Structural Rearrangements of a Dodecameric Ketol-Acid Reductoisomerase Isolated from a Marine Thermophilic Methanogen

Ketol-acid reductoisomerase (KARI) orchestrates the biosynthesis of branched-chain amino acids, an elementary reaction in prototrophic organisms as well as a valuable process in biotechnology. Bacterial KARIs belonging to class I organise as dimers or dodecamers and were intensively studied to understand their remarkable specificity towards NADH or NADPH, but also to develop antibiotics. Here, we present the first structural study on a KARI natively isolated from a methanogenic archaea. The dodecameric structure of 0.44-MDa was obtained in two different conformations, an open and close state refined to a resolution of 2.2-Å and 2.1-Å, respectively. These structures illustrate the conformational movement required for substrate and coenzyme binding. While the close state presents the complete NADP bound in front of a partially occupied Mg2+-site, the Mg2+-free open state contains a tartrate at the nicotinamide location and a bound NADP with the adenine-nicotinamide protruding out of the active site. Structural comparisons show a very high conservation of the active site environment and detailed analyses point towards few specific residues required for the dodecamerisation. These residues are not conserved in other dodecameric KARIs that stabilise their trimeric interface differently, suggesting that dodecamerisation, the cellular role of which is still unknown, might have occurred several times in the evolution of KARIs

Campholenic aldehyde ozonolysis: a mechanism leading to specific biogenic secondary organic aerosol constituents

In the present study, campholenic aldehyde ozonolysis was performed to investigate pathways leading to specific biogenic secondary organic aerosol (SOA) marker compounds. Campholenic aldehyde, a known α-pinene oxidation product, is suggested to be a key intermediate in the formation of terpenylic acid upon α-pinene ozonolysis. It was reacted with ozone in the presence and absence of an OH radical scavenger, leading to SOA formation with a yield of 0.75 and 0.8, respectively. The resulting oxidation products in the gas and particle phases were investigated employing a denuder/filter sampling combination. Gas-phase oxidation products bearing a carbonyl group, which were collected by the denuder, were derivatised by 2,4-dinitrophenylhydrazine (DNPH) followed by liquid chromatography/negative ion electrospray ionisation time-of-flight mass spectrometry analysis and were compared to the gas-phase compounds detected by online proton-transfer-reaction mass spectrometry. Particle-phase products were also analysed, directly or after DNPH derivatisation, to derive information about specific compounds leading to SOA formation. Among the detected compounds, the aldehydic precursor of terpenylic acid was identified and its presence was confirmed in ambient aerosol samples from the DNPH derivatisation, accurate mass data, and additional mass spectrometry (MS² and MS³ fragmentation studies). Furthermore, the present investigation sheds light on a reaction pathway leading to the formation of terpenylic acid, involving α-pinene, α-pinene oxide, campholenic aldehyde, and terpenylic aldehyde. Additionally, the formation of diaterpenylic acid acetate could be connected to campholenic aldehyde oxidation. The present study also provides insights into the source of other highly functionalised oxidation products (e.g. <i>m</i> / <i>z</i> 201, C₉H₁₄O₅ and <i>m</i> / <i>z</i> 215, C₁₀H₁₆O₅), which have been observed in ambient aerosol samples and smog chamber-generated monoterpene SOA. The <i>m</i> / <i>z</i> 201 and 215 compounds were tentatively identified as a C₉- and C₁₀-carbonyl-dicarboxylic acid, respectively, based on reaction mechanisms of campholenic aldehyde and ozone, as well as detailed interpretation of mass spectral data, in conjunction with the formation of corresponding DNPH derivatives

Das biegesteife Seil unter Fliehkraftbeanspruchung

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Enhanced Performance of Dye-Sensitized Solar Cells based on TiO2 Nanotube Membranes using Optimized Annealing Profile

We use free-standing TiO2 nanotube membranes that are transferred onto FTO slides in front-side illuminated dye-sensitized solar cells (DSSCs). We investigate the key parameters for solar cell arrangement of self-ordered anodic TiO2 nanotube layers on the FTO substrate and namely the influence of the annealing procedure on the DSSC light conversion efficiency. The results show that using an optimal temperature annealing profile can significantly enhance the DSSC efficiency (in our case 9.8 %), as it leads to a markedly lower density of trapping states in the tube oxide, and thus to strongly improved electron transport properties

