415 research outputs found
Complex-band structure: a method to determine the off-resonant electron transport in oligomers
We validate that off-resonant electron transport across {\it ultra-short}
oligomer molecular junctions is characterised by a conductance which decays
exponentially with length, and we discuss a method to determine the damping
factor via the energy spectrum of a periodic structure as a function of complex
wavevector. An exact mapping to the complex wavevector is demonstrated by
first-principle-based calculations of: a) the conductance of molecular
junctions of phenyl-ethynylene wires covalently bonded to graphitic ribbons as
a function of the bridge length, and b) the complex-band structure of
poly-phenyl-ethynylene.Comment: version to appear in Chem Phys Lett; 8 pages, 4 figures; minor
changes to the 06/08/03 submission (nomenclature and added concluding remark
Pathway level analysis of gene expression using singular value decomposition
BACKGROUND: A promising direction in the analysis of gene expression focuses on the changes in expression of specific predefined sets of genes that are known in advance to be related (e.g., genes coding for proteins involved in cellular pathways or complexes). Such an analysis can reveal features that are not easily visible from the variations in the individual genes and can lead to a picture of expression that is more biologically transparent and accessible to interpretation. In this article, we present a new method of this kind that operates by quantifying the level of 'activity' of each pathway in different samples. The activity levels, which are derived from singular value decompositions, form the basis for statistical comparisons and other applications. RESULTS: We demonstrate our approach using expression data from a study of type 2 diabetes and another of the influence of cigarette smoke on gene expression in airway epithelia. A number of interesting pathways are identified in comparisons between smokers and non-smokers including ones related to nicotine metabolism, mucus production, and glutathione metabolism. A comparison with results from the related approach, 'gene-set enrichment analysis', is also provided. CONCLUSION: Our method offers a flexible basis for identifying differentially expressed pathways from gene expression data. The results of a pathway-based analysis can be complementary to those obtained from one more focused on individual genes. A web program PLAGE (Pathway Level Analysis of Gene Expression) for performing the kinds of analyses described here is accessible at
Identifying differential expression in multiple SAGE libraries: an overdispersed log-linear model approach
BACKGROUND: In testing for differential gene expression involving multiple serial analysis of gene expression (SAGE) libraries, it is critical to account for both between and within library variation. Several methods have been proposed, including the t test, t(w )test, and an overdispersed logistic regression approach. The merits of these tests, however, have not been fully evaluated. Questions still remain on whether further improvements can be made. RESULTS: In this article, we introduce an overdispersed log-linear model approach to analyzing SAGE; we evaluate and compare its performance with three other tests: the two-sample t test, t(w )test and another based on overdispersed logistic linear regression. Analysis of simulated and real datasets show that both the log-linear and logistic overdispersion methods generally perform better than the t and t(w )tests; the log-linear method is further found to have better performance than the logistic method, showing equal or higher statistical power over a range of parameter values and with different data distributions. CONCLUSION: Overdispersed log-linear models provide an attractive and reliable framework for analyzing SAGE experiments involving multiple libraries. For convenience, the implementation of this method is available through a user-friendly web-interface available at
Complex band structure and plasmon lattice Green's function of a periodic metal-nanoparticle chain
When the surface plasmon resonance in a metal-nanoparticle chain is excited
at one point, the response signal will generally decay down the chain due to
absorption and radiation losses. The decay length is a key parameter in such
plasmonic systems. By studying the plasmon lattice Green's function, we found
that the decay length is generally governed by two exponential decay constants
with phase factors corresponding to guided Bloch modes and one power-law decay
with a phase factor corresponding to that of free space photons. The results
show a high level of similarity between the absorptive and radiative decay
channels. By analyzing the poles (and the corresponding residues) of the
Green's function in a transformed complex reciprocal space, the dominant decay
channel of the real-space Green's function is understood.Comment: 19 pages, 3 figure
Analytical study of non-linear transport across a semiconductor-metal junction
In this paper we study analytically a one-dimensional model for a
semiconductor-metal junction. We study the formation of Tamm states and how
they evolve when the semi-infinite semiconductor and metal are coupled
together. The non-linear current, as a function of the bias voltage, is studied
using the non-equilibrium Green's function method and the density matrix of the
interface is given. The electronic occupation of the sites defining the
interface has strong non-linearities as function of the bias voltage due to
strong resonances present in the Green's functions of the junction sites. The
surface Green's function is computed analytically by solving a quadratic matrix
equation, which does not require adding a small imaginary constant to the
energy. The wave function for the surface states is given
Amine-Gold Linked Single-Molecule Junctions: Experiment and Theory
The measured conductance distribution for single molecule benzenediamine-gold
junctions, based on 59,000 individual conductance traces recorded while
breaking a gold point contact in solution, has a clear peak at 0.0064 G
with a width of 40%. Conductance calculations based on density functional
theory (DFT) for 15 distinct junction geometries show a similar spread.
