Increase In Time Constant Research Articles

Fluorescence properties of pyrene derivative aggregates formed in polymer matrix depending on concentration

We investigated the fluorescence properties of dye aggregates formed in a poly(vinylalcohol) (PVA) matrix by phase separation. Trimethyl-(2-oxo-2-pyrene-1-yl-ethyl)-ammonium bromide (PyAm) was used as a fluorescent dye molecule. The size of PyAm aggregates in the PVA thin films were increased with increasing dye concentration, which was confirmed by atomic force microscope (AFM) measurements. The fluorescence spectra of PyAm in the PVA film at a lower concentration of 0.001 mol% only showed the monomer emission. The fluorescence peak shifted to the red with increasing dye concentration, which was assigned to a dimer or excimer-like emission. Changes in the fluorescence spectra relate to the formation of aggregates in the films. The fluorescence anisotropy decay time constant increases with increasing PyAm concentration up to the order of 100 ps. It is suggested that the exciton efficiently diffuses within the aggregates, and then was trapped at the dimer sites. We also demonstrated the application for gas sensing of nitroaromatics: 2,4-dinitrotoulene (DNT) based on the fluorescence quenching by the photoinduced electron transfer. The quenching efficiency of PyAm fluorescence reached about 43% under concentration of 2.0 mol%. The fluorescence intensity efficiently quenched at the dimer or excimer-like band. These results indicated that the efficient fluorescence quenching increases the reaction probability between PyAm and DNT by the exciton diffusion in the aggregates, called "amplified quenching". The nano-sized aggregates of PyAm formed in the PVA films are responsible for high sensitivity as an artificial fluorescent chemosensor for vapors of the nitroaromatics.

All Together Now

All Together Now

Synthesis, characterization and hydrogen adsorption in mixed crystals of MOF-5 and MOF-177

Mixed MOF crystals with morphology similar to that of pure MOF-5 and pure MOF-177 were synthesized using two organic solvents: dimethylformamide (DMF) and diethylformamide (DEF). The mixed crystals were characterized with XRD, SEM and TGA for their physical properties and also evaluated for their hydrogen adsorption properties. The XRD and SEM results suggest that the mixed crystals are different from pure MOF-5 and pure MOF-177. The DMF-derived mixed MOF crystals have a slightly higher specific surface area, smaller pore diameter and greater pore volume than those of the DEF-derived crystals, and seem to be a better adsorbent than the DEF-derived crystals, which was confirmed by the higher hydrogen and nitrogen adsorption capacities on the DMF-derived crystals. The hydrogen adsorption capacities on the mixed MOF crystals are lower than those of pure MOF-5 and MOF-177. It was also observed that the hydrogen diffusion time constant increases with hydrogen pressure, and the heat of hydrogen adsorption decreases with adsorbed hydrogen amount on both mixed crystals.

Possibilities of Assessment of the Control Potential of Aircraft Engines

While an aircraft engine is being used, its wear takes place. Its gain coefficients decrease and at the same time its time constants increase (engine becomes „weak“ and „slow“). Therefore, the rigorously definite adjustment of engine according to the principles of automatics and manufacturer recommendations is carried out periodically during the ground service of engine. The practice shows that during ground tests an engine works in „easy“ surrounding, however, during flight an engine works in „difficult“ surrounding. After good adjustment of an engine from changing the set value w during the ground service, this engine may not meet requirements of the work quality by variable disturbances z during the flight. Therefore arises a problem of the synthesis of quantification of the control possibility of engine which will result in connecting the operation status of engine (automatics) with the maintenance status of engine (diagnostic). The measure of this relationship can be named „the control potential“ of the engine. The control potential can be determined from the equation of state: , where: y – operational signal of engine, u – diagnostic signal of engine, b – coefficient of intensity of influence of the maintenance status of engine on its control possibilities, a – control potential of engine,  - operation time (Bergson’s time). The parameter „a“ calculated with help of static and dynamic identification methods will permit to estimate the functional value of engine during its ground tests.

