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)--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, Δ, 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.

Kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cages.

The kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cage compounds, measured in terms of ΔΔ‡on, is determined by the planar-chirality of the host and influenced by the size, as measured by ion mobility-mass spectrometry, but not the chirality of its substituents.

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.

Enantiodivergent epoxidation of alkenes with a photoswitchable phosphate manganese-salen complex.

The development of enantiodivergent catalysts capable of preparing both enantiomeric products from one substrate in a controlled fashion is challenging. Introducing a switching function into the catalyst can address this challenge, allowing the chiral reaction environment to reversibly change during catalysis. Here we report a photoswitchable phosphate ligand, derived from 2,2'-biphenol, which axially coordinates as the counter ion to an achiral manganese(III) salen catalyst, providing the latter with the ability to switch stereoselectivity in the epoxidation of alkenes.

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.

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

Cadmium porphyrin cage compounds and have been synthesized from the free base porphyrin cage derivative and Cd(OAc) ⋅ 2 HO or Cd(OAc) ⋅ 2 HO, respectively. The compounds form allosteric complexes with the positively charged guests '-dimethylimidazolium hexafluorophosphate () and '-dimethylviologen dihexafluorophosphate (), which bind in the cavity of the cage, and , which coordinates as an axial ligand to the outside of the cage. In the presence of , the binding of in is enhanced by a factor of ∼31, while the presence of or in the cavity of enhances the binding of by factors of 55 and 85, respectively.

Paramagnetic relaxation enhancement NMR as a tool to probe guest binding and exchange in metallohosts.

Paramagnetic metallohost systems can bind guest molecules and find application as biomimetic catalysts. Due to the presence of the paramagnetic metal center, rigorous characterization of these systems by NMR spectroscopy can be very difficult. We report here that metallohost-guest systems can be studied by using the paramagnetic relaxation enhancement (PRE) effect.

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests.

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric -symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔ = 3 kJ mol) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini.

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

Chiral zirconium(IV) double cage sandwich complex has been synthesized in one step from porphyrin cage 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.

Engineered protein cages for selective heparin encapsulation.

A heparin-specific binding peptide was conjugated to a cowpea chlorotic mottle virus (CCMV) capsid protein, which was subsequently allowed to encapsulate heparin and form capsid-like protein cages. The encapsulation is specific and the capsid-heparin assemblies display negligible hemolytic activity, indicating proper blood compatibility and promising possibilities for heparin antidote applications.

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.

Molecular motor-functionalized porphyrin macrocycles.

Molecular motors and switches change conformation under the influence of an external stimulus, e.g. light.

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.

Absolute configuration and host-guest binding of chiral porphyrin-cages by a combined chiroptical and theoretical approach.

Porphyrin cage-compounds are used as biomimetic models and substrate-selective catalysts in supramolecular chemistry. In this work we present the resolution of planar-chiral porphyrin cages and the determination of their absolute configuration by vibrational circular dichroism in combination with density functional theory calculations. The chiral porphyrin-cages form complexes with achiral and chiral viologen-guests and upon binding one of the axial enantiomorphs of the guest is bound selectively, as is indicated by induced-electronic-dichroism-spectra in combination with calculations.

Nonequilibrium Reshaping of Polymersomes Polymer Addition.

Polymersomes are a class of artificial liposomes, assembled from amphiphilic synthetic block copolymers, holding great promise toward applications in nanomedicine. The diversity in polymersome morphological shapes and, in particular, the precise control of these shapes, which is an important aspect in drug delivery studies, remains a great challenge. This is due to a lack of general methodologies that can be applied and the inability to capture the morphologies at the nanometer scale.

Porphyrin cage compounds based on glycoluril - from enzyme mimics to functional molecular machines.

The study of artificial receptor molecules with the intention to mimic enzyme-substrate binding processes and catalysis in nature has always been a traditional area of research in supramolecular chemistry. Along this line, our group has developed a family of porphyrin cage compounds based on glycoluril and employed these in host-guest binding studies, as components of allosterically controlled self-assembled processes, in which structural changes in the cage upon complexation of a guest or a ligand change binding equilibria, and as enzyme mimics in supramolecular catalysis. In a recently started research program aimed at developing a new molecular approach to long-term data storage, porphyrin cage compounds are studied as molecular machines to encode information into synthetic polymer chains.

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 H-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.

Directional threading of a chiral porphyrin cage compound onto viologen guests.

We report the face-selective threading of a chiral porphyrin cage compound onto viologen guests that are provided on both ends with substituents of different sizes. Depending on the types of terminal groups on the guest the cage compound orients itself in one of two possible directions.

Self-Assembly and Stabilization of Hybrid Cowpea Chlorotic Mottle Virus Particles under Nearly Physiological Conditions.

Capsids of the cowpea chlorotic mottle virus (CCMV) hold great promise for use as nanocarriers in vivo. A major drawback, however, is the lack of stability of the empty wild-type virus particles under physiological conditions. Herein, the assembly behavior and stability under nearly physiological conditions of protein-based block copolymers composed of the CCMV capsid protein and two hydrophobic elastin-like polypeptides are reported.

Direct Synthesis of Chiral Porphyrin Macrocyclic Receptors via Regioselective Nitration.

Nitration of tetraphenylporphyrin cage compound 1, at -40 °C, leads to the regioselective formation of the chiral mononitro compound 2 (75% isolated yield) and, at -30 °C, to the achiral syn-dinitro-derivative 3 and the chiral anti-dinitro derivative 4 in a diastereomeric ratio of 5:2, which were separated by chromatography (46 and 20% yields, respectively). The structures of the compounds were confirmed by X-ray crystallography.

Self-assembly of porphyrin hexamers via bidentate metal-ligand coordination.

The supramolecular assembly of metal-porphyrin hexamers with bidentate ligands in chloroform solutions is demonstrated by UV/Vis and 1H NMR-titrations, and Small Angle Neutron Scattering (SANS) experiments. Titrations of zinc porphyrin hexamer Zn1 with 1,4-diazabicyclo[2,2,2]octane (DABCO) revealed that at a DABCO/Zn1 molar ratio of 3, intermolecular sandwich complexes are formed, which can be considered as "circular-shaped porphyrin ladders". These supramolecular complexes further aggregate into larger polymeric stacks, as a result of a combination of cooperativity effects, π-π stacking interactions, and chelate effects.

A manganese porphyrin-α-cyclodextrin conjugate as an artificial enzyme for the catalytic epoxidation of polybutadiene.

We describe a manganese porphyrin-α-cyclodextrin conjugate as a catalyst for the epoxidation of cis-polybutadiene with trans-epoxide preference, which is a reverse stereoselectivity as compared to normal porphyrin catalysts. A clamp-like mechanism is proposed based on a combination of circular dichroism analysis, molecular modeling, and a series of epoxidation experiments.

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.