Bart Jan Ravoo

University of Münster, Muenster, North Rhine-Westphalia, Germany

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Publications (135)732.09 Total impact

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    ABSTRACT: Aluminium- and gallium-functionalised alkenylalkynylgermanes, R12Ge(CCR2)[C{E(CMe3)2}C(H)R2] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α-C atoms of the intact ethynyl groups. These interactions activate the GeC(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the EC(vinyl) bonds by means of 1,1-carbalumination or 1,1-carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes. Germacyclobutenes with an unsaturated GeC3 heterocycle and endo- and exocyclic CC bonds resulted from concomitant GeC bond formation to the β-C atoms of the alkynyl groups. These heterocyclic compounds show an interesting photoluminescence behaviour with Stokes shifts of >110 nm. The fascinating properties are based on extended π-delocalisation including σ*-orbitals localised at Ge and Al. High-level quantum chemical DFT and TD-DFT calculations for an Al compound were applied to elucidate their absorption and emission properties. They revealed a biradical excited state with the transfer of a π-electron into the empty p-orbital at Al and a pyramidalisation of the metal atom.
    Chemistry - A European Journal 12/2014; · 5.93 Impact Factor
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    ABSTRACT: Friction and adhesion between two β-cyclodextrin functionalized surfaces can be switched reversibly by external light stimuli. The interaction between the cyclodextrin molecules attached to the tip of an atomic force microscope and a silicon wafer surface is mediated by complexation of ditopic azobenzene guest molecules. At the single molecule level, the rupture force of an individual complex is 61±10 pN.
    Chemical Communications 12/2014; · 6.38 Impact Factor
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    ABSTRACT: A novel method to prepare microstructured polymer brushes using TiO2 nanoparticles and photocatalytic microcontact printing is described. It is shown that ethanol amine can be polymerized to linear polyethyleneimine (PEI) driven by the photocatalytic action of TiO2. Upon UV irradiation during microcontact printing of ethanol amine with a stamp coated with TiO2 nanoparticles, patterned polymer brushes with a length of around 50 nm are obtained.
    Chemical Communications 11/2014; · 6.38 Impact Factor
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    ABSTRACT: In this article we report the preparation and characterization of a peptide-based hydrogel, which possesses characteristic rheological properties, is pH responsive and can be functionalized at its thiol function. The tripeptide N-(fluorenyl-9-methoxycarbonyl)-L-Cys(acetamidomethyl)-L-His-L-Cys-OH 1 forms stable supramolecular aggregates in water leading to hydrogels above 1.5 wt%. Rheological analysis of the hydrogel revealed visco-elastic and shear thinning properties of samples containing 1.5 wt% of peptide 1. The hydrogel reversibly responds to pH changes. Below and above pH 6, electrostatic repulsion of the peptide results in a weakening of the three- dimensional gel network. Based on atomic force microscopy, small angle X-ray scattering and molecular dynamics simulations, it is proposed that the peptide assembles in bilayers, which form tapes that tend to entangle at higher concentrations in water. The development of functional materials based on the peptide assemblies was possible via thiol-ene-click chemistry of the free thiol function at the C-terminal cysteine unit. As a proof of concept, the functionalization with adamantyl units to give 1-Ad was shown by molecular recognition of β cyclodextrin vesicles. These vesicles were used as supramolecular cross-linkers of the assemblies of peptide 1 mixed with peptide 1-Ad leading to gel networks at a reduced peptide concentration.
    Organic & Biomolecular Chemistry 10/2014; · 3.57 Impact Factor
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    ABSTRACT: Controlled fusion events between natural membranes composed of phospholipids with synthetic unnatural membranes will yield valuable fundamental information on the mechanism of membrane fusion. Here, fusion between vastly different phospholipid liposomes and cyclodextrin amphiphile based vesicles (CDVs) controlled by a pair of coiled coil forming lipidated peptides was investigated. Fusion events were characterized using lipid and content mixing assays and the resulting hybrid assemblies were characterized with cryo-TEM imaging. The secondary/quaternary structure of the lipidated peptides at the membrane interface was studied using circular dichroism spectroscopy. This is the first example of targeted fusion between natural and non-natural bilayer membranes and the in situ formation of hybrid CDV/liposome structures is of interest as it yields fundamental information about the mechanism through which fusion proceeds.