Ultrafast Intersystem Crossing: Excited State Dynamics of Platinum Acetylide Complexes

Femtosecond time-resolved fluorescence and transient absorption measurements have been carried out on a series of platinum acetylide complexes to unravel the dynamics of intersystem crossing and the formation of triplet states in real time as a function of chain length. Ultrafast inter system crossing with a time constant less than 100 fs has been observed for the case of short chain length platinum acetylide complex and this time constant increases with increasing the chain length. Apart from the singlet to triplet intersystem crossing, additional triplet state relaxation has also been observed which happens in picosecond time scale.

Flow and dispersion in an urban cubical cavity

Flow and dispersion in an urban cubical cavity are numerically investigated using a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k– ɛ turbulence closure model. The urban cubical cavity is surrounded by flank walls that are parallel to the streamwise direction, called end-walls, as well as upstream and downstream walls. A primary vortex and secondary vortices including end-wall vortices are formed in the cavity. Because of the end-wall drag effect, the averaged mean-flow kinetic energy in the cavity is smaller than that in an urban street canyon that is open in the along-canyon direction. A trajectory analysis shows that the end-wall vortices cause fluid particles to move in the spanwise direction, indicating that flow in the cavity is essentially three-dimensional. The iso-surfaces of the Okubo–Weiss criterion capture cavity vortices well. The pollutant concentration is high near the bottom of the upstream side in the case of continuous pollutant emission, whereas it is high near the center of the primary vortex in the case of instantaneous pollutant emission. To get some insight into the degree of pollutant escape from the cavity according to various meteorological factors, extensive numerical experiments with different ambient wind speeds and directions, inflow turbulence intensities, and cavity-bottom heating intensities are performed. For each experiment, we calculate the time constant, which is defined as the time taken for the pollutant concentration to decrease to e −1 of its initial value. The time constant decreases substantially with increasing ambient wind speed, and tends to decrease with increasing inflow turbulence intensity and cavity-bottom heating intensity. The time constant increases as the ambient wind direction becomes oblique. High ambient wind speed is found to be the most crucial factor for ventilating the cavity, thus improving air quality in an urban cubical cavity.

Cartilage stress-relaxation proceeds slower at higher compressive strains

Articular cartilage is the connective tissue which covers bone surfaces and deforms during in vivo activity. Previous research has investigated flow-dependent cartilage viscoelasticity, but relatively few studies have investigated flow-independent mechanisms. This study investigated polymer dynamics as an explanation for the molecular basis of flow-independent cartilage viscoelasticity. Polymer dynamics predicts that stress-relaxation will proceed more slowly at higher volumetric concentrations of polymer. Stress-relaxation tests were performed on cartilage samples after precompression to different strain levels. Precompression increases the volumetric concentration of cartilage biopolymers, and polymer dynamics predicts an increase in relaxation time constant. Stress-relaxation was slower for greater precompression. There was a significant correlation between the stress-relaxation time constant and cartilage volumetric concentration. Estimates of the flow-dependent timescale suggest that flow-dependent relaxation occurs on a longer timescale than presently observed. These results are consistent with polymer dynamics as a mechanism of cartilage viscoelasticity.

Transport Property and Photoinduced Relaxation Effect in Sm0.5Ca0.5MnO3 Film

Abstract Charge-ordering Sm0.5Ca0.5MnO3 film has been deposited on a (100)-LaAlO3 substrate by a RF magnetron sputtering method, and the X-ray diffraction analysis shows that the film has better epitaxial character. The film exhibits the semiconductor-type conduction with the increase of the temperature. The magnetic field and laser irradiation induce the reduction in the resistance of the film, which is attributed to the delocalization effect of manganites. The photoinduced relaxation effect of the film reveals that the resistance is exponentially dependent on the time and the time constant increases with the temperature increasing due to the growing thermal fluctuation.

