74 research outputs found
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Verification of impurity-related photocatalytic activity of insulating oxide supports
Photocatalysts composed of vanadium oxide species supported on commercial MgO and ZrO2 are investigated in selective methanol oxidation. Both support oxides are insulators, so the vanadium oxide species are expected as sole active component in photocatalysis. However, the pure supports showed considerable activity: Bare MgO was more active than MgO-supported vanadia catalysts, and ZrO2 showed intermediate activity. By various characterization methods, the presence of TiO2 (anatase) in the MgO support, and the presence of Zn, possibly as ZnO, in ZrO2 is demonstrated. The present study highlights that photocatalysts containing commercial supports must be carefully checked for impurity-related photocatalytic performance. © 2021 The Author
Analysis of the structure and dynamics of the type I interferon receptor
Der Typ I Interferonrezeptor, der aus den Transmembranproteinen ifnar1 und ifnar2 besteht, nimmt eine wichtige Rolle bei der angeborenen und erworbenen Immunantwort ein. Durch Bindung von Typ I Interferonen werden antivirale, antiproliferative und immunmodulatorische Aktivitäten in der Zelle induziert. Die Wirkung der Interferone wird bereits bei der Behandlung einer Vielzahl von Krankheiten eingesetzt. Es ist bislang nicht bekannt, wie die verschiedenen Typ I Interferone nach Bindung an einen gemeinsamen Rezeptor, unterschiedliche Zellantworten induzieren. So unterscheiden die Typ I Interferone sich nicht hinsichtlich ihrer Bindungsstelle oder der Stöchiometrie der Bindung an ifnar1 bzw. ifnar2. Sie weisen jedoch unterschiedliche Affinitäten zu den Rezeptoruntereinheiten auf, wobei ihnen eine niedrigere Affinität zu ifnar1 gemeinsam ist. Bislang konnte keine Interaktion zwischen den Rezeptoruntereinheiten nachgewiesen werden. Es wird angenommen, dass bei der Rezeptorassemblierung das Interferon zunächst an ifnar2 bindet und anschließend ifnar1 rekrutiert. Es wird postuliert, dass die unterschiedlichen Zellantworten für verschiedene Typ I Interferone auf Unterschieden in der Stabilität der ternären Komplexe beruhen könnten. Im Rahmen dieser Arbeit wurden daher die Struktur und Dynamik des Interferonrezeptors in vitro für die Typ I Interferone IFNa2 und IFNb charakterisiert. Die Struktur des ternären Komplexes aus den extrazellulären Domänen von ifnar1 und ifnar2 mit IFNa2 wurde mittels Elektronenmikroskopie untersucht. Über Einzelpartikelanalyse aufgereinigter Komplexe von IFN mit den extrazellulären Domänen von ifnar1 (ifnar1-EC) und ifnar2 (ifnar2-EC) konnte ein Strukturmodell des ternären Komplexes erstellt werden. Dieses zeigte eine Verschiebung der membranproximalen Domänen von ifnar1-EC und ifnar2-EC wie sie bereits für den Rezeptor für Erythropoietin und den Wachstumsfaktor beobachtet wurden, welche zu den Typ I Zytokinrezeptoren gehören. Die Struktur des ternären Komplexes ermöglicht als erste Struktur eines Typ II Zytokinrezeptors einen Einblick in die Architektur des Komplexes und mögliche Aktivierungsmechanismen. Die Strukturen der Komplexe für die verschiedenen Typ I Interferone IFNa2 und IFNb wiesen keine fundamentalen Unterschiede auf, was auf einen gemeinsamen Aktivierungsmechanismus hinweist. Temperatur-abhängige Messungen von Bindungskinetik und –affinität ergaben sehr unterschiedliche Energiehyperflächen für die Ligandenbindung an ifnar1- und ifnar2-EC, und wiesen auf einen mehrstufigen Prozess und mögliche Konformationsänderungen bei der Bindung an ifnar1-EC hin. Zur Analyse der Dynamik von ifnar1-EC wurden daher verschiedene fluoreszenzbasierte Assays etabliert. Eine besondere Herausforderung bestand darin, das Protein ortsspezifisch und stöchiometrisch mit zwei verschiedenen Fluorophoren zu koppeln. Ifnar1-EC wurde an verschiedenen Stellen kovalent mit Fluoreszenzfarbstoffen markiert. Es wurde gezeigt, dass nach Bindung eines geeigneten tris-NTA-Fluorophor-Konjugats an den C-terminalen His-Tag die Fluoreszenz abstandsabhägig durch Förster-Resonanz-Energie-Transfer gelöscht wurde. Für ifnar1-EC wurde eine ligandeninduzierte Abstandsänderung detektiert. Die detaillierte Analyse ergab nach Bindung von IFNa2 eine Abstandszunahme von 13 A vom N- zum C-Terminus. Durch die Interferonbindung nimmt demnach ifnar1-EC eine gestrecktere Konformation ein. Ähnliche Ergebnisse wurden auch in Anwesenheit von ifnar2-EC und für IFNb erhalten. Die Einzelmolekülanalysen mittels Fluoreszenz Korrelationsspektroskopie (FCS) zeigten sowohl einen Verlust der Flexibilität von ifnar1-EC nach Ligandenbindung als auch ein ligandeninduziertes Rearangement der Ig-ähnlichen Domänen. Die Änderung der Flexibilität wurde durch Messungen der Fluoreszenzlebensdauer bestätigt. Untersuchungen der Kinetik der Ligand-induzierten Konformationsänderung mittels Stopped-Flow Messungen bestätigten eine mehrstufige Umorientierung der Ig-ähnlichen Domänen nach Ligandenbindung. Zusätzlich konnte gezeigt werden, dass sich nach Ligandenbindung die Zugänglichkeit des Tryptophans in der membranproximalen Domäne von ifnar1-EC ändert. Da die membranproximale Domäne nicht bei der Ligandenbindung beteiligt ist, deutet dieser Effekt auf eine Propagation der