    Soft Matter 10/2014; · 4.15 Impact Factor
  • Avik Samanta, Bart Jan Ravoo
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    ABSTRACT: The easy and effective separation of proteins from a mixture is crucial in proteomics. A supramolecular method is described to selectively capture and precipitate one protein from a protein mixture upon application of a magnetic field. A multivalent complex self-assembles in a dilute aqueous solution of three components: magnetic nanoparticles capped with cyclodextrin, non-covalent cross-linkers with an adamantane and a carbohydrate moiety, and lectins. The self-assembled ternary complex is precipitated in a magnetic field and readily redispersed with the aid of a non-ionic surfactant and competitive binding agents. This strategy to purify proteins by supramolecular magnetic precipitation is highly selective and efficient.
    Angewandte Chemie International Edition 09/2014; · 11.34 Impact Factor
  • Avik Samanta, Bart Jan Ravoo
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    ABSTRACT: Für die Proteomik ist die einfache und effiziente Isolierung von Proteinen aus einer Mischung essenziell. Wir beschreiben hier einen supramolekularen Ansatz, um selektiv Proteine aus einer Proteinmischung einzufangen und mithilfe eines Magnetfeldes auszufällen. Dies geschieht durch die Bildung eines multivalenten selbstorganisierten Komplexes aus einer verdünnten Lösung der folgenden drei Komponenten: Cyclodextrin-beschichtete magnetische Nanopartikel, Adamantan- und Kohlenhydrat-funktionalisierte nichtkovalente Vernetzer und Lectine. Der selbstorganisierte ternäre Komplex wird im Magnetfeld ausgefällt und kann durch die Zugabe eines konkurrierenden Bindungspartners leicht wieder in Lösung gebracht werden. Wir zeigen, dass dieser supramolekulare Ansatz zur Aufreinigung von Proteinen mithilfe magnetischer Extraktion hoch selektiv und effizient erfolgt.
    Angewandte Chemie 09/2014;
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    ABSTRACT: Exploiting metal-organic framework (MOF) materials as novel building blocks to construct superstructures with extended and enhanced functions represents a big challenge. In biological systems, the ordering of many components is not achieved by interaction of the components with each other, but by interaction of each component with the host protein which provides a matrix to support the entire assembly. Inspired by biological systems, in this work, a general strategy for efficient spatial arrangement of MOF materials was developed by using spherical colloidal crystals as host matrices, affording a new class of highly tunable MOF composite spheres with a series of distinctive properties. It was found that the synergetic combination of the unique features of both MOF and photonic colloidal crystal imparted these hierarchically structured spheres intrinsic optical properties, specific molecular recognition with self-reporting signalling, derivatization capability, and anisotropy. More importantly, the unique photonic band-gap structure integrated in these composite spheres provides a more convenient means to manipulate the photophysical and photochemical behaviour of the trapped guest molecules in MOF nanocavities.
    Nanoscale 09/2014; · 6.73 Impact Factor
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    ABSTRACT: Bioorthogonal ligation methods are the focus of current research due to their versatile applications in biotechnology and materials science for post-functionalization and immobilization of biomolecules. Recently, inverse electron demand Diels-Alder (iEDDA) reactions employing 1,2,4,5-tetrazines as electron deficient dienes emerged as powerful tools in this field. We adapted iEDDA in microcontact chemistry (μCC) in order to create enhanced surface functions. μCC is a straightforward soft-lithography technique which enables fast and large area patterning with high pattern resolutions. In this work, tetrazine functionalized surfaces were reacted with carbohydrates conjugated with norbornene or cyclooctyne acting as strained electron rich dienophiles employing μCC. It was possible to create monofunctional as well as bifunctional substrates which were specifically addressable by proteins. Furthermore we structured glass supported alkene terminated self-assembled monolayers with a tetrazine conjugated atom transfer radical polymerization (ATRP) initiator enabling surface grafted polymerizations of poly(methylacrylate) brushes. The success of the surface initiated iEDDA via μCC as well as the functionalization with natural and synthetic polymers was verified via fluorescence and optical microscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).