Thermal conduction between a heated microcantilever and a surrounding air environment

This paper investigates transient heat conduction between a heated microcantilever and its air environment. Continuum finite element simulations allow detailed analysis of heat flow within and from the resistively heated microcantilever. Heat generation of 8 mW excites the cantilever with heating that is either steady, pulse, sinusoidal, or pulse duty cycle. The time-averaged heat conduction from the cantilever leg to the nearby air is typically two to six times greater than the heat conduction from the heater to the air. The cooling time constant increases as the pulse heating time increases; for heating times of 1–1000 μs, the cooling time ranges 4.6–70 μs. The effective heat transfer coefficients around the heater and around the leg are considerably large; on the order of 1 kW/m 2 K. This study of heat transfer between a microcantilever and its surrounding air environment will aid the design and operation of microcantilever heaters.

Transport and photoinduced properties in La0.8Sn0.2MnO3 thin film

Abstract A systematic investigation of transport, photoinduced and current–voltage properties is carried out in the La 0.8 Sn 0.2 MnO 3 film prepared on a LaAlO 3 (0 1 2) substrate by a pulsed laser deposition method. The film exhibits a typical metal-insulator transition at T = 340 K. The laser irradiation results in an increase of the resistivity of the film in the metallic state and a decrease in the insulating state. The photoinduced relaxation character of the film indicates that the time constant increases with increasing the temperature because of the growing thermal fluctuation. It is found that the film shows nonlinear current–voltage characteristics and a negative resistance effect at high voltage, which is attributed to the localization of e g electrons in manganites.

Increasing Extracellular Potassium Results in Subthalamic Neuron Activity Resembling That Seen in a 6-Hydroxydopamine Lesion

Abnormal neuronal activity in the subthalamic nucleus (STN) plays a crucial role in the pathophysiology of Parkinson's disease (PD). Although altered extracellular potassium concentration ([K+]o) and sensitivity to [K+]o modulates neuronal activity, little is known about the potassium balance in the healthy and diseased STN. In vivo measurements of [K+]o using ion-selective electrodes demonstrated a twofold increase in the decay time constant of lesion-induced [K+]o transients in the STN of adult Wistar rats with a unilateral 6-hydroxydopamine (6-OHDA) median forebrain bundle lesion, employed as a model of PD, compared with nonlesioned rats. Various [K+]o concentrations (1.5-12.5 mM) were applied to in vitro slice preparations of three experimental groups of STN slices from nonlesioned control rats, ipsilateral hemispheres, and contralateral hemispheres of lesioned rats. The majority of STN neurons of nonlesioned rats and in slices contralateral to the lesion fired spontaneously, predominantly in a regular pattern, whereas those in slices ipsilateral to the lesion fired more irregularly or even in bursts. Experimentally increased [K+]o led to an increase in the number of spontaneously firing neurons and action potential firing rates in all groups. This was accompanied by a decrease in the amplitude of post spike afterhyperpolarization (AHP) and the amplitude and duration of the posttrain AHP. Lesion effects in ipsilateral neurons at physiological [K+]o resembled the effects of elevated [K+]o in nonlesioned rats. Our data suggest that changed potassium sensitivity due to conductivity alterations and delayed clearance may be critical for shaping STN activity in parkinsonian states.

Light Harvesting in Photosystem II Core Complexes Is Limited by the Transfer to the Trap: Can the Core Complex Turn into a Photoprotective Mode?

