SEARCH · PubMed Health

Results for “spatial dependency”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources

PubMed2004

Spatial encoding and the acquisition of high-resolution NMR spectra in inhomogeneous magnetic fields.

A scheme enabling the acquisition of high-resolution nuclear magnetic resonance (NMR) spectra within inhomogeneous magnetic fields is introduced and exemplified. The scheme is based on the spatial encoding protocol recently introduced for collecting multidimensional NMR data within a single scan, which retrieves spectral information via interference phenomena between spin packets located at distinct positions within the sample. This in turn enables compensating for field inhomogeneities over the sample as a whole by shifting the phases of the radio-frequency pulses involved in the spatial encoding, rather than by demanding an extreme uniformity in the employed magnetic field. The upper tolerable field inhomogeneity limit thus becomes orders of magnitude higher than that in conventional time-domain acquisitions. No particular spatial dependencies are demanded by the new scheme, which can yield its high-resolution results on a single-scan basis.

Journal Article↗

PubMed2005

Multilevel microfluidics via single-exposure photolithography.

This communication reports a new method to form multilevel features in a single layer of SU-8 photoresist to facilitate the generation of 3D microfluidic chips. The method utilizes the spatial dependence of diffracted light intensity to selectively overexpose masked regions of photoresist and requires only a UV light source and a single transparency mask. 3D structures are formed within microfluidic channels using this selective overexposure method, with feature sizes being determined by the exposure dose and mask feature sizes. The dimensions of the internal features and the microfluidic channels can be varied independently according to these parameters, and any number of different heights can be obtained in a single exposure step. The method provides a simple means of forming 3D microfluidic structures with integrated features, including mixing structures, flow stabilization ridges, and separation weirs to increase the capabilities of microfluidic chips in a variety of microchemical applications.

Journal Article↗

PubMed2002

Photoinduced degradation of carbaryl in a wetland surface water.

The photoinduced degradation of carbaryl (1-naphthyl-N-methyl carbamate) was studied in a wetland's surface water to examine the photochemical processes influencing its transformation. For this particular wetland water, at high pH, it was difficult to delineate the photolytic contribution to the overall degradation of carbaryl. At lower pH values, the extent of the degradation attributable to indirect pathways, that is, in the presence of naturally occurring photosensitizers, increased significantly. Moreover, the photoenhanced degradation at the lower pH values was found to be seasonally and spatially dependent. Analysis of water samples revealed two primary constituents responsible for the observed indirect photolytic processes: nitrate and dissolved natural organic matter (NOM). Nitrate in the wetland appears at high concentrations (> or =1 mM) seasonally after the application of fertilizers in the watershed and promotes contaminant destruction through the photochemical production of the hydroxyl radical (HO*). The extent of the observed indirect photolysis pathway appears to be dependent upon the concentration of nitrates and the presence of HO* scavengers such as dissolved NOM and carbonate alkalinity. Paradoxically, during low-nitrate events (<50 microM), NOM becomes the principal photosensitizer through either the production of HO*, direct energy transfer from the excited triplet state, and/or production of an unidentified transient species.

Carbaryl↗

PubMed2005

Local atomic structure of partially ordered NiMn in NiMn/NiFe exchange-coupled layers: 2. Electronic structure calculations.

Local electronic and magnetic structure calculations for NiMn exchange bias alloys are reported for clusters containing NiMn in both the chemically disordered face-centered cubic and the chemically and magnetically ordered L1(0) phases. The results of these calculations are consistent with our local structure measurements that point toward the existence of nanometer-scale ordered clusters at the beginning stages of chemical ordering. The spatial dependence of both the local density of states and the magnetization is strongly influenced by the existence of magnetic order on short length scales, giving rise to an inhomogeneous profile for these quantities across the material with the greatest change at the interface that is still small enough within the domain to imply that the magnetization is still highly developed.

Journal Article↗

PubMed2005

Variation of the resonant transfer rate when passing from nonadiabatic to adiabatic electron transfer.

