain attached to a substrate and perform multiple rounds of catalysis. They are abundant in nature. This review highlights artificial processive catalytic systems, which can be divided into (A) catalytic rings that move along a polymer chain, (B) catalytic pores that hold polymer chains and decompose them, (C) catalysts that remain attached to and move around a cyclic substrate via supramolecular interactions, and (D) anchored catalysts that remain in contact with a substrate via multiple catalytic interactions (see frontispiece).

The average magnetic anisotropy of polystyrene in polymersomes self-assembled from poly(ethylene glycol)-b-polystyrene.

Using the diamagnetic anisotropy of polymers for the characterization of polymers and polymer aggregates is a relatively new approach in the field of soft-matter and polymer research. So far, a good and thorough quantitative description of these diamagnetic properties has been lacking. Using a simple equation that links the magnetic properties of an average polymer repeating unit to those of the polymer vesicle of any shape, we measured, using magnetic birefringence, the average diamagnetic anisotropy of a polystyrene (PS) repeating unit, ΔχPS, inside a poly(ethylene glycol)-polystyrene (PEG-PS) polymersome membrane as a function of the PS-length and as a function of the preparation method. All obtained values of ΔχPS have a negative sign which results in polymers tending to align perpendicular to an applied magnetic field. Combined, the same order of magnitude of ΔχPS (10-12 m3 mol-1) for all polymersome shapes proves that the individual polymers are organized similarly regardless of the PS length and polymersome shape. Furthermore, the value found is only a fraction (∼1%) of what it can maximally be due to the random coiling of the polymers. We, therefore, predict that further ordering of the polymers within the membrane could lead to similar responses at much lower magnetic fields, possibly obtainable with permanent magnets, which would be highly advantageous for practical applications.

Enantiodivergent Sulfoxidation Catalyzed by a Photoswitchable Iron Salen Phosphate Complex.

Here we describe a photoswitchable iron(III) salen phosphate catalyst, which is able to catalyze the enantiodivergent oxidation of prochiral aryl alkyl sulfides to chiral aryl alkyl sulfoxides. The stable (S)-axial isomer of the catalyst produced enantioenriched sulfoxides with the (R)-configuration in up to 75 % e.e., whereas the photoisomerized metastable (R)-axial isomer of the catalyst favored the formation of (S)-sulfoxides in up to 43 % e.e. The maximum Δe.e. value obtained in the enantiodivergent sulfoxidation was 118 %, which is identical to the maximum Δe.e. value that was measured in the enantiodivergent epoxidation of alkenes by a related recently described Mn1 catalyst. This iron-based catalyst broadens the scope of photoswitchable enantiodivergent catalysts and may be used in the future to develop a photoswitchable catalytic system that can write digital information on a polymer chain in the form chiral sulfoxide functions.

113Cd as a Probe in NMR Studies of Allosteric Host-Guest-Ligand Complexes of Porphyrin Cage Compounds.

Cadmium porphyrin cage compounds Cd1 and 113 Cd1 have been synthesized from the free base porphyrin cage derivative H21 and Cd(OAc)2 ⋅ 2 H2O or 113Cd(OAc)2 ⋅ 2 H2O, respectively. The compounds form allosteric complexes with the positively charged guests N,N'-dimethylimidazolium hexafluorophosphate (DMI) and N,N'-dimethylviologen dihexafluorophosphate (Me2V), which bind in the cavity of the cage, and tbupy, which coordinates as an axial ligand to the outside of the cage. In the presence of tbupy, the binding of DMI in Cd1 is enhanced by a factor of ∼31, while the presence of DMI or Me2V in the cavity of Cd1 enhances the binding of tbupy by factors of 55 and 85, respectively. The X-ray structures of the coordination complexes of Cd1 with acetone, acetonitrile, and pyridine, the host-guest complex of Cd1 with a bound viologen guest, and the ternary allosteric complex of Cd1 with a bound DMI guest and a coordinated tbupy ligand, were solved. These structures revealed relocations of the cadmium center in and out of the porphyrin plane, depending on whether a guest or a ligand is present. 113Cd NMR could be employed as a tool to quantify the binding of guests and ligands to 113 Cd1. 1D EXSY experiments on the ternary allosteric system Cd1-tbupy-Me2V revealed that the coordination of tbupy significantly slowed down the dissociation of the Me2V guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts.

Macrocyclic metal porphyrin complexes can act as shape-selective catalysts mimicking the action of enzymes. To achieve enzyme-like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20-88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes.

