111 research outputs found
The band structure of BeTe - a combined experimental and theoretical study
Using angle-resolved synchrotron-radiation photoemission spectroscopy we have
determined the dispersion of the valence bands of BeTe(100) along ,
i.e. the [100] direction. The measurements are analyzed with the aid of a
first-principles calculation of the BeTe bulk band structure as well as of the
photoemission peaks as given by the momentum conserving bulk transitions.
Taking the calculated unoccupied bands as final states of the photoemission
process, we obtain an excellent agreement between experimental and calculated
spectra and a clear interpretation of almost all measured bands. In contrast,
the free electron approximation for the final states fails to describe the BeTe
bulk band structure along properly.Comment: 21 pages plus 4 figure
Schottky barrier heights at polar metal/semiconductor interfaces
Using a first-principle pseudopotential approach, we have investigated the
Schottky barrier heights of abrupt Al/Ge, Al/GaAs, Al/AlAs, and Al/ZnSe (100)
junctions, and their dependence on the semiconductor chemical composition and
surface termination. A model based on linear-response theory is developed,
which provides a simple, yet accurate description of the barrier-height
variations with the chemical composition of the semiconductor. The larger
barrier values found for the anion- than for the cation-terminated surfaces are
explained in terms of the screened charge of the polar semiconductor surface
and its image charge at the metal surface. Atomic scale computations show how
the classical image charge concept, valid for charges placed at large distances
from the metal, extends to distances shorter than the decay length of the
metal-induced-gap states.Comment: REVTeX 4, 11 pages, 6 EPS figure
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.
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
Toll-like receptor 2 expression is decreased on alveolar macrophages in cigarette smokers and COPD patients
BACKROUND: Cigarette smoke exposure including biologically active lipopolysaccharide (LPS) in the particulate phase of cigarette smoke induces activation of alveolar macrophages (AM) and alveolar epithelial cells leading to production of inflammatory mediators. This represents a crucial mechanism in the pathogenesis of chronic obstructive pulmonary disease (COPD). Respiratory pathogens are a major cause of exacerbations leading to recurrent cycles of injury and repair. The interaction between pathogen-associated molecular patterns and the host is mediated by pattern recognition receptors (PRR's). In the present study we characterized the expression of Toll-like receptor (TLR)- 2, TLR4 and CD14 on human AM compared to autologous monocytes obtained from patients with COPD, healthy smokers and non-smokers. METHODS: The study population consisted of 14 COPD patients without evidence for acute exacerbation, 10 healthy smokers and 17 healthy non-smokers stratified according to age. The expression of TLR2, TLR4 and CD14 surface molecules on human AM compared to autologous monocytes was assessed ex vivo using FACS analysis. In situ hybridization was performed on bronchoalveolar lavage (BAL) cells by application of the new developed HOPE-fixative. RESULTS: The expression of TLR2, TLR4 and CD14 on AM from COPD patients, smokers and non-smokers was reduced as compared to autologous monocytes. Comparing AM we detected a reduced expression of TLR2 in COPD patients and smokers. In addition TLR2 mRNA and protein expression was increased after LPS stimulation on non-smokers AM in contrast to smokers and COPD patients. CONCLUSION: Our data suggest a smoke related change in the phenotype of AM's and the cellular response to microbial stimulation which may be associated with impairment of host defenses in the lower respiratory tract
Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals
Epitaxial strain energies of epitaxial films and bulk superlattices are
studied via first-principles total energy calculations using the local-density
approximation. Anharmonic effects due to large lattice mismatch, beyond the
reach of the harmonic elasticity theory, are found to be very important in
Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that
is the elastically soft direction for biaxial expansion of Cu and Ni, but it is
for large biaxial compression of Cu, Ag, and Au. The stability of
superlattices is discussed in terms of the coherency strain and interfacial
energies. We find that in phase-separating systems such as Cu-Ag the
superlattice formation energies decrease with superlattice period, and the
interfacial energy is positive. Superlattices are formed easiest on (001) and
hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the
formation energy of superlattices increases with period, and interfacial
energies are negative. These superlattices are formed easiest on (001) or (110)
and hardest on (111) substrates. For Ni-Au we find a hybrid behavior:
superlattices along and like in phase-separating systems, while for
they behave like in ordering systems. Finally, recent experimental
results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys,
immiscible in the bulk form, are explained in terms of destabilization of the
phase separated state due to lattice mismatch between the substrate and
constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in
Physical Review
Methylation-associated down-regulation of RASSF1A and up-regulation of RASSF1C in pancreatic endocrine tumors