Two competing theories are used for bridging the gap between the nonadiabatic and the deeply adiabatic electron transfer between symmetric parabolic wells. For the high friction limit, a simple analytic interpolation is proposed as a reasonable alternative to them, well-fitted to the results of numerical simulations. It provides a continuous description of the electron transfer rate in the whole range of variation of the nonadiabatic coupling between the diabatic states. For lower friction, the original theories are used for the same goal. With an increase in coupling, the cusped barrier transforms into the parabolic one. Correspondingly, the pre-exponent of the Arrhenius transfer rate first increases with coupling, then levels off approaching the "dynamic solvent effect" plateau but finally reduces reaching the limit of the adiabatic Kramers theory for the parabolic barrier. These changes proceeding with a reduction in the particle separation affect significantly the spatial dependence of the total transfer rate. When approaching the contact distance, the exact rate becomes smaller than in the theory of dynamical solvent effects and much smaller than predicted by perturbation theory (golden rule), conventionally used in photochemistry and electrochemistry.

Journal Article↗

PubMed2006

Electronic structure of metal/molecule//metal junctions: a density functional theory study of the influence of the molecular terminal group.

We report on density functional theory calculations of the electronic structure of Au(111)/molecule//Au(111) junctions in which thiol molecules are chemically bound at one end to a gold electrode (the "substrate"), while the other end has a separation of a few to several angstroms from a second gold electrode (the "tip"). Our goal is to investigate the role of different molecular terminal groups and of the tip-molecule distance either on the spatial dependence of the local density of states (LDOS) at the Fermi energy E(f) or on the energy dependence of the projected density of states onto different molecular subunits. We consider conjugated diphenylthiol (SPh2R) molecules with terminal groups R = H, SH, CH3, or CF3 as well as "mixed" conjugated-saturated phenylthiol-pentane (SPhC4CH3) and butanethiol-toluene (SC4PhCH3) molecules. For SPh2R molecules, the LDOS at E(f) exhibits an oscillatory exponential decay along the molecule, with an average decay constant that depends weakly on the R terminal group. For the mixed aromatic-aliphatic molecules instead, there are large differences in the LDOS at E(f), with SC4PhCH3 showing a much larger LDOS in the proximity of the terminal CH3 group than SPhC4CH3.

Journal Article↗

PubMed2005

Observation of diffusion and tunneling recombination of dye-photoinjected electrons in ultrathin TiO2 layers by surface photovoltage transients.

Surface photovoltage transients were used to monitor both the short time dynamics (>10 ns) and the spatial distribution of electrons photoinjected in thin (2-20 nm) TiO2 layers from dye molecules adsorbed at the surface. At low temperatures (100-250 K), the dynamics are governed exclusively by spatially dependent tunneling recombination, with a rate that varies with the distance from the surface x as exp(-2x/a), and an initial exponential distribution of photoinjected electrons, n0 exp(-x/b). This model is confirmed by the observation of power law decay in time t(-a/2b) with a ratio a/b = 0.28 +/- 0.04. The stability of cis-di(isothiocyanato)-N-bis(2,2'-bipyridine-4,4'-dicarboxy) ruthenium(II) (N3) dye molecules on TiO2 during treatment in a vacuum at high temperatures was proven. For high temperatures (250-540 K), the thickness dependence of the decays indicates that the dynamics of surface recombination are retarded by the diffusion of electrons toward the interior of the film. The implications for thin layer coating in dye-sensitized solar cells are discussed.

Journal Article↗

PubMed2006

Direct measurement of the internal electron quasi-Fermi level in dye sensitized solar cells using a titanium secondary electrode.

The spatial dependence of the electron quasi-Fermi level (QFL) in the interior of dye sensitized nanocrystalline solar cells (DSC) under short circuit conditions can be inferred from calculations based on a diffusive electron transport model. The calculations predict that the difference in the QFL between the electrolyte and contact sides of the TiO(2) layer under short circuit conditions at 1 sun could be as much as 0.5-0.7 eV. The predicted QFL profiles depend on assumptions made about energy positions, electron mobility, and the conduction band density of states. In this work, the position of the QFL at the electrolyte side of the dye sensitized TiO(2) film in a DSC has been measured using a thin passivated titanium contact deposited on top of the nanocrystalline TiO(2) by evaporation. The method allows changes in the electron QFL at all points on the IV characteristic of the cell to be monitored under dark and photostationary conditions. In addition, cells incorporating the titanium electrode can give information about the behavior of the QFL under dynamic conditions.