    Organic & Biomolecular Chemistry 08/2014; · 3.57 Impact Factor
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    ABSTRACT: In this article, we report a light-responsive supramolecular system based on the host–guest interaction of β-cyclodextrin covered silica nanoparticles and a bifunctional, noncovalent azobenzene linker in dilute aqueous solution. β-Cyclodextrin was immobilized onto silica nanoparticles either by nucleophilic substitution or by thiol–ene click chemistry. Azobenzene has two isomers which can be converted into each other by alternating irradiation with visible light (λ = 465 nm) and UV light (λ = 350 nm). When using visible light, the trans isomer of the azobenzene moieties on the linker bind to the β-cyclodextrin cavities on the nanoparticle surface leading to aggregation of the nanoparticles. The aggregation is reversible, since irradiation with UV light leads to the formation of the cis isomer of the azobenzene linker, dissociation of the azobenzene and the cyclodextrin, and subsequent dispersion of the nanoparticles. The light-responsive supramolecular system is investigated by using optical density measurements (OD600), DLS and TEM measurements.
    J. Mater. Chem. A. 06/2014; 2(25).
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    ABSTRACT: Basierend auf der Kombination der einzigartigen Eigenschaften von polyionischen Flüssigkeiten und kugelförmigen, kolloidalen Kristallen wurde eine neue Klasse von Opalkugeln mit einer Reihe besonderer Eigenschaften hergestellt. Diese photonischen Kugeln konnten nicht nur als responsive photonische Mikrogele verwendet werden, sondern auch als multifunktionelle Mikrokugeln, die die Haupteigenschaften herkömmlicher Moleküle wie optische Eigenschaften, spezifische molekulare Erkennung, Reaktivität, Derivatisierung und Anisotropie nachahmen.
    Angewandte Chemie 04/2014; 126(15).
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    ABSTRACT: Functionalization of metal, glass, and semiconductor substrates with polymers and nanoparticles is a key challenge for surface-based material science. Such substrates have the potential to find widespread application in optical and electronic devices, microarrays, and materials for information storage. We report site-specific immobilization of nitroxide-mediated polymerization initiators via microcontact chemistry employing thiol–ene click chemistry on glass-supported alkene-terminated self-assembled monolayers. The polymer initiator covered substrates are used for preparation of poly(styrene) and poly(acrylate) brushes with defined and tunable brush thicknesses. Brush thickness dependent site-specific protein adsorption of streptavidin and concanavalin A on structured polystyrene brushes is reported. Poly(styrene) brushes with a thickness of 40 nm or larger showed protein repellence whereas brushes below 15 nm thickness reveal protein adhesive properties. We also disclose the site selective host–guest assisted immobilization of β-cyclodextrin-coated silica nanoparticles and the tethering of liposomes modified with amphiphilic β-cyclodextrin onto adamantane-functionalized poly(acrylate) brushes. Selective immobilization of these supramolecular colloids via the multivalent hydrophobic inclusion complex of β-cyclodextrin and adamantane can be readily verified by fluorescence microscopy imaging, atomic force microscopy, and quartz crystal microbalance with dissipation monitoring.
    Macromolecules 03/2014; 47(7):2411–2419. · 5.93 Impact Factor
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    ABSTRACT: Cyclodextrin vesicles (CDVs) consist of a bilayer of amphiphilic cyclodextrins (CDs). CDVs exhibit CD cavities at their surface that are able to recognize and bind hydrophobic guest molecules via size-selective inclusion. In this study, the permeability of α- and β-CDVs is investigated by pulsed field gradient-stimulated echo (PFG-STE) NMR. Diffusion experiments with water and two types of water-soluble polymers, polyethylene glycol (PEG) and polypropylene glycol (PPG), revealed three main factors that influence the exchange rate and permeability of CDVs. First, the length of the hydrophobic chain of the CD amphiphile plays a crucial role. Reasonably, vesicles consisting of amphiphiles with a longer aliphatic chain are less permeable since both membrane thickness and melting temperature Tm increase. Second, the exchange rate through the bilayer membrane depends on the molecular weight of the polymer and decreases with increasing weight of the polymer. Most interestingly, a size-selective distinction of permeation due to the embedded CDs in the bilayer membrane was found. The mechanism of permeation is shown to occur through the CD cavity, such that depending on the size of the cavity, permeation of polymers with different cross-sectional diameter takes place. Whereas PPG permeates through the membrane of β-CD vesicles, it does not permeate α-CD vesicles.
