High-temperature quantum oscillations of the Hall resistance in bulk Bi2Se3.

Helically spin-polarized Dirac fermions (HSDF) in protected topological surface states (TSS) are of high interest as a new state of quantum matter. In three-dimensional (3D) materials with TSS, electronic bulk states often mask the transport properties of HSDF. Recently, the high-field Hall resistance and low-field magnetoresistance indicate that the TSS may coexist with a layered two-dimensional electronic system (2DES). Here, we demonstrate quantum oscillations of the Hall resistance at temperatures up to 50 K in nominally undoped bulk Bi2Se3 with a high electron density n of about 2·1019 cm-3. From the angular and temperature dependence of the Hall resistance and the Shubnikov-de Haas oscillations we identify 3D and 2D contributions to transport. Angular resolved photoemission spectroscopy proves the existence of TSS. We present a model for Bi2Se3 and suggest that the coexistence of TSS and 2D layered transport stabilizes the quantum oscillations of the Hall resistance.

2D layered transport properties from topological insulator Bi2Se3 single crystals and micro flakes.

Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼10(19) cm(-3)) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi2Se3 single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.

Temporally resolved GC-MS-based metabolic profiling of herbicide treated plants treated reveals that changes in polar primary metabolites alone can distinguish herbicides of differing mode of action.

We conducted a comprehensive metabolic phenotyping of primary metabolism of photosynthetic tissue of Arabidopsis thaliana following spray treatment with a number of commercially used herbicides using a well established gas-chromatography mass-spectrometry profiling method. Applying this technique we were able to identify and quantify in excess of 80 polar metabolites and based on a combination of co-elution with standards and prediction from the mass spectra a similar number of lipophillic components within two chromatographic runs. The herbicides selected were glufosinate, sulcotrione, AE944 [N2-(1-ethyl-3-phenylpropyl)-6-(1-fluoro-1-methylethyl)-1,3,5-triazine-2,4-diamine], foramsulfuron, benfuresate and glyphosate. We determined causal changes in the metabolite profiles by following their time-dependent changes using a serial sampling strategy. The resultant profiles were compared both by looking at the largest changes in a metabolite by metabolite manner and by performance of statistical analyses. These data revealed that analysis of the polar metabolites allows clear separation of the compounds under test. This finding is discussed in the context of current strategies for agrochemical discovery. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-008-0149-8) contains supplementary material, which is available to authorized users.

Crystal structure of 1-deoxy-D-xylulose 5-phosphate synthase, a crucial enzyme for isoprenoids biosynthesis.

Isopentenyl pyrophosphate (IPP) is a common precursor for the synthesis of all isoprenoids, which have important functions in living organisms. IPP is produced by the mevalonate pathway in archaea, fungi, and animals. In contrast, IPP is synthesized by a mevalonate-independent pathway in most bacteria, algae, and plant plastids. 1-Deoxy-D-xylulose 5-phosphate synthase (DXS) catalyzes the first and the rate-limiting step of the mevalonate-independent pathway and is an attractive target for the development of novel antibiotics, antimalarials, and herbicides. We report here the first structural information on DXS, from Escherichia coli and Deinococcus radiodurans, in complex with the coenzyme thiamine pyrophosphate (TPP). The structure contains three domains (I, II, and III), each of which bears homology to the equivalent domains in transketolase and the E1 subunit of pyruvate dehydrogenase. However, DXS has a novel arrangement of these domains as compared with the other enzymes, such that the active site of DXS is located at the interface of domains I and II in the same monomer, whereas that of transketolase is located at the interface of the dimer. The coenzyme TPP is mostly buried in the complex, but the C-2 atom of its thiazolium ring is exposed to a pocket that is the substrate-binding site. The structures identify residues that may have important roles in catalysis, which have been confirmed by our mutagenesis studies.

Structure and properties of an engineered transketolase from maize.

The gene specifying plastid transketolase (TK) of maize (Zea mays) was cloned from a cDNA library by southern blotting using a heterologous probe from sorghum (Sorghum bicolor). A recombinant fusion protein comprising thioredoxin of Escherichia coli and mature TK of maize was expressed at a high level in E. coli and cleaved with thrombin, affording plastid TK. The protein in complex with thiamine pyrophoshate was crystallized, and its structure was solved by molecular replacement. The enzyme is a C2 symmetric homodimer closely similar to the enzyme from yeast (Saccharomyces cerevisiae). Each subunit is folded into three domains. The two topologically equivalent active sites are located in the subunit interface region and resemble those of the yeast enzyme.

Functional analysis of the early steps of carotenoid biosynthesis in tobacco.

Carotenoids contribute to energy transduction in the light harvesting complexes and serve in protection from excess light fluence. Because of the importance of carotenoids, the genes encoding enzymes of carotenoid biosynthesis in higher plants are potential targets for herbicides. To obtain further insight into tobacco carotenoid biosynthesis and to investigate and prioritize potential herbicide targets in the pathway, the effects of changed phytoene synthase (PSY) and phytoene desaturase (PDS) gene expression were studied in transgenic tobacco (Nicotiana tabacum Petit Havana SR1) plants. Genes for both enzymes were cloned from tobacco, and surprisingly two functional PSY genes were found. Transgenic tobacco plants constitutively expressing these genes in both sense and antisense orientations were examined regarding phenotype, carotenoid content and transcript levels of carotene biosynthesis genes. Overexpression of either psy gene resulted in severe phenotypic effects including dwarfism, altered leaf morphology, and pigmentation. A correlation among phenotype, transcript level, and metabolic profile was demonstrated by comparison of hemizygous and homozygous plants from the same transformation event. Antisense expression of PSY and PDS also caused lethal phenotypes. Transcript levels of other carotene biosynthesis genes remained unaltered in the transgenic mutant. Phytoene accumulated in plants expressing antisense RNA to pds. However, elevated levels of phytoene were detected suggesting an increase in metabolic flux into this pathway.