Quantum Monte Carlo calculations of H2_2 dissociation on Si(001)

We present quantum Monte Carlo calculations for various reaction pathways of H$_2$ with Si(001), using large model clusters of the surface. We obtain reaction energies and energy barriers noticeably higher than those from approximate exchange-correlation functionals. In improvement over previous studies, our adsorption barriers closely agree with experimental data. For desorption, the calculations give barriers for conventional pathways in excess of the presently accepted experimental value, and pinpoint the role of coverage effects and desorption from steps.Comment: 4 pages, 1 figur

Identification of the family of aquaporin genes and their expression in upland cotton (Gossypium hirsutum L.)

<p>Abstract</p> <p>Background</p> <p>Cotton (<it>Gossypium spp</it>.) is produced in over 30 countries and represents the most important natural fiber in the world. One of the primary factors affecting both the quantity and quality of cotton production is water. A major facilitator of water movement through cell membranes of cotton and other plants are the aquaporin proteins. Aquaporin proteins are present as diverse forms in plants, where they function as transport systems for water and other small molecules. The plant aquaporins belong to the large major intrinsic protein (MIP) family. In higher plants, they consist of five subfamilies including plasma membrane intrinsic proteins (PIP), tonoplast intrinsic proteins (TIP), NOD26-like intrinsic proteins (NIP), small basic intrinsic proteins (SIP), and the recently discovered X intrinsic proteins (XIP). Although a great deal is known about aquaporins in plants, very little is known in cotton.</p> <p>Results</p> <p>From a molecular cloning effort, together with a bioinformatic homology search, 71 upland cotton (<it>G. hirsutum</it>) aquaporin genes were identified. The cotton aquaporins consist of 28 PIP and 23 TIP members with high sequence similarity. We also identified 12 NIP and 7 SIP members that showed more divergence. In addition, one XIP member was identified that formed a distinct 5^thsubfamily. To explore the physiological roles of these aquaporin genes in cotton, expression analyses were performed for a select set of aquaporin genes from each subfamily using semi-quantitative reverse transcription (RT)-PCR. Our results suggest that many cotton aquaporin genes have high sequence similarity and diverse roles as evidenced by analysis of sequences and their expression.</p> <p>Conclusion</p> <p>This study presents a comprehensive identification of 71 cotton aquaporin genes. Phylogenetic analysis of amino acid sequences divided the large and highly similar multi-gene family into the known 5 aquaporin subfamilies. Together with expression and bioinformatic analyses, our results support the idea that the genes identified in this study represent an important genetic resource providing potential targets to modify the water use properties of cotton.</p

Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote’s centre

Isolation and characterisation of the first microsatellite markers for \u3ci\u3eCyperus rotundus\u3c/i\u3e

This is the first report of microsatellite markers for Cyperus rotundus. A total of 191 sequence-specific microsatellite markers were isolated and used to screen12 accessions of C. rotundus and one accession of Cyperus esculentus collected from 10 different countries. Polymorphisms were observed in 49% of the markers tested, 22% of the markers were monomorphic and 29% had weak or no amplification. The best 57 markers are reported, and cluster analysis was used to analyse their resolving power. BLASTx screening of the contig sequences was also performed. Multiallelic loci over all samples ranged from 24% to 60%. The maximum number of alleles detected by the markers suggests a polyploidy nature of all C. rotundus accessions tested, except for the sample N25-Brazil. Chromosome number was determined for N12-Taiwan and used as an internal flow cytometry standard to estimate the amount of DNA within haploid nuclei of the remaining material. Chromosome numbers estimated for C. rotundus were 16 and 24. The markers identified in this study can be used for the identification of biotypes and detection of potential crosses of C. rotundus, to implement management practices for the effective control of this weed

Electronic and structural properties of vacancies on and below the GaP(110) surface

We have performed total-energy density-functional calculations using first-principles pseudopotentials to determine the atomic and electronic structure of neutral surface and subsurface vacancies at the GaP(110) surface. The cation as well as the anion surface vacancy show a pronounced inward relaxation of the three nearest neighbor atoms towards the vacancy while the surface point-group symmetry is maintained. For both types of vacancies we find a singly occupied level at mid gap. Subsurface vacancies below the second layer display essentially the same properties as bulk defects. Our results for vacancies in the second layer show features not observed for either surface or bulk vacancies: Large relaxations occur and both defects are unstable against the formation of antisite vacancy complexes. Simulating scanning tunneling microscope pictures of the different vacancies we find excellent agreement with experimental data for the surface vacancies and predict the signatures of subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Purification of immature neuronal cells from neural stem cell progeny