Letter↗

PubMed2004

Fabrication of gradient hydrogels using a microfluidics/photopolymerization process.

A method of fabricating photo-cross-linked hydrogels with gradients of immobilized molecules and crosslinking densities is introduced. Two macromer/initiator solutions are injected into a unique poly(dimethylsiloxane) channel system that produces a prepolymer gradient that is subsequently polymerized into a water-swollen hydrogel with ultraviolet light exposure. The gradient is controlled by the injection flow rate (optimized to 0.3 microL/min per inlet to produce a linear gradient). The technique is investigated both through fabrication of adhesive ligand gradients that modulate spatial distribution of attached endothelial cells and gradients of cross-linking densities that led to unique hydrogel architectures and spatially dependent swelling.

Cell Adhesion↗

PubMedDate not supplied

Stochastic models for prodrug targeting. 1. Diffusion of the efflux drug.

In modeling prodrug targeting using the stochastic approach, we first modeled diffusion of the efflux drug. Drug efflux is one of the major reasons for the failure of prodrug strategy: the active agent is pumped out the membrane ("efflux"), causing an insufficient amount to be delivered to the targeted sites and thus diminishing the efficacy of chemotherapy. Because the biological body is a nonlinear nonequilibrium complex system, the molecular transport taking place in vivo often showed stochasticity. The model described here for diffusion of the efflux drug is basically a diffusion process with reflecting/absorbing boundary conditions, divided into two distinctive regions with one allowing the particles to jump to the origin as a result of efflux pumping. We study discrete time birth-death Markov chain and compute the time-dependent spatial probability density function (PDF) of particles. The results showed that the jumping probability, although small, has a significant impact on the evolution of PDF of the efflux drug. The implications of this model were discussed.

Biological Transport↗

PubMed2005

Charge control in a model biphenyl molecular transistor.

We study charge control in a gated 4,4'-biphenyl diradical molecular transistor using ab initio density functional theory calculations. I-V curves and intrinsic gate capacitances were derived. We find charge control in this transistor to be strongly affected by polarization of the sigma-states of the molecule, leading to strong electrostatic coupling of the internal potentials to the source and drain electrodes, and relatively weak coupling to the gate. We suggest that this spatially dependent and anisotropic polarization is an essential element in the operation of molecular transistors.

Biphenyl Compounds↗

PubMed2003

Clines: a reductionist model.

A piecewise linear model is used to provide a caricature of the nonlinear equation describing genetic variation due to migration and local natural selection in an inhomogeneous bounded habitat. The conditions for which nontrivial spatially-dependent steady state solutions exist are analytically determined together with these solutions for three distinct scenarios. For the usual case of no flux (Neumann) boundary conditions, explicit solutions require additional information to fix the genetic frequency within the habitat. This difficulty can be bypassed in two of the scenarios considered (this is an artifact of the model), but in the remaining scenario it is necessary to take into account the connection with the initial data to explicitly determine the steady state frequency. Some numerical examples are considered for each of the scenarios to illustrate the analytical results that are the primary focus of the work presented here.

Gene Frequency↗

PubMed2006

Impaired trace fear conditioning following neonatal ethanol: reversal by choline.

Neonatal ethanol exposure in animals results in performance deficits on tests of hippocampus-dependent spatial memory, and recent studies have shown that extra dietary choline can ameliorate some of these impairments. In this experiment, rats were administered 5.25 g/kg ig ethanol per day or sham intubations on Postnatal Days (PD) 4-9 and choline (0.1 ml of an 18.8 mg/ml solution) or saline subcutaneously on PD 4-20. On PD 30, rats were given delay or trace fear conditioning trials and were tested for conditioned stimulus-elicited freezing 24 hr later. Neonatal ethanol produced a profound impairment in trace conditioning that was reversed by choline. Groups did not differ in delay conditioned responding, indicating that neonatal ethanol produces a relatively selective cognitive deficit that can be alleviated with supplemental choline.

Animals↗

PubMed2000

Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2CaCu2O8+delta

Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental and theoretical studies. Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems. Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu2O8+delta. We find intense quasiparticle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within approximately 15 A of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle 'cloud' aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials.

Journal Article↗

PubMed2003

Organization of cell assemblies in the hippocampus.