A structure-based modeling and analysis of the primary photophysical reactions in photosystem II (PS-II) core complexes is presented. The modeling is based on a description of stationary and time-resolved optical spectra of the CP43, CP47, and D1-D2-cytb559 subunits and whole core complexes. It shows that the decay of excited states in PS-II core complexes with functional (open) reaction centers (RCs) is limited by the excitation energy transfer from the CP43 and CP47 core antennae to the RC occurring with a time constant of 40-50 ps at room temperature. The chlorophylls responsible for the low energy absorbance bands in the CP43 and CP47 subunits are assigned, and their signatures in hole burning, fluorescence line narrowing, and triplet-minus-singlet spectra are explained. The different locations of these trap states in the CP43 and CP47 antennae with respect to the reaction center lead to a dramatic change of the transfer dynamics at low temperatures. The calculations predict that, compared to room temperature, the fluorescence decay at 77 K should reveal a faster transfer from CP43 and a much slower and highly dispersive transfer from CP47 to the RC. A factor of 3 increase in the fastest decay time constant of fluorescence that was reported to occur when the RC is closed (the plastoquinone QA is reduced) is understood in the present model by assuming that the intrinsic rate constant for primary electron transfer decreases from 100 fs-1 for open RCs to 6 ps-1 for closed RCs, leading to a reduction of the primary electron acceptor PheoD1, in 300 fs and 18 ps, respectively. The model suggests that the reduced QA switches the photosystem into a photoprotective mode in which a large part of the excitation energy of the RC returns to the CP43 and CP47 core antennae, where the physiologically dangerous triplet energy of the chlorophylls can be quenched by the carotenoids. Experiments are suggested to test this hypothesis. The ultrafast primary electron transfer inferred for open RCs provides further support for the accessory chlorophyll ChlD1 to be the primary electron donor in photosystem II.

Cooling load reduction by using thermal mass and night ventilation

We provide a quantitative understanding of the relationship between thermal mass and cooling load, i.e. the effect of thermal mass on energy consumption of air-conditioning in office buildings. A simple office-building model with air-conditioning at daytime and free cooling at nighttime is analyzed in detail to quantify the hourly and overall variation of cooling load of air-conditioning. As an important parameter, an increase of time constant can effectively reduce the cooling load, by as much as more than 60% when the time constant is more than 400 h. However, when the time constant is larger than 1000 h, a further increase may slightly increase the cooling load, as a too large time constant may also postpone the heat release of thermal mass until the daytime. For the most effective reduction of cooling load, the interior and exterior convective heat transfer numbers need to be matched.

Effect of synaptic plasticity on sensory coding and steady-state filtering properties in the electric sense

Our modeling study examines short-term plasticity at the synapse between afferents from electroreceptors and pyramidal cells in the electrosensory lateral lobe (ELL) of the weakly electric fish Apteronotus leptorhynchus. It focusses on steady-state filtering and coherence-based coding properties. While developed for electroreception, our study exposes general functional features for different mixtures of depression and facilitation. Our computational model, constrained by the available in vivo and in vitro data, consists of a synapse onto a deterministic leaky integrate-and-fire (LIF) neuron. The synapse is either depressing ( D), facilitating ( F) or both ( FD), and is driven by a sinusoidally or randomly modulated Poisson process. Due to nonlinearity, numerically computed input–output transfer functions are used to determine the filtering properties. The gain of the response at each sinusoidally modulated frequency is computed by dividing the fitted amplitudes of the input and output cycle histograms of the LIF models. While filtering is always low-pass for F alone, D alone exhibits a gain resonance (non-monotonicity) at a frequency that decreases with increasing recovery time constant of synaptic depression ( τ d ). This resonance is mitigated by the presence of F. For D, F and FD, coherence improves as the synaptic conductance time constant ( τ g ) increases, yet the mutual information per spike decreases. The information per spike for D and F follows opposite trends as their respective time constants increase. The broadband but non-monotonic gain and coherence functions seen in vivo suggest that D and perhaps FD dynamics are involved at this synapse. Our results further predict that the likely synaptic configuration is a slower τ g , e.g. via a mixture of AMPA and NMDA synapses, and a relatively smaller synaptic facilitation time constant ( τ f ) and larger τ d (with τ f smaller than τ d and τ g ). These results are compatible with known physiology.

