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Biomedical subjects

S G Louie

Publications and source records attributed to S G Louie.

At least 19 recordsLinked to original sources
PubMed2004

Orientation-dependent C60 electronic structures revealed by photoemission spectroscopy.

We observe, with angle-resolved photoemission, a dramatic change in the electronic structure of two C60 monolayers, deposited, respectively, on Ag (111) and (100) substrates, and similarly doped with potassium to half filling of the C60 lowest unoccupied molecular orbital. The Fermi surface symmetry, the bandwidth, and the curvature of the dispersion at Gamma point are different. Orientations of the C60 molecules on the two substrates are known to be the main structural difference between the two monolayers, and we present new band-structure calculations for some of these orientations. We conclude that orientations play a key role in the electronic structure of fullerides.

Journal Article↗
PubMed2002

Cellular expression of estrogen and progesterone receptors in mammary glands: regulation by hormones, development and aging.

At present, there is an extensive body of literature documenting the participation of estrogen receptors (ER) and progesterone receptors (PR) in mammary gene expression. Yet, the precise roles of these receptors in regulating mammary development, carcinogenesis and the growth of a subset of tumors still remain unclear. Mammary glands are composed of various cell types with different developmental potentials. Further, ultimately, that it is their mutual interactions which dictate the behavior of mammary epithelial cells. Therefore, to resolve the roles of ER and PR in normal mammary growth, differentiation and carcinogenesis, analyses for the expression of these receptors at the level of individual cell types is of paramount importance. Accordingly, in the present studies using immunolocalization techniques, we document the ontogeny and cellular distribution of ER and PR during mammary development and in response to ovarian hormones and aging. In addition, we discuss the potential biological significances of the expression patterns of ER and PR during various physiological states. We believe that the observations reported here should provide a conceptual framework(s) for elucidating the roles of ER and PR in normal and neoplastic mammary tissues.

Aging↗
PubMed2001

Anomalous quasiparticle lifetime in graphite: band structure effects.

We report ab initio calculations of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW approximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

Journal Article↗
PubMed2001

Coupling of nonlocal potentials to electromagnetic fields.

Nonlocal Hamiltonians are used widely in first-principles quantum calculations; the nonlocality stems from eliminating undesired degrees of freedom, e.g., core electrons. To date, attempts to couple nonlocal systems to external electromagnetic (EM) fields have been heuristic or limited to weak or long wavelength fields. Using Feynman path integrals, we derive an exact, closed-form coupling of arbitrary EM fields to nonlocal systems. Our results justify and clarify the couplings used to date and are essential for systematic computation of linear and especially nonlinear responses.

Journal Article↗
PubMed2001

Mechanical instability and ideal shear strength of transition metal carbides and nitrides.

The ideal shear strength of transition metal carbides and nitrides is calculated with the use of the ab initio pseudopotential density functional method. The microscopic mechanism that limits the ideal strength is studied using full atomic and structural relaxation and the results of electronic structure calculations. It is shown that plasticity in perfect crystals can be triggered by electronic instabilities at finite strains. Our study explicitly demonstrates that the ideal strength in these materials is limited by the elastic instability which is in turn initiated by electronic instabilities. The potential application of alloy hardening due to the onset of instabilities at different strains is also discussed.

Journal Article↗
PubMed2001

Vacancy hardening and softening in transition metal carbides and nitrides.

The effects of vacancies on mechanical properties of the transition metal carbides and nitrides are studied using the ab initio pseudopotential approach. Calculated shear elastic stiffness and electronic structures show that the vacancy produces entirely different effects on the mechanical strength of groups IVb nitrides and Vb carbides. It is found that the occupation of shear-unstable metallic dd bonding states changes essentially in an opposite way for the carbides and nitrides in the presence of vacancies, resulting in different responses to shear stress. Our study provides an atomistic understanding of the anomaly in hardness for these substoichiometric materials.

Journal Article↗
PubMed2001

Quasiparticle effects on tunneling currents: a study of C2H4 adsorbed on the Si(001)- (2 x 1) surface.

We present a first-principles calculation of the quasiparticle electronic structure of ethylene adsorbed on the dimer reconstructed Si(001)-(2x1) surface. Within the GW approximation, the self-energy corrections for the adsorbate states are found to be about 1.5 eV larger than those for the states derived from bulk silicon. The calculated quasiparticle band structure is in excellent agreement with photoemission spectra. Finally, the effects of the quasiparticle corrections on the scanning tunneling microscope images of the adsorbed molecules are shown to be important as the lowering of the C2H4 energy levels within GW strongly reduces their tunneling probability.