Selektion anbauwürdiger Saflor-Formen für den Ökologischen Landbau aus einem zweijährigen Screening-Experiment

Als alternative Ölpflanze mit hervorragender Speiseölqualität könnte Saflor für den ökologischen Landbau dort in Frage kommen, wo Raps und Sonnenblumen weniger hohe Erträge zu liefern imstande sind. In der vorliegenden Studie sollte die Anbauwürdigkeit des Saflors unter den Bedingungen des Ökologischen Landbaues mit einem Screening von 741 ausgewählten Formen dieser Art überprüft werden. Das Experiment fand 2002 an zwei Standorten statt. Eine hieraus getroffene Auswahl von 65 geeigneten Herkünften wurde 2003 in einer dreiortigen Leistungsprüfung weiter getestet. 2002 wurden sowohl bei morphologischen als auch agronomisch wichtigen Merkmalen große Unterschiede zwischen den Herkünften gefunden. Besonders in der Anfälligkeit für verschiedene Krankheiten zeigte das Material eine große Variabilität. Neben ertragslosen Formen konnten eine Reihe von Genotypen mit ausreichendem Kornansatz geerntet werden, die zusätzlich im Mittel einen Kernanteil bis zu 40% aufwiesen. Das weitergeprüfte Material zeigte 2003 eine mit den Daten aus 2002 gut übereinstimmende Krankheitsanfälligkeit. Es kann hieraus gefolgert werden, dass es bei Saflor anbauwürdige Formen gibt, die auch unter unseren eher humiden Klimabedingungen noch ausreichende Leistungen erbringen können

Interfacial charge transfer in functionalized multi-walled carbon nanotube@TiO2 nanofibres

A new insight into photoinduced charge transfer processes across carbon nanotube@TiO2 interfaces has been gained based on experimental details from transient absorption spectroscopy. We show that photoinduced, interfacial hole transfer to carboxylic acid-functionalized multiwalled carbon nanotubes (oxMWCNTs) from TiO2 results in hole-doped oxMWCNTs and reduced TiO2. The latter is inferred from femto- and nanosecond transient absorption spectroscopy performed with oxMWCNT@TiO2 dispersions and complemented with investigations using methyl viologen and N,N,N\u2032,N\u2032-tetramethyl-p-phenylenediamine as an electron scavenger and a hole scavenger, respectively. The results of ultraviolet photoemission spectroscopy (UPS) of the compounds corroborate the findings, highlighting the strong coupling between oxMWCNTs and TiO2 in these hybrids

Long-Living Holes in Grey Anatase TiO2 Enable Noble-Metal-Free and Sacrificial-Agent-Free Water Splitting

Titanium dioxide has been the benchmark semiconductor in photocatalysis for more than 40 years. Full water splitting, that is, decomposing water into H2 and O2 in stoichiometric amounts and with an acceptable activity, still remains a challenge, even when TiO2-based photocatalysts are used in combination with noble-metal co-catalysts. The bottleneck of anatase-type TiO2 remains the water oxidation, that is, the hole transfer reaction from pristine anatase to the aqueous environment. In this work, we report that “grey” (defect engineered) anatase can provide a drastically enhanced lifetime of photogenerated holes, which, in turn, enables an efficient oxidation reaction of water to peroxide via a two-electron pathway. As a result, a Ni@grey anatase TiO2 catalyst can be constructed with an impressive performance in terms of photocatalytic splitting of neutral water into H2 and a stoichiometric amount of H2O2 without the need of any noble metals or sacrificial agents. The finding of long hole lifetimes in grey anatase opens up a wide spectrum of further photocatalytic applications of this material. © 2020 The Authors. Published by Wiley-VCH Gmb

Resistivity of a Metal between the Boltzmann Transport Regime and the Anderson Transition

We study the transport properties of a finite three dimensional disordered conductor, for both weak and strong scattering on impurities, employing the real-space Green function technique and related Landauer-type formula. The dirty metal is described by a nearest neighbor tight-binding Hamiltonian with a single s-orbital per site and random on-site potential (Anderson model). We compute exactly the zero-temperature conductance of a finite size sample placed between two semi-infinite disorder-free leads. The resistivity is found from the coefficient of linear scaling of the disorder averaged resistance with sample length. This ``quantum'' resistivity is compared to the semiclassical Boltzmann expression computed in both Born approximation and multiple scattering approximation.Comment: 5 pages, 3 embedded EPS figure