Publications

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The room-temperature crack-growth properties of anin situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900°C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the β to α transformation. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first ∼ 600 μm of crack extension, leading to a “plateau” fracture toughness ofKc ∼ 9.1MPa√m; this represents more than a threefold increase over the toughness of Hexology, where aKc value of 2.5 MPa√m was measured with no evidence of a resistance curve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistance was found to be far superior to that of Hexology. Whereas cracking in the commercial SiC became unstable when the maximum stress intensityKmax exceeded ∼ 2 MPa√m, thresholds for fatigue-crack growth in thein situ toughened material exceeded aKmax of 7 MPa√m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly α-phase grains, which combine to both bridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding due to frictional-wear degradation of the zone of grain bridging ligaments in the crack wake. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.

Abstract Laboratory evolution is a powerful approach to search for genetic adaptations to new or improved phenotypes, yet either relies on labour-intensive human-guided iterative rounds of mutagenesis and selection, or prolonged adaptation regimes based on naturally evolving cell populations. Here we present CRISPR- and RNA-assisted in vivo directed evolution (CRAIDE) of genomic loci using evolving chimeric donor gRNAs continuously delivered from an error-prone T7 RNA polymerase, and directly introduced as RNA repair donors into genomic targets under either Cas9 or dCas9 guidance. We validate CRAIDE by evolving novel functional variants of an auxotrophic marker gene, and by conferring resistance to a toxic amino acid analogue in baker’s yeast Saccharomyces cerevisiae with a mutation rate >3,000-fold higher compared to spontaneous native rate, thus enabling the first demonstrations of in vivo delivery and information transfer from long evolving RNA donor templates into genomic context without the use of in vitro supplied and pre-programmed repair donors.

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and K JIc fracture-toughness values above 200 MPa m 1/2 ; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and K JIc values of 275 MPa m 1/2 . Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.

The biosynthesis of iron sulfur (FeS) clusters, their trafficking from initial assembly on scaffold proteins via carrier proteins to final incorporation into FeS apoproteins, is a highly coordinated process enabled by multiprotein systems encoded in iscRSUAhscBAfdx and sufABCDSE operons in Escherichia coli. Although these systems are believed to encode all factors required for initial cluster assembly and transfer to FeS carrier proteins, accessory factors such as monothiol glutaredoxin, GrxD, and the FeS carrier protein NfuA are located outside of these defined systems. These factors have been suggested to function both as shuttle proteins acting to transfer clusters between scaffold and carrier proteins and in the final stages of FeS protein assembly by transferring clusters to client FeS apoproteins. Here we implicate both of these factors in client protein interactions. We demonstrate specific interactions between GrxD, NfuA, and the methylthiolase MiaB, a radical S-adenosyl-L-methionine-dependent enzyme involved in the maturation of a subset of tRNAs. We show that GrxD and NfuA physically interact with MiaB with affinities compatible with an in vivo function. We furthermore demonstrate that NfuA is able to transfer its cluster in vitro to MiaB, whereas GrxD is unable to do so. The relevance of these interactions was demonstrated by linking the activity of MiaB with GrxD and NfuA in vivo. We observe a severe defect in in vivo MiaB activity in cells lacking both GrxD and NfuA, suggesting that these proteins could play complementary roles in maturation and repair of MiaB.

We describe the construction and performance of dye-sensitized solar cells (DSCs) based on arrays of ZnO nanowires coated with thin shells of amorphous Al(2)O(3) or anatase TiO(2) by atomic layer deposition. We find that alumina shells of all thicknesses act as insulating barriers that improve cell open-circuit voltage (V(OC)) only at the expense of a larger decrease in short-circuit current density (J(SC)). However, titania shells 10-25 nm in thickness cause a dramatic increase in V(OC) and fill factor with little current falloff, resulting in a substantial improvement in overall conversion efficiency, up to 2.25% under 100 mW cm(-2) AM 1.5 simulated sunlight. The superior performance of the ZnO-TiO(2) core-shell nanowire cells is a result of a radial surface field within each nanowire that decreases the rate of recombination in these devices. In a related set of experiments, we have found that TiO(2) blocking layers deposited underneath the nanowire films yield cells with reduced efficiency, in contrast to the beneficial use of blocking layers in some TiO(2) nanoparticle cells. Raising the efficiency of our nanowire DSCs above 2.5% depends on achieving higher dye loadings through an increase in nanowire array surface area.

The metal-oxygen bond plays a critical role in some of the most important biological and synthetic reactions. However, the majority of these processes result in the oxidation of the target organic substrate; applications of this class of metal complexes to other organic transformations are extremely rare. In this paper, we report a new type of catalytic process in which complexes with metal-oxygen multiple bonds are used as reductants rather than oxidants. The overall reaction provides a highly chemoselective reduction/protection of carbonyl groups. In addition to providing a new way of catalyzing organic reactions, these catalysts can be used in the presence of a wide range of other functional groups such as amino, cyano, nitro, aryl halo, ester, and alkene; unlike many of their late metal relatives, they are inexpensive as well as air and moisture tolerant.

Mit vielfach funktionalisierten Kettenenden versehene Pfropf‐ und dendrimerartige Pfropfcopolymere (Beispiel rechts, PS = Polystyrol, PBMA = Poly‐ n ‐ butylmethacrylat) können mit aufeinanderfolgenden „lebenden”︁ radikalischen Polymerisationen hergestellt werden. Die erste Polymerisation liefert das lineare Polymerrückgrat, in den folgenden werden die verzweigten Ketten eingeführt. magnified image