Journal Article↗
PubMed2001

Structural deformation and intertube conductance of crossed carbon nanotube junctions.

We present a first-principles study of the structure and quantum electronic conductance of junctions consisting of two crossed (5,5) single-walled carbon nanotubes. The structures are determined by constrained minimization of total energy at a given force between the two tubes, simulating the effects of substrate-tube attraction or an applied force. We find that the intertube contact distance is very sensitive to the applied force in the range of 0--10 nN. The intertube conductance is sizable for realistic deformation expected from substrate interaction. The results explain the recent transport data on crossed nanotubes and show that these systems may be potentially useful as electromechanical devices.

Journal Article↗
PubMed2001

High accuracy many-body calculational approaches for excitations in molecules.

Two state-of-the-art computational approaches: quantum Monte Carlo and GW with exciton effects [GW-BSE (Bethe-Salpeter equation)] are employed to calculate ionization potentials, electron affinities, and first excited singlet and triplet energies for the silane and methane molecules. Results are in excellent agreement between these dramatically different approaches and with available experiment. The optically forbidden triplet excitation in silane is predicted to lie roughly 1 eV higher than previously reported. In the GW-BSE method, we demonstrate that inclusion of off-diagonal matrix elements in the self-energy operator is crucial for an accurate picture.

Journal Article↗
PubMed2001

Computational design of direct-bandgap semiconductors that lattice-match silicon.

Crystalline silicon is an indirect-bandgap semiconductor, making it an inefficient emitter of light. The successful integration of silicon-based electronics with optical components will therefore require optically active (for example, direct-bandgap) materials that can be grown on silicon with high-quality interfaces. For well ordered materials, this effectively translates into the requirement that such materials lattice-match silicon: lattice mismatch generally causes cracks and poor interface properties once the mismatched overlayer exceeds a very thin critical thickness. But no direct-bandgap semiconductor has yet been produced that can lattice-match silicon, and previously suggested structures pose formidable challenges for synthesis. Much recent work has therefore focused on introducing compliant transition layers between the mismatched components. Here we propose a more direct solution to integrating silicon electronics with optical components. We have computationally designed two hypothetical direct-bandgap semiconductor alloys, the synthesis of which should be possible through the deposition of specific group-IV precursor molecules and which lattice-match silicon to 0.5-1% along lattice planes with low Miller indices. The calculated bandgaps (and hence the frequency of emitted light) lie in the window of minimal absorption in current optical fibres.

Journal Article↗
PubMed2001

Population pharmacokinetics and use of Monte Carlo simulation to evaluate currently recommended dosing regimens of ciprofloxacin in adult patients with cystic fibrosis.

Pharmacodynamic data on ciprofloxacin indicate that a target area under the concentration-time curve from 0 to 24 h (AUC(0-24))/MIC ratio of >or=125 is necessary to achieve optimal bactericidal activity for the treatment of gram-negative pneumonia. The purpose of this prospective study was to (i) develop a pharmacokinetic (PK) model to be utilized for therapeutic drug monitoring (TDM) of ciprofloxacin and (ii) evaluate current ciprofloxacin dosing regimens for pneumonias in cystic fibrosis (CF) patients. Twelve adult CF patients received a single 400-mg dose of IV ciprofloxacin. Six blood samples were obtained over a 12-h interval. Serum drug concentrations were determined by high-pressure liquid chromotography and were fitted to one- and two-compartment models by using NPEM2. Ciprofloxacin MIC data for Pseudomonas aeruginosa were obtained from 1,213 CF patients enrolled in a large clinical trial. A Monte Carlo simulation was performed to estimate the fractional attainment of an AUC(0-24)/MIC ratio of >or=125. A two-compartment model best describes the serum drug concentration data. The mean fitted PK parameter values are volume of distribution in the central compartment, 0.29 liter/kg; volume of distribution at steady state, 1.1 liters/kg; total clearance, 0.34 liter/h/kg; distributional clearance, 0.89 liter/h/kg; half-life at alpha phase, 0.16 h; and half-life at beta phase, 2.9 h. The overall fractional attainment of achieving an AUC(0-24)/MIC ratio of >or=125 against P. aeruginosa isolates with ciprofloxacin (400 mg every 12 h [q12h] and 8 qh) were 10 and 30%, respectively. A clinical breakpoint MIC of <0.5 microg/ml for susceptibility is suggested, based on an examination of the fractional attainment of the AUC(0-24)/MIC target at each MIC. The recommended doses of 400 mg q8h or q12h may be inadequate to treat an acute pulmonary exacerbation when given alone. The poor and variable AUC(0-24)/MIC ratios support the use of TDM to monitor and adjust the dosage to optimize the efficacy of ciprofloxacin therapy in these patients.