Ligand-induzierten Konformationsänderung in diese Domäne hin. Das Tryptophan könnte mit der Membran interagieren, was auf eine wichtige Rolle der membranproximalen Domäne für die korrekte Orientierung von ifnar1 in der Membran hindeut. Die Stopped-Flow Analyse zeigte, dass es sich hierbei um einen einstufigen Prozess handelt, der mit der Interferonbindung korreliert. Die Ergebnisse wiesen insgesamt auf eine Ligand-induzierte Flexibilitätsänderung und Umorientierung der Ig-ähnlichen Domänen bei ifnar1-EC hin. Vermutlich wird nach Ligandenbindung das Signal in die membranproximale Domäne von ifnar1-EC propagiert. Die Strukturen der ternären Komplexe mit den verschiedenen Typ I Interferonen wiesen keine fundamentalen Unterschiede auf. Auch die Ergebnisse der fluoreszenzbasierten Assays zeigten keine Unterschiede für IFNa2 und IFNb, was die Hypothese stützt, dass die differentielle Aktivität der Interferone nicht auf grundsätzlichen Unterschieden in der Architektur des ternären Komplexes beruht, sondern in der unterschiedlichen Dynamik der Komplexe codiert sein könnte.The type I interferon receptor, which is comprised of the transmembrane proteins ifnar1 and ifnar2, plays a key role in mediating innate and adaptive immune responses. Binding of type I interferons induce antiviral, antiproliferative and immunmodulatory activities in the cell. Interferons have been used to treat various diseases, but it remains unclear, how binding of type I interferons to the same receptor leads to differential activities. Type I interferons share a common binding site on ifnar1 and ifnar2 and bind with the same stoichiometry. Different members of the family bind with different affinities to the receptor subunits, whereas the affinity to ifnar1 is always lower. Interaction between the receptor subunits has not been detected. Interferons bind first to ifnar2. The binary complex then recruits ifnar1. Differences in the stability of the ternary complexes could lead to differential cell activities. In this work the structure and dynamics of the type I interferon receptor have been characterized in vitro for the type I interferons IFNa2 and IFNb. Electron microscopy was used to obtain a model of the structure of the ternary complex including the extracellular domains of ifnar1 (ifnar1-EC) and ifnar2 (ifnar2-EC) and IFNa2. The model of the ternary complex shows a characteristic shift of the membrane proximal domain of ifnar1-EC and ifnar2-EC as it is also observed for the receptor complexes for erythropoietin and for growth hormone, which belong to the class I cytokine receptor family. The structure of the type I interferon receptor is the first structure of a class II cytokine receptor. This gives a first impression of the architecture of the complex and possible activation mechanisms. No fundamental differences in the structure of the complexes with IFNa2 and IFNb suggest a common activation mechanism of the receptor. Temperature-dependent measurements of the binding kinetics and affinities revealed different energy landscapes for the ligand binding to ifnar1- and ifnar2-EC, which points to a possible ligand induced conformational change in ifnar1-EC. Several fluorescence-based assays have been established in this work to analyze the conformational dynamics of ifnar1-EC. For this purpose, the proteins were site-specifically labeled with two different fluorophores. Ifnar1-EC was covalently coupled at different positions with fluorescence donor dyes using cysteine-specific chemistry. Binding of a suitable acceptor dye conjugated to tris-NTA to the C-terminal His-Tag led to a distance-dependent quenching of the fluorescence via fluorescence resonance energy transfer. A ligand-induced change of the distance was detected for ifnar1-EC, but not ifnar2-EC. Detailed analysis of the FRET efficiencies revealed an increase of the distance between N- and C-terminus of 13 A after binding of IFNa2. Therefore, after ligand binding ifnar1-EC adopts a more straight conformation. Experiments with IFNb and IFNa2 in complex with ifnar2-EC gave similar results. Fluorescence correlation spectroscopy (FCS) on single molecule level showed a decrease of flexibility of ifnar1-EC after ligand binding as well as a ligand induced rearrangement of the Ig-like domains. A change of conformational flexibility was confirmed by fluorescence-lifetime measurements. Stopped-flow-analysis of the kinetics of the ligand-induced conformational change confirmed a multistep rearrangement of the Ig-like domains after ligand binding. Moreover it was shown that upon ligand binding the accessibility of the tryptophan in the membrane-proximal domain of ifnar1-EC is changed. This domain is not involved in ligand recognition, which suggests a propagation of the ligand-induced conformational change into this domain. The tryptophan may interact with the membrane, which could be important for the correct orientation of ifnar1-EC in the membrane. Stopped-flow-analysis showed a single step process, which correlated with IFN-binding. Taken together, these results suggest a ligand-induced loss of conformational flexibility and a rearrangement of the Ig-like domains of ifnar1-EC. Probably the signal is propagated to the membrane-proximal domain of ifnar1-EC. The structures of the ternary complexes with different type I interferons were very similar. Highly similar conformational changes were observed for IFNa2 and IFNb. This supports the hypothesis that the differential activity of type I interferons relies on different dynamics of the ternary complexes and is not based on fundamental differences in the architecture of the complexes