Large-scale proliferation and multi-lineage differentiation capabilities make neural stem cells (NSCs) a promising renewable source of cells for therapeutic applications. However, the practical application for neuronal cell replacement is limited by heterogeneity of NSC progeny, relatively low yield of neurons, predominance of astrocytes, poor survival of donor cells following transplantation and the potential for uncontrolled proliferation of precursor cells. To address these impediments, we have developed a method for the generation of highly enriched immature neurons from murine NSC progeny. Adaptation of the standard differentiation procedure in concert with flow cytometry selection, using scattered light and positive fluorescent light selection based on cell surface antibody binding, provided a near pure (97%) immature neuron population. Using the purified neurons, we screened a panel of growth factors and found that bone morphogenetic protein-4 (BMP-4) demonstrated a strong survival effect on the cells in vitro, and enhanced their functional maturity. This effect was maintained following transplantation into the adult mouse striatum where we observed a 2-fold increase in the survival of the implanted cells and a 3-fold increase in NeuN expression. Additionally, based on the neural-colony forming cell assay (N-CFCA), we noted a 64 fold reduction of the bona fide NSC frequency in neuronal cell population and that implanted donor cells showed no signs of excessive or uncontrolled proliferation. The ability to provide defined neural cell populations from renewable sources such as NSC may find application for cell replacement therapies in the central nervous system

Frustrated H-Induced Instability of Mo(110)

Using helium atom scattering Hulpke and L"udecke recently observed a giant phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An explanation which is able to account for this and other experiments is still lacking. Below we present density-functional theory calculations of the atomic and electronic structure of the clean and hydrogen-covered Mo(110) surfaces. For the full adsorbate monolayer the calculations provide evidence for a strong Fermi surface nesting instability. This explains the observed anomalies and resolves the apparent inconsistencies of different experiments.Comment: 4 pages, 2 figures, submitted to PR

Highly site-specific H2 adsorption on vicinal Si(001) surfaces

Experimental and theoretical results for the dissociative adsorption of H_2 on vicinal Si(001) surfaces are presented. Using optical second-harmonic generation, sticking probabilities at the step sites are found to exceed those on the terraces by up to six orders of magnitude. Density functional theory calculations indicate the presence of direct adsorption pathways for monohydride formation but with a dramatically lowered barrier for step adsorption due to an efficient rehybridization of dangling orbitals.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. (1998). Other related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

ATN profile classification across two independent prospective cohorts

BACKGROUND The ATN model represents a research framework used to describe in subjects the presence or absence of Alzheimer's disease (AD) pathology through biomarkers. The aim of this study was to describe the prevalence of different ATN profiles using quantitative imaging biomarkers in two independent cohorts, and to evaluate the pertinence of ATN biomarkers to identify comparable populations across independent cohorts. METHODS A total of 172 subjects from the Geneva Memory Clinic and 113 volunteers from a study on healthy aging at the University Hospital of Zurich underwent amyloid (A) and tau (T) PET, as well as T1-weigthed MRI scans using site-specific protocols. Subjects were classified by cognition (cognitively unimpaired, CU, or impaired, CI) based on clinical assessment by experts. Amyloid data converted into the standardized centiloid scale, tau PET data normalized to cerebellar uptake, and hippocampal volume expressed as a ratio over total intracranial volume ratio were considered as biomarkers for A, T, and neurodegeneration (N), respectively. Positivity for each biomarker was defined based on previously published thresholds. Subjects were then classified according to the ATN model. Differences among profiles were tested using Kruskal-Wallis ANOVA, and between cohorts using Wilcoxon tests. RESULTS Twenty-nine percent of subjects from the Geneva cohorts were classified with a normal (A-T-N-) profile, while the Zurich cohort included 64% of subjects in the same category. Meanwhile, 63% of the Geneva and 16% of the Zurich cohort were classified within the AD continuum (being A+ regardless of other biomarkers' statuses). Within cohorts, ATN profiles were significantly different for age and mini-mental state examination scores, but not for years of education. Age was not significantly different between cohorts. In general, imaging A and T biomarkers were significantly different between cohorts, but they were no longer significantly different when stratifying the cohorts by ATN profile. N was not significantly different between cohorts. CONCLUSION Stratifying subjects into ATN profiles provides comparable groups of subjects even when individual recruitment followed different criteria

The Effect of the Environment on alpha-Al_2O_3 (0001) Surface Structures

We report that calculating the Gibbs free energy of the alpha-Al_2O_3 (0001) surfaces in equilibrium with a realistic environment containing both oxygen and hydrogen species is essential for obtaining theoretical predictions consistent with experimental observations. Using density-functional theory we find that even under conditions of high oxygen partial pressure, the metal terminated surface is surprisingly stable. An oxygen terminated alpha-Al_2O_3 (0001) surface becomes stable only if hydrogen is present on the surface. In addition, including hydrogen on the surface resolves discrepancies between previous theoretical work and experimental results with respect to the magnitude and direction of surface relaxations.Comment: 4 pages including 2 figures. Submitted to Phys. Rev. Lett. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm