155 research outputs found
Band bending and quasi-2DEG in the metallized -SiC(001) surface
We study the mechanism leading to the metallization of the -SiC(001)
Si-rich surface induced by hydrogen adsorption. We analyze the effects of band
bending and demonstrate the existence of a quasi-2D electron gas, which
originates from the donation of electrons from adsorbed hydrogen to bulk
conduction states. We also provide a simple model that captures the main
features of the results of first-principles calculations, and uncovers the
basic physics of the process.Comment: accepted for publication in physica status solidi - Rapid Research
Letter
Cohesive and magnetic properties of grain boundaries in bcc Fe with Cr additions
Structural, cohesive, and magnetic properties of two symmetric
and tilt grain boundaries (GBs) in pure bcc Fe and in dilute
FeCr alloys are studied from first principles. Different concentration and
position of Cr solute atoms are considered. We found that Cr atoms placed in
the GB interstice enhance the cohesion by 0.5-1.2 J/m. Substitutional Cr in
the layers adjacent to the boundary shows anisotropic effect on the GB
cohesion: it is neutral when placed in the (111) oriented Fe grains, and
enhances cohesion (by 0.5 J/m) when substituted in the boundary layer of
the (210) grains. The strengthening effect of the Cr solute is dominated by the
chemical component of the adhesive binding energy. Our calculations show that
unlike the free iron surfaces, Cr impurities segregate to the boundaries of the
Fe grains. The magnetic moments on GB atoms are substantially changed and their
variation correlates with the corresponding relaxation pattern of the GB
planes. The moments on Cr additions are 2-4 times enhanced in comparison with
that in a Cr crystal and are antiparallel to the moments on the Fe atoms
Potential Energy Surfaces for H Adsorbed at 4H-SiC{0001} Surfaces
The constant adsorption energy surfaces for hydrogen adsorbed on Si-and C-terminated hexagonal 4H-SiC{0001} surfaces have been calculated within density functional theory framework. The two unreconstructed and one reconstructed √ 3 × √ 3 surfaces were taken into account. We show that on all surfaces there is a global energy minimum indicating the most favourable adsorption site corresponding to H atom adsorption on-top of the topmost substrate layer atom. In case of reconstructed surface, there is another small and shallow local minimum. Moreover, the diusion barrier is much higher at reconstructed surface than at unreconstructed ones
An Extended, Boolean Model of the Septation Initiation Network in S.Pombe Provides Insights into Its Regulation.
Cytokinesis in fission yeast is controlled by the Septation Initiation Network (SIN), a protein kinase signaling network using the spindle pole body as scaffold. In order to describe the qualitative behavior of the system and predict unknown mutant behaviors we decided to adopt a Boolean modeling approach. In this paper, we report the construction of an extended, Boolean model of the SIN, comprising most SIN components and regulators as individual, experimentally testable nodes. The model uses CDK activity levels as control nodes for the simulation of SIN related events in different stages of the cell cycle. The model was optimized using single knock-out experiments of known phenotypic effect as a training set, and was able to correctly predict a double knock-out test set. Moreover, the model has made in silico predictions that have been validated in vivo, providing new insights into the regulation and hierarchical organization of the SIN
Analysis of S. pombe SIN protein association to the SPB reveals two genetically separable states of the SIN.
The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis, and asymmetric association of SIN proteins with the mitotic spindle pole bodies (SPBs) is important for its regulation. Here, we have used semi-automated image analysis to study SIN proteins in large numbers of wild-type and mutant cells. Our principal conclusions are: first, that the association of Cdc7p with the SPBs in early mitosis is frequently asymmetric, with a bias in favour of the new SPB; second, that the early association of Cdc7p-GFP to the SPB depends on Plo1p but not Spg1p, and is unaffected by mutations that influence its asymmetry in anaphase; third, that Cdc7p asymmetry in anaphase B is delayed by Pom1p and by activation of the spindle assembly checkpoint, and is promoted by Rad24p; and fourth, that the length of the spindle, expressed as a fraction of the length of the cell, at which Cdc7p becomes asymmetric is similar in cells dividing at different sizes. These data reveal that multiple regulatory mechanisms control the SIN in mitosis and lead us to propose a two-state model to describe the SIN
O adsorption and incipient oxidation of the Mg(0001) surface
First principles density functional calculations are used to study the early
oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <=
3 monolayers. It is found that at very low coverages O is incorporated below
the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in
on-surface sites is increased but at one monolayer coverage the on-surface
binding is still about 60 meV weaker than for subsurface sites. The subsurface
octahedral sites are found to be unfavorable compared to subsurface tetrahedral
sites and to on-surface sites. At higher coverages oxygen adsorbs both under
the surface and up. Our calculations predict island formation and clustering of
incorporated and adsorbed oxygen in agreement with previous calculations. The
calculated configurations are compared with the angle-scanned x-ray