Differences in local structure have a limited influence on conductance because
the amine-Au bonding motif is well-defined and flexible. The average calculated
conductance (0.046 G) is seven times larger than experiment, suggesting
the importance of many-electron corrections beyond DFT
MCM-test: a fuzzy-set-theory-based approach to differential analysis of gene pathways
Abstract
Background
Gene pathway can be defined as a group of genes that interact with each other to perform some biological processes. Along with the efforts to identify the individual genes that play vital roles in a particular disease, there is a growing interest in identifying the roles of gene pathways in such diseases.
Results
This paper proposes an innovative fuzzy-set-theory-based approach, Multi-dimensional Cluster Misclassification test (MCM-test), to measure the significance of gene pathways in a particular disease. Experiments have been conducted on both synthetic data and real world data. Results on published diabetes gene expression dataset and a list of predefined pathways from KEGG identified OXPHOS pathway involved in oxidative phosphorylation in mitochondria and other mitochondrial related pathways to be deregulated in diabetes patients. Our results support the previously supported notion that mitochondrial dysfunction is an important event in insulin resistance and type-2 diabetes.
Conclusion
Our experiments results suggest that MCM-test can be successfully used in pathway level differential analysis of gene expression datasets. This approach also provides a new solution to the general problem of measuring the difference between two groups of data, which is one of the most essential problems in most areas of research
Romantic Relationship Quality and Mental Health in Pregnancy During the COVID-19 Pandemic
Introduction: Social capital is important for good mental health and the quality of close relationships is one key indicator of social capital. Examining the association between relationship quality and mental health may be particularly important during pregnancy as mental health concerns during this period pose significant risk to families. The COVID-19 pandemic has contributed to increased mental health problems among pregnant individuals. The resulting lockdown protocols of the pandemic has also disrupted larger social networks and couples spent more time together in the context of ongoing chronic stress, highlighting the particular importance of romantic relationship quality. This study explored longitudinal associations between relationship satisfaction, depression, and anxiety among pregnant individuals during the first wave of the COVID-19 pandemic. Methods: Pregnant individuals (n = 1842) from the Pregnancy During the Pandemic Study were surveyed monthly (April-July 2020). Depression and anxiety symptoms, and relationship satisfaction were self-reported. Cross-lagged panel models were conducted to examine bidirectional associations between relationship satisfaction and mental health symptoms over time. Results: Relationship satisfaction was significantly correlated with depression and anxiety at all time points. Longitudinally, relationship satisfaction predicted later depression and anxiety symptoms, but depressive and anxiety symptoms did not predict later relationship satisfaction. Discussion: This study suggests that poor relationship satisfaction was linked to subsequent elevations in prenatal depressive and anxiety symptoms during the COVID-19 pandemic. Relationship enhancement interventions during pregnancy may be a means of improving the mental health of pregnant individuals, and interrupting transgenerational transmission, during times of prolonged psychological distress.Social Sciences and Humanities Research Council (SSHRC)Alberta Innovates - Research GrantCanadian Institutes of Health Research (CIHR)Othe
Current-Induced Effects in Nanoscale Conductors
We present an overview of current-induced effects in nanoscale conductors
with emphasis on their description at the atomic level. In particular, we
discuss steady-state current fluctuations, current-induced forces, inelastic
scattering and local heating. All of these properties are calculated in terms
of single-particle wavefunctions computed using a scattering approach within
the static density-functional theory of many-electron systems. Examples of
current-induced effects in atomic and molecular wires will be given and
comparison with experimental results will be provided when available.Comment: revtex, 10 pages, 8 figure
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