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Photocatalytic Reduction of CO2 by Metal-Free-Based Materials: Recent Advances and Future Perspective
Photocatalytic CO2 reduction to produce valuable chemicals and fuels using solar energy provides an appealing route to alleviate global energy and environmental problems. Searching for photocatalysts with high activity and selectivity for CO2 conversion is the key to achieving this goal. Among the various proposed photocatalysts, metal-free materials, such as graphene, nitrides, carbides, and conjugated organic polymers, have gained extensive research interest for photocatalytic CO2 reduction, due to their earth abundance, cost-effectiveness, good electrical conductivity, and environmental friendliness. They exhibit prominent catalytic activity, impressive selectivity, and long durability for the conversion of CO2 to solar fuels. Herein, the recent progress on metal-free photocatalysis of CO2 reduction is systematically reviewed. Opportunities and challenges on modification of nonmetallic catalysts to enhance CO2 transformation are presented. Theoretical calculations on possible reduction mechanisms and pathways as well as the potential in situ and operando techniques for mechanistic understanding are also summarized and discussed. Based on the aforementioned discussions, suitable future research directions and perspectives for the design and development of potential nonmetallic photocatalysts for efficient CO2 reduction are provided. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
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Photocatalytic nitrogen reduction to ammonia: Insights into the role of defect engineering in photocatalysts
Engineering of defects in semiconductors provides an effective protocol for improving photocatalytic N2 conversion efficiency. This review focuses on the state-of-the-art progress in defect engineering of photocatalysts for the N2 reduction toward ammonia. The basic principles and mechanisms of thermal catalyzed and photon-induced N2 reduction are first concisely recapped, including relevant properties of the N2 molecule, reaction pathways, and NH3 quantification methods. Subsequently, defect classification, synthesis strategies, and identification techniques are compendiously summarized. Advances of in situ characterization techniques for monitoring defect state during the N2 reduction process are also described. Especially, various surface defect strategies and their critical roles in improving the N2 photoreduction performance are highlighted, including surface vacancies (i.e., anionic vacancies and cationic vacancies), heteroatom doping (i.e., metal element doping and nonmetal element doping), and atomically defined surface sites. Finally, future opportunities and challenges as well as perspectives on further development of defect-engineered photocatalysts for the nitrogen reduction to ammonia are presented. It is expected that this review can provide a profound guidance for more specialized design of defect-engineered catalysts with high activity and stability for nitrogen photochemical fixation
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Role of SrCO3 on Photocatalytic Performance of SrTiO3-SrCO3 Composites
Perovskites such as SrTiO3 are interesting for photocatalytic applications due to their structure-related and electronic properties. These properties are influenced by the presence of SrCO3 which is often formed simultaneously during the hydrothermal synthesis of SrTiO3. In this study, SrTiO3-SrCO3 composites with different contents of SrCO3 (5–24 wt%) were synthesized. Their morphological, structural, and optical properties were investigated using complementary methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen sorption, and diffuse reflectance spectroscopy (DRS). Their photocatalytic activity was assessed during the degradation of diclofenac (DCFNa) in aqueous solution and CO2 photoreduction under Xe lamp irradiation. Improved photocatalytic efficiency in DCFNa degradation was observed for all the studied composites in comparison with SrTiO3, and the highest mineralization efficiency was obtained for the sample with 21 wt% SrCO3 content. The presence of SrCO3 led to an increased concentration of active species, such as •OH radicals. Otherwise, its presence inhibits CH4 and C2H6 production during CO2 photoreduction compared with pure SrTiO3
Experimental data-driven and phenomenological modeling approaches targeting the enhancement of CaTiO3 photocatalytic efficiency
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Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy
By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase
The Lantern Vol. 73, No. 1, Fall 2005
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The Lantern Vol. 74, No. 2, Spring 2007
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