photoelectron diffraction experiment to determine the geometrical structure of
the oxidized Mg(0001) surface.Comment: 10 pages, 5 figure
The Level of Isoprostanes as a Non-invasive Marker for in vivo Lipid Peroxidation in Secondary Progressive Multiple Sclerosis
Oxidative stress leads to lipid peroxidation and may contribute to the pathogenesis of lesions in multiple sclerosis (MS), an autoimmune disease characterized by inflammatory as well as degenerative phenomena. Isoprostanes are prostaglandin-like compounds which are formed by free radical catalysed peroxidation of arachidonic acid esterified in membrane phospholipids. They are a new class of sensitive specific markers for in vivo lipid peroxidation. In this study 26 patients (15 females and 11 males; mean age 48.2 ± 15.2 year; mean disease duration 10.0 ± 6.5 year) with secondary progressive MS (SPMS) and 12 healthy controls were enrolled. In patients with multiple sclerosis the lipid peroxidation as the level of urine isoprostanes and the level of thiobarbituric acid reactive species (TBARS) in plasma were estimated. Moreover, we estimated the total antioxidative status (TAS) in plasma. It was found that the urine isoprostanes level was over 6-fold elevated in patients with SPMS than in control (P < 0.001). In SPMS patients TBARS level was also statistically higher than in controls (P < 0.01). However, we did not observed any difference of TAS level in serum between SPMS patients and controls (P > 0.05). In patients with SPMS the lipid peroxidation and oxidative stress measured as the increased level of isoprostanes was observed. Thus, we suggest that the level of isoprostanes may be used as non-invasive marker for a determination of oxidative stress what in turn, together with clinical symptoms, may determine an specific antioxidative therapy in SPMS patients
Heavy Coronal Ions in the Heliosphere: I. Global Distribution of Charge-states of C, N, O, Mg, Si and S
Aims: Investigate/Study de-charging of solar wind C, N, O, Mg, Si and S ions
and assess fluxes of resulting ENA in the heliosphere. Methods: The model
treats the heavy ions as test particles convected by (and in a particular case
also diffusing through) a hydrodynamically calculated background plasma flow,
from 1 AU to the termination shock (TS), to heliosheath (HS) and finally to
heliospheric tail (HT). The ions undergo radiative and dielectronic
recombinations, charge exchanges, photo- and electron impact ionizations with
plasma particles, interstellar neutral atoms (calculated on a Monte-Carlo
model) and solar photons. Results: Highly-charged heavy coronal ions flowing
with the solar wind undergo successive de-ionizations, mainly in the
heliosheath, leading to charge-states much lower than in the supersonic solar
wind. If Coulomb scattering is the main ion energy loss mechanism, the end
product of these deionizations are fluxes of ENA of ~1 keV/nucleon originating
in the upwind heliosheath that for C, Mg, Si and S may constitute sources of
pickup ions (PUI) significantly exceeding the interstellar supply. Conclusions:
Discussed processes result in (i) distinct difference of ion charge q in the
supersonic solar wind (approximately q >= +Z/2, Z = atomic number) compared to
that in the HS (approximately 0 <= q <= +Z/2)), (ii) probable concentration of
singly ionized atoms (q = +1) in the heliosheath towards the heliopause (HP)
and in the HT, (iii) possible significant production of ENA in the HS offering
natural explanation for production of PUI, and -- after acceleration at the TS
-- anomalous cosmic rays (ACR) of species (like C, Mg, Si, S) unable to enter
the heliospheric cavity from outside because of their total ionization in the
local interstellar medium.Comment: Reduced-resolution version, submitted to A&A; full resolution:
http://articles.cbk.waw.pl/0printed/g/grzedzielski_etal08a.pdf. One figure
correcte
Proteome changes in platelets activated by arachidonic acid, collagen, and thrombin
<p>Abstract</p> <p>Background</p> <p>Platelets are small anucleated blood particles that play a key role in the control of bleeding. Platelets need to be activated to perform their functions and participate in hemostasis. The process of activation is accompanied by vast protein reorganization and posttranslational modifications. The goal of this study was to identify changes in proteins in platelets activated by different agonists. Platelets were activated by three different agonists - arachidonic acid, collagen, and thrombin. 2D SDS-PAGE (pI 4-7) was used to separate platelet proteins. Proteomes of activated and resting platelets were compared with each other by Progenesis SameSpots statistical software; and proteins were identified by nanoLC-MS/MS.</p> <p>Results</p> <p>190 spots were found to be significantly different. Of these, 180 spots were successfully identified and correspond to 144 different proteins. Five proteins were found that had not previously been identified in platelets: protein CDV3 homolog, protein ETHE1, protein LZIC, FGFR1 oncogene partner 2, and guanine nucleotide-binding protein subunit beta-5. Using spot expression profile analysis, we found two proteins (WD repeat-containing protein 1 and mitochondrial glycerol-3-phosphate dehydrogenase) that may be part of thrombin specific activation or signal transduction pathway(s).</p> <p>Conclusions</p> <p>Our results, characterizing the differences within proteins in both activated (by various agonists) and resting platelets, can thus contribute to the basic knowledge of platelets and to the understanding of the function and development of new antiplatelet drugs.</p
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