293 research outputs found
Carrier transport coefficients across GaAs-GaAIAs (100) interfaces
We present calculations of reflection and transmission coefficients for electrons and holes at (100) interfaces for the GaAsâGa_(1âx)Al_xAs system. We consider semiâinfinite crystals of the two semiconductors joined at an abrupt or compositionally graded interface. The calculations are performed using the empirical tight binding approximation. The transport coefficients were computed as a function of the components of the incident carrier wavevector normal and parallel to the interface. We have investigated the transport coefficients for incident states near various band minima into different final state channels. The transmission into states with qualitatively similar character to the incident state is found to be much greater than transmission into states of different character. For example, an electron near the X minimum normal to the interface in Ga_(1âxAl_xAs transmits into the X valley of GaAs with much greater probability than it transmits into the Î minimum of GaAs. We have investigated the dependence of the transport coefficients on alloy composition. The effect of compositional grading of the interface on the transport coefficients has also been investigated
plantiSMASH: automated identification, annotation and expression analysis of plant biosynthetic gene clusters
Plant specialized metabolites are chemically highly diverse, play key roles in host-microbe interactions, have important nutritional value in crops and are frequently applied as medicines. It has recently become clear that plant biosynthetic pathway-encoding genes are sometimes densely clustered in specific genomic loci: Biosynthetic gene clusters (BGCs). Here, we introduce plantiSMASH, a versatile online analysis platform that automates the identification of candidate plant BGCs. Moreover, it allows integration of transcriptomic data to prioritize candidate BGCs based on the coexpression patterns of predicted biosynthetic enzyme-coding genes, and facilitates comparative genomic analysis to study the evolutionary conservation of each cluster. Applied on 48 high-quality plant genomes, plantiSMASH identifies a rich diversity of candidate plant BGCs. These results will guide further experimental exploration of the nature and dynamics of gene clustering in plant metabolism. Moreover, spurred by the continuing decrease in costs of plant genome sequencing, they will allow genome mining technologies to be applied to plant natural product discovery.</p
Hole effective mass in remote doped Si/Si1âxGex quantum wells with 0.05x0.3
The effective masses in remote doped Si/Si1âxGex hole quantum wells with 0.05<=x<=0.3, have been determined from the temperature dependence of the Shubnikovâde Haas oscillations. The values are lower than previously observed by other workers, but still somewhat higher than the theoretical Gamma-point values for the ground-state heavy hole subband. The differences are attributed to finite carrier sheet densities and can be satisfactorily accounted for by nonparabolicity corrections
Biocatalytic preparation and absolute configuration of enantiomerically pure fungistatic anti-2-benzylindane derivatives. Study of the detoxification mechanism by Botrytis cinerea
Enantiomerically pure 2-benzylindane derivatives were prepared using biocatalytic methods and their
absolute configuration determined. (1R,2S)-2-Benzylindan-1-ol ((1R,2S)-2) and
(S)-2-benzylindan-1-one ((S)-3) were produced by fermenting bakerâs yeast. Lipase-mediated
esterifications and hydrolysis of the corresponding racemic substrates gave rise to the enantiopure
compounds (1S,2R)-2-benzylindan-1-ol ((1S,2R)-2) and (1R,2S)-2-benzylindan-1-ol ((1R,2S)-2),
respectively. The antifungal activity of these products against two strains of the plant pathogen Botrytis
cinerea was tested. The metabolism of anti-(±)-2-benzylindan-1-ol (anti-(±)-2) by B. cinerea as part of
the fungal detoxification mechanism is also described and revealed interesting differences in the genome
of both strains
Hole effective mass in remote doped Si/Si1âxGex quantum wells with 0.05x0.3
The effective masses in remote doped Si/Si1âxGex hole quantum wells with 0.05<=x<=0.3, have been determined from the temperature dependence of the Shubnikovâde Haas oscillations. The values are lower than previously observed by other workers, but still somewhat higher than the theoretical Gamma-point values for the ground-state heavy hole subband. The differences are attributed to finite carrier sheet densities and can be satisfactorily accounted for by nonparabolicity corrections
OeBAS and CYP716C67 catalyze the biosynthesis of health-beneficial triterpenoids in olive (Olea europaea) fruits
center dot The bioactive properties of olive (Olea europaea) fruits and olive oil are largely attributed to terpenoid compounds, including diverse triterpenoids such as oleanolic, maslinic and ursolic acids, erythrodiol, and uvaol. They have applications in the agri-food, cosmetics, and pharmaceutical industries. Some key steps involved in the biosynthesis of these compounds are still unknown.center dot Genome mining, biochemical analysis, and trait association studies have been used to identify major gene candidates controlling triterpenoid content of olive fruits.center dot Here, we identify and functionally characterize an oxidosqualene cyclase (OeBAS) required for the production of the major triterpene scaffold beta-amyrin, the precursor of erythrodiol, oleanolic and maslinic acids, and a cytochrome P450 (CYP716C67) that mediates 2 alpha oxidation of the oleanane- and ursane-type triterpene scaffolds to produce maslinic and corosolic acids, respectively. To confirm the enzymatic functions of the entire pathway, we have reconstituted the olive biosynthetic pathway for oleanane- and ursane-type triterpenoids in the heterologous host, Nicotiana benthamiana. Finally, we have identified genetic markers associated with oleanolic and maslinic acid fruit content on the chromosomes carrying the OeBAS and CYP716C67 genes.center dot Our results shed light on the biosynthesis of olive triterpenoids and provide new gene targets for germplasm screening and breeding for high triterpenoid content
Linear and Second-order Optical Response of the III-V Mono-layer Superlattices
We report the first fully self-consistent calculations of the nonlinear
optical properties of superlattices. The materials investigated are mono-layer
superlattices with GaP grown on the the top of InP, AlP and GaAs (110)
substrates. We use the full-potential linearized augmented plane wave method
within the generalized gradient approximation to obtain the frequency dependent
dielectric tensor and the second-harmonic-generation susceptibility. The effect
of lattice relaxations on the linear optical properties are studied. Our
calculations show that the major anisotropy in the optical properties is the
result of strain in GaP. This anisotropy is maximum for the superlattice with
maximum lattice mismatch between the constituent materials. In order to
differentiate the superlattice features from the bulk-like transitions an
improvement over the existing effective medium model is proposed. The
superlattice features are found to be more pronounced for the second-order than
the linear optical response indicating the need for full supercell calculations
in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.
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Quartz crystal microbalance (QCM) arrays for solution analysis
Quartz crystal microbalances (QCMs) are piezoelectric thickness-shear-mode resonators where the resonant frequency has long been known to vary linearly with the mass of rigid layers on the surface when the device is in contact with air. This reports summarizes the results from a Laboratory Directed Research and Development effort to use an array of QCMs to measure and identify volatile organic compounds (VOCs) in water solutions. A total of nine polymer-coated QCMs were tested with varying concentrations of twelve VOCs while frequency and damping voltage were measured. Results from these experiments were analyzed using a Sandia-developed pattern recognition technique called visually empirical region of influence (VERI) developed at Sandia. The VERI analyses of data with up to 16% and 50% sensitivity drifts were carried out on an array with six signals obtained from five sensors. The results indicate that better than 98% and 88% correct chemical recognition is maintained for the 16% and 50% drifts, respectively. These results indicate a good degree of robustness for these sensor films
Structural study of GaSb/AlSb strained-layer superlattice
Owing to the lattice mismatch between GaSb and AlSb, a superlattice consisting of alternating layers of these materials will be strained. We have carried out ion-channeling measurements by backscattering of 1.76-MeV He ions, and present an experimental procedure and a data-analysis technique to measure the difference in strain between the two individual layers of the superlattice. The data analysis is based on computer simulations of channeling, the accuracy of which is supported by the many fine details of the experiments reproduced in the simulations. X-ray rocking-curve analysis yielded detailed profiles of strains in directions perpendicular and parallel to the surface. The x-ray value for the strain present at an unirradiated spot on the crystal is in excellent agreement with the value calculated by elasticity theory. In the bombarded region, the values of strain are less than the value calculated by elasticity theory. It appears that bombardment by the He ions reduced the strain by 50% and created lateral inhomogeneities in the crystal structure
High toxicity and specificity of the saponin 3-GlcA-28-AraRhaxyl-medicagenate, from Medicago truncatula seeds, for Sitophilus oryzae
<p>Abstract</p> <p>Background</p> <p>Because of the increasingly concern of consumers and public policy about problems for environment and for public health due to chemical pesticides, the search for molecules more safe is currently of great importance. Particularly, plants are able to fight the pathogens as insects, bacteria or fungi; so that plants could represent a valuable source of new molecules.</p> <p>Results</p> <p>It was observed that <it>Medicago truncatul</it>a seed flour displayed a strong toxic activity towards the adults of the rice weevil <it>Sitophilus oryzae</it> (Coleoptera), a major pest of stored cereals. The molecule responsible for toxicity was purified, by solvent extraction and HPLC, and identified as a saponin, namely 3-GlcA-28-AraRhaxyl-medicagenate. Saponins are detergents, and the CMC of this molecule was found to be 0.65âmg per mL. Neither the worm <it>Caenorhabditis elegans</it> nor the bacteria <it>E. coli</it> were found to be sensitive to this saponin, but growth of the yeast <it>Saccharomyces cerevisiae</it> was inhibited at concentrations higher than 100âÎŒg per mL. The purified molecule is toxic for the adults of the rice weevils at concentrations down to 100âÎŒg per g of food, but this does not apply to the others insects tested, including the coleopteran <it>Tribolium castaneum</it> and the Sf9 insect cultured cells.</p> <p>Conclusions</p> <p>This specificity for the weevil led us to investigate this saponin potential for pest control and to propose the hypothesis that this saponin has a specific mode of action, rather than acting <it>via</it> its non-specific detergent properties.</p
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