    Langmuir 03/2014; · 4.38 Impact Factor
  • Avik Samanta, Bart Jan Ravoo
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    ABSTRACT: Chemical, photochemical and electrical stimuli are versatile possibilities to exert external control on self-assembled materials. Here, a trifunctional molecule that switches between an "adhesive" and a "non-adhesive" state in response to metal ions, or light, or oxidation is presented. To this end, an azobenzene-ferrocene conjugate with a flexible N,N'-bis(3-aminopropyl)ethylenediamine spacer was designed as a multistimuli-responsive guest molecule that can form inclusion complexes with β-cyclodextrin. In the absence of any stimulus the guest molecule induces reversible aggregation of host vesicles composed of amphiphilic β-cyclodextrin due to the formation of intervesicular inclusion complexes. In this case, the guest molecule operates as a noncovalent cross-linker for the host vesicles. In response to any of three external stimuli (metal ions, UV irradiation, or oxidation), the conformation of the guest molecule changes and its affinity for the host vesicles is strongly reduced, which results in the dissociation of intervesicular complexes. Upon elimination or reversal of the stimuli (sequestration of metal ion, visible irradiation, or reduction) the affinity of the guest molecules for the host vesicles is restored. The reversible cross-linking and aggregation of the cyclodextrin vesicles in dilute aqueous solution was confirmed by isothermal titration calorimetry (ITC), optical density measurements at 600 nm (OD600 ), dynamic light scattering (DLS), ζ-potential measurements and cyclic voltammetry (CV). To the best of our knowledge, a dynamic supramolecular system based on a molecular switch that responds orthogonally to three different stimuli is unprecedented.
    Chemistry - A European Journal 03/2014; · 5.93 Impact Factor
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    ABSTRACT: Based on the combination of the unique features of both polyionic liquids and spherical colloidal crystals, a new class of inverse opaline spheres with a series of distinct properties was fabricated. It was found that such photonic spheres could not only be used as stimuli-responsive photonic microgels, but also serve as multifunctional microspheres that mimic the main characteristics of conventional molecules, including intrinsic optical properties, specific molecular recognition, reactivity and derivatization, and anisotropy.
    Angewandte Chemie International Edition 03/2014; · 11.34 Impact Factor
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    ABSTRACT: A broad spectrum of physiological processes is mediated by highly specific noncovalent interactions of carbohydrates and proteins. In a recent communication we identified several cyclic hexapeptides in a dynamic combinatorial library that interact selectively with carbohydrates with high binding constants in water. Herein, we report a detailed investigation of the noncovalent interaction of two cyclic hexapeptides (Cys-His-Cys (which we call HisHis) and Cys-Tyr-Cys (which we call TyrTyr)) with a selection of monosaccharides and disaccharides in aqueous solution. The parallel and antiparallel isomers of HisHis or TyrTyr were synthesized separately, and their interaction with monosaccharides and disaccharides in aqueous solution was studied by isothermal titration calorimetry, NMR spectroscopic titrations, and circular dichroism spectroscopy. From these measurements, we identified particularly stable complexes (Ka >1000 M(-1) ) of the parallel isomer of HisHis with N-acetylneuraminic acid and with methyl-α-D-galactopyranoside as well as of both isomers of TyrTyr with trehalose. To gain further insight into the structure of the peptide-carbohydrate complexes, structure prediction was performed using quantum chemical methods. The calculations confirm the selectivity observed in the experiments and indicate the formation of multiple intermolecular hydrogen bonds in the most stable complexes.
    Chemistry - A European Journal 02/2014; · 5.93 Impact Factor
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    ABSTRACT: N-heterocyclic carbenes (NHCs) represent a leading class of ligands in organometallic chemistry, but have been rarely exploited as stabilizers for metal nanoparticles (NPs). We report the first example of NHC stabilized Pd-NPs that demonstrate long term stability. These NHC Pd-NPs were synthesized by a facile ligand exchange protocol using rationally designed long chained NHCs (LC-NHCs). Furthermore, we demonstrate that the surface modification of Pd-NPs results in significant chemoselectivity in a model reaction.