Neurons can produce action potentials with high temporal precision. A fundamental issue is whether, and how, this capability is used in information processing. According to the 'cell assembly' hypothesis, transient synchrony of anatomically distributed groups of neurons underlies processing of both external sensory input and internal cognitive mechanisms. Accordingly, neuron populations should be arranged into groups whose synchrony exceeds that predicted by common modulation by sensory input. Here we find that the spike times of hippocampal pyramidal cells can be predicted more accurately by using the spike times of simultaneously recorded neurons in addition to the animals location in space. This improvement remained when the spatial prediction was refined with a spatially dependent theta phase modulation. The time window in which spike times are best predicted from simultaneous peer activity is 10-30 ms, suggesting that cell assemblies are synchronized at this timescale. Because this temporal window matches the membrane time constant of pyramidal neurons, the period of the hippocampal gamma oscillation and the time window for synaptic plasticity, we propose that cooperative activity at this timescale is optimal for information transmission and storage in cortical circuits.

Action Potentials↗

PubMed2004

Conditional transgenic suppression of M channels in mouse brain reveals functions in neuronal excitability, resonance and behavior.

In humans, mutations in the KCNQ2 or KCNQ3 potassium-channel genes are associated with an inherited epilepsy syndrome. We have studied the contribution of KCNQ/M-channels to the control of neuronal excitability by using transgenic mice that conditionally express dominant-negative KCNQ2 subunits in brain. We show that suppression of the neuronal M current in mice is associated with spontaneous seizures, behavioral hyperactivity and morphological changes in the hippocampus. Restriction of transgene expression to defined developmental periods revealed that M-channel activity is critical to the development of normal hippocampal morphology during the first postnatal weeks. Suppression of the M current after this critical period resulted in mice with signs of increased neuronal excitability and deficits in hippocampus-dependent spatial memory. M-current-deficient hippocampal CA1 pyramidal neurons showed increased excitability, reduced spike-frequency adaptation, attenuated medium afterhyperpolarization and reduced intrinsic subthreshold theta resonance. M channels are thus critical determinants of cellular and neuronal network excitability, postnatal brain development and cognitive performance.

Animals↗

PubMed2006

An essential role for beta-actin mRNA localization and translation in Ca2+-dependent growth cone guidance.

Axon pathfinding requires directional responses of growth cones to extracellular cues, which have been shown to involve local synthesis of protein. The identity and functions of the locally produced proteins remain, however, unclear. Here we report that Ca(2+)-dependent bidirectional turning of Xenopus laevis growth cones requires localized distribution and translation of beta-actin messenger RNA. Both beta-actin mRNA and its zipcode-binding protein, ZBP1, are localized at the growth cone and become asymmetrically distributed upon local exposure to brain-derived neurotrophic factor (BDNF). Inhibition of protein synthesis or antisense interference with beta-actin mRNA-ZBP1 binding abolishes both Ca(2+)-mediated attraction and repulsion. In addition, attraction involves a local increase in beta-actin, whereas repulsion is accompanied by a local decrease in beta-actin; thus, both produce a synthesis- and ZBP1 binding-dependent beta-actin asymmetry but with opposite polarities. Together with a similar asymmetry in Src activity during bidirectional responses, our findings indicate that Ca(2+)-dependent spatial regulation of beta-actin synthesis through Src contributes to the directional motility of growth cones during guidance.

Actins↗

PubMed2024

SUN2 mediates calcium-triggered nuclear actin polymerization to cluster active RNA polymerase II.

The nucleoskeleton is essential for nuclear architecture as well as genome integrity and gene expression. In addition to lamins, titin or spectrins, dynamic actin filament polymerization has emerged as a potential intranuclear structural element but its functions are less well explored. Here we found that calcium elevations trigger rapid nuclear actin assembly requiring the nuclear membrane protein SUN2 independently of its function as a component of the LINC complex. Instead, SUN2 colocalized and associated with the formin and actin nucleator INF2 in the nuclear envelope in a calcium-regulated manner. Moreover, SUN2 is required for active RNA polymerase II (RNA Pol II) clustering in response to calcium elevations. Thus, our data uncover a SUN2-formin module linking the nuclear envelope to intranuclear actin assembly to promote signal-dependent spatial reorganization of active RNA Pol II.

RNA Polymerase II↗