Allosteric Guest Binding in Chiral Zirconium(IV) Double Decker Porphyrin Cages.

Chiral zirconium(IV) double cage sandwich complex Zr(1)2 has been synthesized in one step from porphyrin cage H21. Zr(1)2 was obtained as a racemate, which was resolved by HPLC and the enantiomers were isolated in >99.5 % ee. Their absolute configurations were assigned on the basis of X-ray crystallography and circular dichroism spectroscopy. Vibrational circular dichroism (VCD) experiments on the enantiomers of Zr(1)2 revealed that the chirality around the zirconium center is propagated throughout the whole cage structure. The axial conformational chirality of the double cage complex displayed a VCD fingerprint similar to the one observed previously for a related chiral cage compound with planar and point chirality. Zr(1)2 shows fluorescence, which is quenched when viologen guests bind in its cavities. The binding of viologen and dihydroxybenzene derivatives in the two cavities of Zr(1)2 occurs with negative allostery, the cooperativity factors α (=4 K2/K1) being as low as 0.0076 for the binding of N,N'-dimethylviologen. These allosteric effects are attributed to a pinching of the second cavity as a result of guest binding in the first cavity.

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR.

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Double Porphyrin Cage Compounds.

The synthesis and characterization of double porphyrin cage compounds are described. They consist of two porphyrins that are each attached to a diphenylglycoluril-based clip molecule via four ethyleneoxy spacers, and are linked together by a single alkyl chain using "click"-chemistry. Following a newly developed multistep synthesis procedure we report three of these double porphyrin cages, linked by spacers of different lengths, i.e. 3, 5, and 11 carbon atoms. The structures of the double porphyrin cages were fully characterized by NMR, which revealed that they consist of mixtures of two diastereoisomers. Their zinc derivatives are capable of forming sandwich-like complexes with the ditopic ligand 1,4-diazabicyclo[2,2,2]octane (dabco).

Responsive biomimetic networks from polyisocyanopeptide hydrogels.

Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells. The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen. As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum. Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model, we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications, in particular in the biomedical field.

Effect of Chirality on the Binding of Viologen Guests in Porphyrin Macrocycles.

As part of a project aimed at the development of chiral processive catalysts that can write information on a polymer chain we describe the synthesis of two optically active porphyrin macrocycles, which are prepared in 3 steps from an achiral precursor compound. Fluorescence and 1H-NMR studies show that one of the macrocycles displays selectivity in the binding of chiral viologen guest molecules.

Motion Control of Polymeric Nanomotors Based on Host-Guest Interactions.

Controlling the motion of artificial self-propelled micro- and nanomotors independent of the fuel concentration is still a great challenge. Here we describe the first report of speed manipulation of supramolecular nanomotors via blue light-responsive valves, which can regulate the access of hydrogen peroxide fuel into the motors. Light-sensitive polymeric nanomotors are built up via the self-assembly of functional block copolymers, followed by bowl-shaped stomatocyte formation and incorporation of platinum nanoparticles. Subsequent addition of ß-cyclodextrin (ß-CD) leads to the formation of inclusion complexes with the trans-isomers of the azobenzene derivatives grafted from the surfaces of the stomatocytes. ß-CDs attachment decreases the diffusion rate of hydrogen peroxide into the cavities of the motors because of partly blocking of the openings of the stomatocyte. This results in a lowering of the speed of the nanomotors. Upon blue light irradiation, the trans-azobenzene moieties isomerize to the cis-form, which lead to the detachment of the ß-CDs due to their inability to form complexes with the cis-isomer. As a result, the speed of the nanomotors increases accordingly. Such a conformational change provides us with the unique possibility to control the speed of the supramolecular nanomotor via light-responsive host-guest complexation. We envision that such artificial responsive nano-systems with controlled motion could have potential applications in drug delivery.

Modular, Bioorthogonal Strategy for the Controlled Loading of Cargo into a Protein Nanocage.

