Biomedical subjects

M van Schilfgaarde

Publications and source records attributed to M van Schilfgaarde.

10 recordsLinked to original sources

PubMed2006

First-principles combinatorial design of transition temperatures in multicomponent systems: the case of Mn in GaAs.

The transition temperature TC of multicomponent systems--ferromagnetic, superconducting, or ferroelectric--depends strongly on the atomic arrangement, but an exhaustive search of all configurations for those that optimize TC is difficult, due to the astronomically large number of possibilities. Here we address this problem by parametrizing the TC of a set of approximately 50 input configurations, calculated from first principles, in terms of configuration variables ("cluster expansion"). Once established, this expansion allows us to search almost effortlessly the transition temperature of arbitrary configurations. We apply this approach to search for the configuration of Mn dopants in GaAs having the highest ferromagnetic Curie temperature. Our general approach of cluster expanding physical properties opens the way to design based on exploring a large space of configurations.

Journal Article↗

PubMed2006

Quasiparticle self-consistent GW theory.

In past decades the scientific community has been looking for a reliable first-principles method to predict the electronic structure of solids with high accuracy. Here we present an approach which we call the quasiparticle self-consistent approximation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is constructed to minimize the perturbation. We apply it to selections from different classes of materials, including alkali metals, semiconductors, wide band gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds. Apart from some mild exceptions, the properties are very well described, particularly in weakly correlated cases. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. Discrepancies with experiment are systematic, and can be explained in terms of approximations made.

Journal Article↗

PubMed2005

Negative spin polarization and large tunneling magnetoresistance in epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions.

We perform an ab initio study of spin-polarized tunneling in epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes. We predict a large tunneling magnetoresistance in these junctions, originating from a mismatch in the majority- and minority-spin bands both in bulk bcc Co and at the Co/SrTiO(3)/Co interface. The intricate complex band structure of SrTiO(3) enables efficient tunneling of the minority d electrons which causes the spin polarization of the Co/SrTiO(3)/Co interface to be negative in agreement with experimental data. Our results indicate that epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes are a viable alternative for device applications.

Journal Article↗

PubMed2005

Anion adsorption induced reversal of coherency strain.

Experimental results are presented for stress evolution, in vacuum and electrolyte, for the first monolayer of Cu on Au(111). In electrolyte the monolayer is pseudomorphic and the stress-thickness change is -0.60 N/m, while conventional epitaxy theory predicts a value of +7.76 N/m. In vacuum, the monolayer is incoherent with the underlying gold. Using a combination of first-principles based calculations and molecular dynamic simulations we analyzed these results and demonstrate that in electrolyte, overlayer coherency is maintained owing to anion adsorption.

Journal Article↗

PubMed2005

Role of disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN.

We present calculations of magnetic exchange interactions and critical temperature T(c) in Ga1-xMnxAs, Ga1-xCrxAs, and Ga1-xCrxN. The local spin-density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg Hamiltonion, but no significant further approximations are made. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T(c); (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T(c), while ordering the dopants on a lattice increases it. With all the factors taken into account, T(c) is reasonably predicted by the local spin-density approximation in Mn:GaAs without the need to invoke compensation by donor impurities.

Journal Article↗

PubMed2001

Pressure-induced Invar effect in Fe-Ni alloys.

We have measured the pressure-volume (P-V) relations for cubic iron-nickel alloys for three different compositions: Fe 0.64Ni (0.36), Fe 0.55Ni (0.45), and Fe 0.20Ni (0.80). It is observed that for a certain pressure range the bulk modulus does not change or can even decrease to some minimum value, after which it begins to increase under still higher pressure. In our experiment, we observe for the first time a new effect, namely, that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range.

Journal Article↗

PubMed2000

Cloning of genes of nontypeable Haemophilus influenzae involved in penetration between human lung epithelial cells.

Haemophilus influenzae penetrates between epithelial cells via an unknown mechanism. A chromosomal library of nonencapsulated H. influenzae strain A960053 DNA was constructed in Escherichia coli DH5alpha to identify bacterial genes contributing to this paracytosis. Two E. coli clones that contained open reading frames (ORFs) homologous to HI0636 to HI0641 of H. influenzae strain Rd and that showed an increased penetration in epithelial cell layers of the human bronchial epithelial cell line NCI-H292 were identified. ORFs HI0636 and HI0638, encoding two small proteins of unknown functions, were further investigated. The clone containing ORFs HI0636 and HI0637 as well as the clone containing ORF HI0638 showed a significant increase in penetration. Disruption of HI0638 by kanamycin box insertion in H. influenzae strain A960053 resulted in loss of penetration into the epithelial cell layers. Disruption of HI0636 had no effect on penetration in this model system. Since a role for HI0637 in the paracytosis of H. influenzae is very unlikely because it encodes TrpS, we conclude that the protein encoded by ORF HI0638 may function as a paracytin, while that encoded by HI0636 may have an auxiliary function.