<p>Abstract</p> <p>Background</p> <p><it>RASSF1A </it>gene silencing by DNA methylation has been suggested as a major event in pancreatic endocrine tumor (PET) but <it>RASSF1A </it>expression has never been studied. The <it>RASSF1 </it>locus contains two CpG islands (<it>A </it>and <it>C</it>) and generates seven transcripts (<it>RASSF1A</it>-<it>RASSF1G</it>) by differential promoter usage and alternative splicing.</p> <p>Methods</p> <p>We studied 20 primary PETs, their matched normal pancreas and three PET cell lines for the (i) methylation status of the <it>RASSF1 </it>CpG islands using methylation-specific PCR and pyrosequencing and (ii) expression of <it>RASSF1 </it>isoforms by quantitative RT-PCR in 13 cases. CpG island A methylation was evaluated by methylation-specific PCR (MSP) and by quantitative methylation-specific PCR (qMSP); pyrosequencing was applied to quantify the methylation of 51 CpGs also encompassing those explored by MSP and qMSP approaches.</p> <p>Results</p> <p>MSP detected methylation in 16/20 (80%) PETs and 13/20 (65%) normal pancreas. At qMSP, 11/20 PETs (55%) and 9/20 (45%) normals were methylated in at least 20% of <it>RASSF1A </it>alleles.</p> <p>Pyrosequencing showed variable distribution and levels of methylation within and among samples, with PETs having average methylation higher than normals in 15/20 (75%) cases (<it>P </it>= 0.01). The evaluation of mRNA expression of <it>RASSF1 </it>variants showed that: i) <it>RASSF1A </it>was always expressed in PET and normal tissues, but it was, on average, expressed 6.8 times less in PET (<it>P </it>= 0.003); ii) <it>RASSF1A </it>methylation inversely correlated with its expression; iii) <it>RASSF1 </it>isoforms were rarely found, except for <it>RASSF1B </it>that was always expressed and <it>RASSF1C </it>whose expression was 11.4 times higher in PET than in normal tissue (<it>P </it>= 0.001). A correlation between <it>RASSF1A </it>expression and gene methylation was found in two of the three PET cell lines, which also showed a significant increase in <it>RASSF1A </it>expression upon demethylating treatment.</p> <p>Conclusions</p> <p><it>RASSF1A </it>gene methylation in PET is higher than normal pancreas in no more than 75% of cases and as such it cannot be considered a marker for this neoplasm. <it>RASSF1A </it>is always expressed in PET and normal pancreas and its levels are inversely correlated with gene methylation. Isoform <it>RASSF1C </it>is overexpressed in PET and the recent demonstration of its involvement in the regulation of the Wnt pathway points to a potential pathogenetic role in tumor development.</p
Which Lynch syndrome screening programs could be implemented in the "real world"? A systematic review of economic evaluations
Purpose: Lynch syndrome (LS) screening can significantly reduce cancer morbidity and mortality in mutation carriers. Our aim was to identify cost-effective LS screening programs that can be implemented in the "real world."Methods: We performed a systematic review of full economic evaluations of genetic screening for LS in different target populations; health outcomes were estimated in life-years gained or quality-adjusted life-years.Results: Overall, 20 studies were included in the systematic review. Based on the study populations, we identified six categories of LS screening program: colorectal cancer (CRC)-based, endometrial cancer-based, general population-based, LS family registry-based, cascade testing-based, and genetics clinic-based screening programs. We performed an in-depth analysis of CRC-based LS programs, classifying them into three additional subcategories: universal, age-targeted, and selective. In five studies, universal programs based on immunohistochemistry, either alone or in combination with the BRAF test, were cost-effective compared with no screening, while in two studies age-targeted programs with a cutoff of 70 years were cost-effective when compared with age-targeted programs with lower age thresholds. Conclusion: Universal or <70 years-age-targeted CRC-based LS screening programs are cost-effective and should be implemented in the "real world
MeCP2 binds to nucleosome free (linker DNA) regions and to H3K9/H3K27 methylated nucleosomes in the brain
Methyl-CpG-binding protein 2 (MeCP2) is a chromatin-binding protein that mediates transcriptional regulation, and is highly abundant in brain. The nature of its binding to reconstituted templates has been well characterized in vitro. However, its interactions with native chromatin are less understood. Here we show that MeCP2 displays a distinct distribution within fractionated chromatin from various tissues and cell types. Artificially induced global changes in DNA methylation by 3-aminobenzamide or 5-aza-2′-deoxycytidine, do not significantly affect the distribution or amount of MeCP2 in HeLa S3 or 3T3 cells. Most MeCP2 in brain is chromatin-bound and localized within highly nuclease-accessible regions. We also show that, while in most tissues and cell lines, MeCP2 forms stable complexes with nucleosome, in brain, a fraction of it is loosely bound to chromatin, likely to nucleosome-depleted regions. Finally, we provide evidence for novel associations of MeCP2 with mononucleosomes containing histone H2A.X, H3K9me2 and H3K27me3 in different chromatin fractions from brain cortex and in vitro. We postulate that the functional compartmentalization and tissue-specific distribution of MeCP2 within different chromatin types may be directed by its association with nucleosomes containing specific histone variants, and post-translational modifications
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