Virus capsids, i.e., viruses devoid of their genetic material, are suitable nanocarriers for biomedical applications such as drug delivery and diagnostic imaging. For this purpose, the reliable encapsulation of cargo in such a protein nanocage is crucial, which can be accomplished by the covalent attachment of the compounds of interest to the protein domains positioned at the interior of the cage. This approach is particularly valid for the capsid proteins of the cowpea chlorotic mottle virus (CCMV), which have their N-termini located at the inside of the capsid structure. Here, we examined several site-selective modification methods for covalent attachment and encapsulation of cargo at the N-terminus of the CCMV protein. Initially, we explored approaches to introduce an N-terminal azide functionality, which would allow the subsequent bioorthogonal modification with a strained alkyne to attach the desired cargo. As these methods showed compatibility issues with the CCMV capsid proteins, a strategy based on 2-pyridinecarboxaldehydes for site-specific N-terminal protein modification was employed. This method allowed the successful modification of the proteins, and was applied for the introduction of a bioorthogonal vinylboronic acid moiety. In a subsequent reaction, the proteins could be modified further with a fluorophore using the tetrazine ligation. The application of capsid assembly conditions on the functionalized proteins led to successful particle formation, showing the potential of this covalent encapsulation strategy.

Virus-like particles as crosslinkers in fibrous biomimetic hydrogels: approaches towards capsid rupture and gel repair.

Biological hydrogels can become many times stiffer under deformation. This unique ability has only recently been realised in fully synthetic gels. Typically, these networks are composed of semi-flexible polymers and bundles and show such large mechanical responses at very small strains, which makes them particularly suitable for application as strain-responsive materials. In this work, we introduced strain-responsiveness by crosslinking the architecture with a multi-functional virus-like particle. At high stresses, we find that the virus particles disintegrate, which creates an (irreversible) mechanical energy dissipation pathway, analogous to the high stress response of fibrin networks. A cooling-heating cycle allows for re-crosslinking at the damaged site, which gives rise to much stronger hydrogels. Virus particles and capsids are promising drug delivery vehicles and our approach offers an effective strategy to trigger the release mechanically without compromising the mechanical integrity of the host material.

Stabilization of a Virus-Like Particle and Its Application as a Nanoreactor at Physiological Conditions.

Virus-like particles are very interesting tools for application in bionanotechnology, due to their monodisperse features and biocompatibility. In particular, the cowpea chlorotic mottle virus (CCMV) capsid has been studied extensively as it can be assembled and disassembled reversibly, facilitating cargo encapsulation. CCMV is, however, only stable at physiological conditions when its endogenous nucleic acid cargo is present. To gain more flexibility in the type of cargo encapsulated and to broaden the window of operation, it is interesting to improve the stability of the empty virus-like particles. Here, a method is described to utilize the CCMV capsid at close to physiological conditions as a stable, enzyme-filled nanoreactor. As a proof-of-principle, the encapsulation of T4 lysozyme (T4L) was chosen; this enzyme is a promising antibiotic, but its clinical application is hampered by, for example, its cationic character. It was shown that four T4L molecules can successfully be encapsulated inside CCMV capsids, while remaining catalytically active, which could thus improve the enzyme's application potential.

Natural supramolecular protein assemblies.

Supramolecular protein assemblies are an emerging area within the chemical sciences, which combine the topological structures of the field of supramolecular chemistry and the state-of-the-art chemical biology approaches to unravel the formation and function of protein assemblies. Recent chemical and biological studies on natural multimeric protein structures, including fibers, rings, tubes, catenanes, knots, and cages, have shown that the quaternary structures of proteins are a prerequisite for their highly specific biological functions. In this review, we illustrate that a striking structural diversity of protein assemblies is present in nature. Furthermore, we describe structure-function relationship studies for selected classes of protein architectures, and we highlight the techniques that enable the characterisation of supramolecular protein structures.

Multicolor Photoluminescence Including White-Light Emission by a Single Host-Guest Complex.

Achieving multicolor photoluminescence, especially white-light emission, under mild conditions based on a single fluorescent compound is a great challenge. Herein, we report a novel colorful-emission host-guest complex BPCY, which is composed of a two-arm fluorescent guest molecule (BPC) and γ-cyclodextrin (γ-CD) as the host molecule. BPC bears a unique asymmetrical donor-acceptor-donor (D1-A+∼D2)-type structure, where D1, A+, and D2 stand for the binaphthol electron donor, pyridinium electron acceptor, and coumarin electron donor, respectively. The luminescence property of BPC shows dual-sensitivity, i.e., toward the excitation wavelength and the cyclodextrin host molecule. Under certain conditions, the complex shows three different emission wavelengths, allowing the realization of multicolor photoluminescence, including red (R), green (G), and blue (B) as well as various intermediate colors by orthogonally modulating these two stimuli. In this way, nearly pure white-light emission with CIE coordinates (0.33, 0.34) could be generated. A combination of structural, spectroscopic, and computational simulation studies revealed the occurrence of synergetic mechanistic processes for the stimuli-responsive multicolor luminescence of the BPCY complex, namely, host-enhanced intramolecular charge-transfer (ICT) and host-induced restriction of intramolecular rotation (RIR). This new supramolecular complex with superior multicolor emission abilities may find wide applications in the fields of information processing and display media. Furthermore, the molecular design rationale presented here may provide a new design strategy for the development of high performance optical materials using a single supramolecular platform.

