2,397 research outputs found
Observation of a uniform temperature dependence in the electrical resistance across the structural phase transition in thin film vanadium oxide ()
An electrical study of thin films in the vicinity of the structural
phase transition at shows (a) that the electrical resistance
follows over the -range,
covering both sides of the structural transition, and (b) a history dependent
hysteresis loop in upon thermal cycling. These features are attributed here
to transport through a granular network.Comment: 3 pages, 3 color figure
Simplicity of Completion Time Distributions for Common Complex Biochemical Processes
Biochemical processes typically involve huge numbers of individual reversible
steps, each with its own dynamical rate constants. For example, kinetic
proofreading processes rely upon numerous sequential reactions in order to
guarantee the precise construction of specific macromolecules. In this work, we
study the transient properties of such systems and fully characterize their
first passage (completion) time distributions. In particular, we provide
explicit expressions for the mean and the variance of the completion time for a
kinetic proofreading process and computational analyses for more complicated
biochemical systems. We find that, for a wide range of parameters, as the
system size grows, the completion time behavior simplifies: it becomes either
deterministic or exponentially distributed, with a very narrow transition
between the two regimes. In both regimes, the dynamical complexity of the full
system is trivial compared to its apparent structural complexity. Similar
simplicity is likely to arise in the dynamics of many complex multi-step
biochemical processes. In particular, these findings suggest not only that one
may not be able to understand individual elementary reactions from macroscopic
observations, but also that such understanding may be unnecessary
Ergodic and Nonergodic Anomalous Diffusion in Coupled Stochastic Processes
Inspired by problems in biochemical kinetics, we study statistical properties
of an overdamped Langevin process whose friction coefficient depends on the
state of a similar, unobserved process. Integrating out the latter, we derive
the long time behaviour of the mean square displacement. Anomalous diffusion is
found. Since the diffusion exponent can not be predicted using a simple scaling
argument, anomalous scaling appears as well. We also find that the coupling can
lead to ergodic or non-ergodic behaviour of the studied process. We compare our
theoretical predictions with numerical simulations and find an excellent
agreement. The findings caution against treating biochemical systems coupled
with unobserved dynamical degrees of freedom by means of standard, diffusive
Langevin descriptions
Measuring missing heritability: Inferring the contribution of common variants
Genome-wide association studies (GWASs), also called common variant association studies (CVASs), have uncovered thousands of genetic variants associated with hundreds of diseases. However, the variants that reach statistical significance typically explain only a small fraction of the heritability. One explanation for the “missing heritability” is that there are many additional disease-associated common variants whose effects are too small to detect with current sample sizes. It therefore is useful to have methods to quantify the heritability due to common variation, without having to identify all causal variants. Recent studies applied restricted maximum likelihood (REML) estimation to case–control studies for diseases. Here, we show that REML considerably underestimates the fraction of heritability due to common variation in this setting. The degree of underestimation increases with the rarity of disease, the heritability of the disease, and the size of the sample. Instead, we develop a general framework for heritability estimation, called phenotype correlation–genotype correlation (PCGC) regression, which generalizes the well-known Haseman–Elston regression method. We show that PCGC regression yields unbiased estimates. Applying PCGC regression to six diseases, we estimate the proportion of the phenotypic variance due to common variants to range from 25% to 56% and the proportion of heritability due to common variants from 41% to 68% (mean 60%). These results suggest that common variants may explain at least half the heritability for many diseases. PCGC regression also is readily applicable to other settings, including analyzing extreme-phenotype studies and adjusting for covariates such as sex, age, and population structure.National Institutes of Health (U.S.) (NIH HG003067)Broad Institute of MIT and Harvar
Synthesis, characterization and performance of polystyrene/PMMA blend membranes for potential water treatment
© 2017 Elsevier B.V. PS membranes were prepared from polymeric blends of PS and PMMA via a phase inversion induced by an immersion precipitation in water coagulation baths. The effects of the casting parameters (e.g., solvent selection, the composition of the coagulation bath) and the type of polystyrene (substituents at the aromatic ring, tacticity) on the morphology and water permeation flux were studied. The findings reveal that modified PS promoting instantaneous demixing with NMP/water systems result in membranes wit- h macrovoids while the addition of 2% of PS-r-PMMA results in membranes with high water fluxes. The membranes morphologies were governed by a trade-off between thermodynamic and kinetic aspects. The antibacterial effect of (aPS)70-co-aPS(I)30/PMMA-based membrane was examined using static and cross-flow systems. These results illustrate the diversity in the design of these PS/PMMA based membranes and highlight the possibility to control the polymer chemistry for tailoring specific morphology, permeation, and antibacterial properties for the desired function
- …