Adult↗
PubMed2001

Respiratory muscle strength, lung function, and dyspnea in patients with sarcoidosis.

BACKGROUND: Sarcoidosis is a systemic granulomatous disorder that is estimated to involve the skeletal muscles in up to 50% of patients. There is little information on the relationship among respiratory muscle strength, lung volumes, and the degree of dyspnea in patients with sarcoidosis. DESIGN AND PATIENTS: Lung function and maximal respiratory muscle force generation were measured in 36 patients with sarcoidosis (24 patients with pulmonary parenchymal infiltration) and 25 control subjects free of cardiorespiratory disease. Dyspnea in the sarcoidosis patients was quantitated by a score based on an activity tolerance assessment scale (ranging from rest to climbing hills or stairs). SETTING: Outpatient clinics of two teaching hospitals. RESULTS: Mean FVC, maximal voluntary ventilation, total lung capacity (TLC), functional residual capacity, residual volume (RV), and diffusing capacity of the lung for carbon monoxide (DLCO) were all at least 16% less than corresponding control values (in all cases, p < 0.001), while maximal inspiratory mouth pressure (PImax) and maximal expiratory mouth pressure (PEmax) were 37% and 39% less, respectively, than control values (both at p < 0.0001). PImax and PEmax declined with increasing dyspnea in a more graded, steady manner than did spirometric and DLCO values. For all measurements, however, the lowest mean values were found in patients with the most severe level of dyspnea. Strong inverse relationships were observed between PEmax and PImax with dyspnea level (p < 0.0001 and p < 0.01, respectively). Both PImax and PEmax correlated best with absolute values of FVC, while only PEmax correlated with RV (absolute and percent predicted) and percent predicted values of TLC. CONCLUSIONS: Maximal respiratory pressures correlate more closely with dyspnea level than lung volumes and DLCO. Since dyspnea is the most common presentation in early to moderately advanced sarcoidosis, respiratory pressures may be a more reliable index of functional work capacity and reflection of activities of daily living than standard tests of lung function.

Adult↗
PubMed2000

Ductal growth is impeded in mammary glands of C-neu transgenic mice.

The steroid hormone, estradiol, is essential for both the growth of normal breast and induction of mammary carcinomas. The growth promoting effects of estrogen are presumed to be mediated by growth factors, in particular, epidermal growth factor, which mediates its effects through erbB receptors, erbB1 and erbB2/C-neu. C-neu is amplified and over-expressed in a large number of human cancers and transgenic mice over-expressing C-neu also develop mammary tumors. However, as yet, the impact of C-neu over-expression on estrogen action during normal mammary development and hence, its precise role in carcinogenesis, remains unclear. In the present studies, we demonstrate that estradiol-dependent mammary ductal growth accompanying puberty is impaired in transgenic mice expressing wild type Cneu, and is intrinsic to the tissue. The impairment is not due to an overall impairment in estrogen action, since progesterone receptor expression is unaffected in C-neu mice. It is also not due to an intrinsic inability of the epithelial cells to proliferate, since impeded ductal growth co-exists with alveolar growth during pregnancy. Therefore, we propose that, depending on the physiological state, C-neu may either promote or inhibit the growth of mammary epithelial cells, and discuss its potential significance to carcinogenesis.

Aging↗
PubMed2000

Structure and energy of the 90 degrees partial dislocation in diamond: a combined Ab initio and elasticity theory analysis.

The core structure and stability of the 90 degrees partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10-20 A, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis.

Journal Article↗
PubMed2000

Estimation of creatinine clearance in patients with metastatic ovarian cancer.