Metal Ion-Induced Self-Assembly of a Multi-Responsive Block Copolypeptide into Well-Defined Nanocapsules.

Protein cages are an interesting class of biomaterials with potential applications in bionanotechnology. Therefore, substantial effort is spent on the development of capsule-forming designer polypeptides with a tailor-made assembly profile. The expanded assembly profile of a triblock copolypeptide consisting of a metal ion chelating hexahistidine-tag, a stimulus-responsive elastin-like polypeptide block, and a pH-responsive morphology-controlling viral capsid protein is presented. The self-assembly of this multi-responsive protein-based block copolymer is triggered by the addition of divalent metal ions. This assembly process yields monodisperse nanocapsules with a 20 nm diameter composed of 60 polypeptides. The well-defined nanoparticles are the result of the emergent properties of all the blocks of the polypeptide. These results demonstrate the feasibility of hexahistidine-tags to function as supramolecular cross-linkers. Furthermore, their potential for the metal ion-mediated encapsulation of hexahistidine-tagged proteins is shown.

Allosterically controlled threading of polymers through macrocyclic dimers.

As part of an ongoing study to construct a molecular Turing machine in which a polymer chain is encoded via allosteric information transfer between macrocyclic complexes, we describe the thermodynamic and kinetic characterization of a multicomponent self-assembled system based on a zinc porphyrin macrocyclic compound, a bidentate ligand (1,4-diazabicyclo[2.2.2]octane, DABCO), and a viologen-substituted polymer guest. Initial addition of DABCO to the porphyrin macrocycle in chloroform solution leads to the formation of a stable 2:1 (porphyrin:DABCO) dimeric complex, even under dilute conditions, by means of strong cooperative interactions involving hydrogen and metal-ligand bonds. Further titration of the porphyrin-DABCO mixtures with the polymer gives rise to a complex array of species in the solution. The system is analyzed in detail by a combination of spectroscopic measurements and computational modeling. Each association constant in the binding scheme and the fraction of each individual complex that is formed in solution are determined precisely using a mass-balance model. Kinetic studies revealed that the rates of the polymer threading and dethreading in and out of the dimeric system are remarkably slow, indicating that the polymer is locked inside the cavity of the stable 2:1 dimeric complex as a result of strong allosteric interactions.

Sortase A-Mediated N-Terminal Modification of Cowpea Chlorotic Mottle Virus for Highly Efficient Cargo Loading.

A new strategy is described for the modification of CCMV for loading of cargoes inside the viral capsid. Sortase A, an enzyme which is present in Gram-positive bacteria, was used to attach cargo to the glycine-tagged N-termini of several CCMV variants. We show that small molecules and proteins bearing a C-terminal LPETG-motif can be attached in this way. This method allows for the site-specific, covalent, and orthogonal modification of CCMV capsids in a mild fashion, leading to high encapsulation efficiencies. This strategy can easily be expanded to other types of cargoes, labeled with an LPETG-tag without altering protein function.

Slippage of a porphyrin macrocycle over threads of varying bulkiness: implications for the mechanism of threading polymers through a macrocyclic ring.

Threading of a polymer through a macrocyclic ring may occur directly, that is, by finding the end of the polymer chain, or by a process in which the polymer chain first folds and then threads through the macrocyclic ring in a hairpin-like conformation. We present kinetic and thermodynamic studies on the threading of a macrocyclic porphyrin receptor (H2 1) onto molecular threads that are blocked on one side and are open on the other side. The open side is modified by groups that vary in ease of folding and in bulkiness. Additionally, the threads contain a viologen binding site for the macrocyclic receptor, which is located close to the blocking group. The rates of threading of H2 1 were measured under various conditions, by recording as a function of time the quenching of the fluorescence of the porphyrin, which occurs when receptor H2 1 reaches the viologen binding site. The kinetic data suggest that threading is impossible if the receptor encounters an open side that is sterically encumbered in a similar way as a folded polymer chain. This indicates that threading of polymers through macrocyclic compounds through a folded chain mechanism is unlikely.