Bacterial Adhesion↗

PubMed2000

Characterization of adherence of nontypeable Haemophilus influenzae to human epithelial cells.

The adherence of 58 nontypeable Haemophilus influenzae isolates obtained from patients with otitis media or chronic obstructive pulmonary disease (COPD) and obtained from the throats of healthy individuals to Chang and NCI-H292 epithelial cells was compared. Otitis media isolates, but not COPD isolates, adhered significantly more to both cell lines than did throat isolates. Since high-molecular-weight (HMW) proteins are major adhesins of nontypeable H. influenzae, the isolates were screened for HMW protein expression by Western blotting with two polyclonal sera and PCR with hmw-specific primers. Twenty-three of the 32 adhering isolates (72%) and only 1 of the 26 nonadherent strains were HMW protein or hmw gene positive. Among the 32 isolates adhering to either cell line, 5 different adherence patterns were distinguished based on the inhibiting effect of dextran sulfate. Using H. influenzae strain 12 expressing two well-defined HMW proteins (HMW1 and HMW2) and its isogenic mutants as a reference, we observed HMW1-like adherence to both cell lines for 16 of the 32 adherent isolates. Four others showed HMW2-like adherence to NCI-H292. Of the three other patterns of adherence, one probably also involved HMW protein. Screening of the isolates with six HMW-specific monoclonal antibodies in a whole-cell enzyme-linked immunosorbent assay showed that the HMW proteins of COPD isolates and carrier isolates were more distinct from the HMW proteins from H. influenzae strain 12 than those from otitis media isolates. Characterization of the HMW protein of a COPD isolate by adherence and DNA sequence analysis showed that despite large sequence diversity in the hmwA gene, probably resulting in the antigenic differences, the HMW protein mediated the HMW2-like adherence of this strain.

Adhesins, Bacterial↗

PubMed1999

Haemophilus influenzae localized in epithelial cell layers is shielded from antibiotics and antibody-mediated bactericidal activity.

Nonencapsulated Haemophilus influenzae frequently persists in the lungs of chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) patients for prolonged periods of time. The bacteria are not eradicated by antibiotic treatment of the patients or by specific antibodies that are found in the sputum and sera of these patients. We investigated whether H. influenzae, when localized in lung epithelial cell layers, is shielded from antibiotics and from antibody-mediated bactericidal activity of specific antibodies. An in vitro model system consisting of lung epithelial NCI-H292 cells on permeable supports was developed to allow long term association of H. influenzae with the cells. Microscopic examination showed increasing numbers of H. influenzae bacteria between the epithelial cells up to 24 h of incubation. Coinciding with the microscopic observations the maximum number of cell-associated bacteria surviving gentamicin treatment of the cell layers was obtained after 24 h of incubation. All H. influenzae strains, and one Haemophilus parainfluenzae strain tested penetrated into the cell layer as determined by gentamicin killing. Cell-associated bacteria were shielded from the bactericidal activity of several antibiotics and from antibody-mediated bactericidal activity. After prolonged incubation in the cell system in the presence of a specific bactericidal antibody against major outer membrane protein (MOMP) P2, antigenic variation occurred due to a point mutation in the MOMP P2 gene, similar to point mutations observed in vivo. We conclude that penetration of H. influenzae between lung epithelial cells results in shielding the bacteria from killing by antibody dependent defense mechanisms and by antibiotics. Therefore, penetration of H. influenzae between epithelial cells may contribute to the persistence of this microorganism in COPD and CF patients.

Anti-Bacterial Agents↗

PubMed1995

Paracytosis of Haemophilus influenzae through cell layers of NCI-H292 lung epithelial cells.

Haemophilus influenzae penetrates the respiratory epithelium during carriage and invasive disease, including respiratory tract infections. We developed an in vitro model system consisting of lung epithelial NCI-H292 cells on permeable supports to study the passage of H. influenzae through lung epithelial cell layers. The NCI-H292 cells formed tight layers with a Ca(2+)-dependent transepithelial resistance of around 40 omega.cm2. H. influenzae passed through the cell layers without affecting the viability of the cells and [3H]inulin penetration. The passage time was independent of the inoculum of H. influenzae in the apical compartment and was not influenced by the presence of capsule or fimbriae on H. influenzae or by the ability of the bacteria to adhere to the epithelial cells. However, highly adherent strains showed greater paracytosis. Different strains passed through the cell layer independently. The passage time was shorter for rapidly growing strains than for slowly growing strains (10 to 18 h and 30 h, respectively). Microscopic examination revealed the presence of clusters of H. influenzae bacteria between the epithelial cells, indicating that bacterial passage was due to paracytosis. After the addition of chloramphenicol, no bacteria were cultured from the basolateral side, and no bacterial clusters between the epithelial cells were seen, suggesting that de novo bacterial protein synthesis was needed for the bacteria to reach the intercellular space. We conclude that H. influenzae passes through viable cell layers of the human lung epithelial cell line NCI-H292 by paracytosis, requiring bacterial protein synthesis.

Bacterial Adhesion↗