The authors evaluated the predictive performance of four methods used to estimate creatinine clearance (Cl(cr)) in patients with metastatic ovarian cancer. Methods described by Cockcroft and Gault, Jelliffe, and two equations derived from cancer patients by Robinson and Tsubaki, were evaluated. Estimated Cl(cr) values obtained by each method using actual weight (ABW), ideal weight (IBW) and lower of ABW and IBW were compared with measured values determined by a 12- or 24-hour urine collection for 14 patients enrolled in a controlled clinical trial. The mean prediction errors (ME) and mean absolute errors (MAE) were calculated to evaluate the bias and precision, respectively, of each method. The relationship between predicted and measured Cl(cr) is poor (r = 0.38 to 0.54). Cockcroft and Gault using ABW (p = 0.21), Robinson using ABW (p = 0.44), and Jelliffe (p = 0.17) were equally unbiased predictors of measured Cl(cr). All other methods significantly underestimated measured Cl(cr). All methods appeared to be equally imprecise (p<0.05). The use of standard equations for estimating Cl(cr) in patients with ovarian cancer is predictive of the measured 24-hour value. The use of oncology specific equations does not improve the accuracy or precision of these estimates.

Adult↗
PubMed1993

Clinical effects of biologic response modifiers.

The clinical use of the biologic response modifiers filgrastim, sargramostim, and regramostim is reviewed. All circulating blood cells are derived from totipotent hematopoietic stem cells. Various biologic response modifiers, including lymphokines and colony-stimulating factors, regulate and activate the lymphoid and myeloid cells of the blood. One of the more important types of blood cell for fighting infection is the neutrophil. Patients with low neutrophil concentrations are at high risk of developing neutropenic fevers and infections. The colony-stimulating factors filgrastim, sargramostim, and regramostim increase the production of circulating neutrophils, and this action is clinically useful in patients undergoing myelosuppressive antineoplastic therapy or bone marrow transplantation and in patients with the acquired immunodeficiency syndrome. Clinical studies of these agents in comparison with antimicrobial prophylaxis or placebo have shown a decreased rate of neutropenic-associated hospitalizations and infections. These agents are also under study for dose intensification of antineoplastics in patients with various solid tumors and for augmenting patient responses to antimicrobial therapy in situations where there is high risk of morbidity and mortality. Sargramostim and regramostim are both granulocyte-macrophage colony-stimulating factors that differ in their degree of glycosylation and source of production, and at high doses they can cause life-threatening adverse effects because they stimulate the production of a broad range of leukocytes. Filgrastim, which stimulates only the production of neutrophils, has been better tolerated, especially at higher doses. Biologic response modifiers hold much promise for improving therapy of certain clinical conditions by decreasing myelosuppressive complications and enhancing responses to other drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Acquired Immunodeficiency Syndrome↗
PubMed1993

Pneumocystis carinii pneumonia in HIV infected patients: effects of the diseases on glutathione and glutathione disulfide.

Human immunodeficiency virus (HIV) infection is associated with altered levels of glutathione (GSH) in cells and extracellular fluids. GSH is essential for lymphocyte proliferation and inhibits HIV replication. Therefore, determination of GSH and glutathione disulfide (GSSG) levels could be useful as indicators of the progression of the disease. Thyroid hormone levels are altered in acquired immuno-deficiency syndrome (AIDS), such that thyroid hormone might be a useful prognostic indicator of the severity of AIDS. Pneumocystis carinii pneumonia (PCP) is a debilitating disease of the lung that can accompany HIV infection. The effects of pulmonary infections were assessed in AIDS patients on thyroid hormone, GSH, GSSG levels and other parameters. Two groups of AIDS patients were selected, a group with PCP and a control group with other respiratory diseases. GSH was evaluated in plasma, pulmonary lavage fluid, pulmonary biopsy tissue and buccal cells. Levels of GSSG in pulmonary lavage fluid were higher in PCP patients than in controls, which suggests that PCP patients suffer from oxygen radical toxicity in their lungs. PCP patients may have altered plasma GSH utilization such that damaged lung tissue may become less efficient at using plasma GSH. Patients with PCP may have altered CD4 cell functions such that thyroid hormone levels do not correlate with CD4 cell counts. Patients with AIDS and secondary infections of the lung were found to have altered GSH redox states, probably indicative of physiologic adaptation to AIDS.

AIDS-Related Opportunistic Infections↗