    Chemical Communications 02/2014; · 6.38 Impact Factor
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    ABSTRACT: The molecular recognition of carbohydrates and proteins mediates a wide range of physiological processes and the development of synthetic carbohydrate receptors ("synthetic lectins") constitutes a key advance in biomedical technology. In this article we report a synthetic lectin that selectively binds to carbohydrates immobilized in a molecular monolayer. Inspired by our previous work, we prepared a fluorescently labeled synthetic lectin consisting of a cyclic dimer of the tripeptide Cys-His-Cys, which forms spontaneously by air oxidation of the monomer. Amine-tethered derivatives of N-acetylneuraminic acid (NANA), β-D-galactose, β-D-glucose and α-D-mannose were microcontact printed on epoxide-terminated self-assembled monolayers. Successive prints resulted in simple microarrays of two carbohydrates. The selectivity of the synthetic lectin was investigated by incubation on the immobilized carbohydrates. Selective binding of the synthetic lectin to immobilized NANA and β-D-galactose was observed by fluorescence microscopy. The selectivity and affinity of the synthetic lectin was screened in competition experiments. In addition, the carbohydrate binding of the synthetic lectin was compared with the carbohydrate binding of the lectins concanavalin A and peanut agglutinin. It was found that the printed carbohydrates retain their characteristic selectivity towards the synthetic and natural lectins and that the recognition of synthetic and natural lectins is strictly orthogonal.
    Beilstein Journal of Organic Chemistry 01/2014; 10:1354-64. · 2.80 Impact Factor
  • Michael Kurlemann, Bart Jan Ravoo
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    ABSTRACT: Sequence-specific multivalent molecular recognition has been recognized to play a major role in biological processes. Furthermore, sequence-specific recognition motifs have been used in various artificial systems in the last years, e.g., to emulate biological processes or to build up new materials with highly specific recognition domains. In this article, we present the preparation of cyclodextrin (CD)-based strands and complementary and non-complementary strands modified with guest molecules and the investigation of their complexation behavior towards each other by isothermal titration calorimetry (ITC). As complementary binding motifs n-butyl and α-CD and adamantane and β-CD were selected. It was found that it is possible to realize sequence-specific molecular recognition by the use of host-guest chemistry, but the recognition motifs as well as the linkages have to be chosen very carefully. In the case of trivalent systems one adamantane moiety must be included to induce preferred formation of 1:1 adducts. Due to the too weak interaction between n-butyl and α-CD these systems have a negative chelate cooperativity and open adducts are preferentially formed. As soon as two adamantane moieties are present, the complementary systems have a positive chelate cooperativity and double-stranded structures are favored over open adducts. In this system the n-butyl moiety provides insufficient discrimination towards α- and β-CD and no sequence specificity is observed. By the combination of three adamantane moieties sequence specificity can be generated. Exclusively with the complementary CD sequence double-stranded structures are formed, with non-complementary strands aggregates of higher stoichiometry are generated.
    Beilstein Journal of Organic Chemistry 01/2014; 10:2428-40. · 2.80 Impact Factor

Publication Stats

1k Citations
732.09 Total Impact Points

Institutions

  • 2008–2014
    • University of Münster
      • Institute of Organic Chemistry
      Muenster, North Rhine-Westphalia, Germany
  • 2011
    • National University of Cordoba, Argentina
      • Department of Organic Chemistry
      Córdoba, Provincia de Cordoba, Argentina
  • 2009
    • Leiden University
      • Leiden Institute of Chemistry
      Leiden, South Holland, Netherlands
  • 2004–2008
    • Universiteit Twente
      • Institute for Nanotechnology (MESA+)
      Enschede, Overijssel, Netherlands
    • Università degli Studi di Messina
      • Dipartimento di Scienze Chimiche
      Messina, Sicily, Italy
    • Università degli studi di Parma
      • Department of Chemistry
      Parma, Emilia-Romagna, Italy
  • 2007
    • Technische Universiteit Eindhoven
      Eindhoven, North Brabant, Netherlands
    • Materials Science Institute of Barcelona
      Barcino, Catalonia, Spain
  • 2000–2005
    • University College Dublin
      • Centre for Synthesis and Chemical Biology
      Dublin, L, Ireland
  • 1999–2001
    • University of Groningen
      • • Groningen Biomolecular Sciences and Biotechnology Institute (GBB)
      • • Molecular Inorganic Chemistry Group
      Groningen, Province of Groningen, Netherlands