Pathophysiologic insights into motor axonal function in Kennedy disease

Kennedy disease (KD), or spinobulbomuscular atrophy, is a slowly progressive inherited neurodegenerative disorder, marked by prominent fasciculations that typically precede the development of other symptoms. Although the genetic basis of KD relates to triplet (CAG) repeat expansion in the androgen receptor (AR) gene on the X chromosome, the mechanisms underlying the clinical presentation in KD have yet to be established. Consequently, the present study applied axonal excitability techniques to investigate the pathophysiologic mechanisms associated with KD. Peripheral nerve excitability studies were undertaken in 7 patients with KD with compound muscle action potentials (CMAP) recorded from the right abductor pollicis brevis. Strength-duration time constant (KD 0.54 +/- 0.03 msec; controls, 0.41 +/- 0.02 msec, p < 0.01) and the hyperpolarizing current/threshold gradient (KD 0.42 +/- 0.01; controls, 0.37 +/- 0.01, p < 0.05) were significantly increased in KD. Strength-duration time constant correlated with the CMAP amplitude (R = 0.68) and the fasciculation frequency (R = 0.62). Threshold electrotonus revealed greater changes in response to subthreshold depolarizing (KD TEd [90 to 100 msec], 50.75 +/- 1.98%; controls TEd [90 to 100 msec], 45.67 +/- 0.67%, p < 0.01) and hyperpolarizing (KD TEh [90 to 100 msec], 128.5 +/- 6.9%; controls TEh [90 to 100 msec], 120.5 +/- 2.4%) conditioning pulses. Measurements of refractoriness, superexcitability, and late subexcitability changed appropriately for axonal hyperpolarization, perhaps reflecting the effects of increased ectopic activity. In total, the increase in the strength-duration time constant may be the primary event, occurring early in course of the disease, contributing to the development of axonal hyperexcitability in Kennedy disease, and thereby to the generation of fasciculations, a characteristic hallmark of the disease.

Hydrogen bond dynamics in aqueous NaBr solutions.

Hydrogen bond dynamics of water in NaBr solutions are studied by using ultrafast 2D IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments. The hydrogen bond structural dynamics are observed by measuring spectral diffusion of the OD stretching mode of dilute HOD in H(2)O in a series of high concentration aqueous NaBr solutions with 2D IR vibrational echo spectroscopy. The time evolution of the 2D IR spectra yields frequency-frequency correlation functions, which permit quantitative comparisons of the influence of NaBr concentration on the hydrogen bond dynamics. The results show that the global rearrangement of the hydrogen bond structure, which is represented by the slowest component of the spectral diffusion, slows, and its time constant increases from 1.7 to 4.8 ps as the NaBr concentration increases from pure water to approximately 6 M NaBr. Orientational relaxation is analyzed with a wobbling-in-a-cone model describing restricted orientational diffusion that is followed by complete orientational randomization described as jump reorientation. The slowest component of the orientational relaxation increases from 2.6 ps (pure water) to 6.7 ps (approximately equal to 6 M NaBr). Vibrational population relaxation of the OD stretch also slows significantly as the NaBr concentration increases.

Kinetics and functional characterization of the glycine receptor α2 and α3 subunit

In recent studies one serine residue (Ser-346) within the protein-kinase-A (R-E-S-R) consensus sequence of the GlyRalpha3 intracellular loop has proven to be an essential target for prostaglandin-E(2)-mediated phosphorylation, which further modulates spinal nociceptive transmission and central inflammatory pain sensitization. In the present study we investigated the effect of Ser-346 phosphorylation and Ser-346 mutation on receptor kinetics and function using whole-cell patch-clamp recordings in transfected HEK 293 T cells. We compared biophysical properties of wild type GlyRalpha3 and two site-directed mutants, where Ser-346 was replaced by alanine or aspartate, in the absence and presence of prostaglandin-E(2). The mutation to alanine was accompanied by significantly altered dose-response and desensitization properties. Mutation to aspartate had only minor effects on receptor kinetics and function. Phosphorylation of Ser-346 slowed desensitization and decreased glycinergic currents in GlyRalpha3/mEP2 transfected cells. In addition, we demonstrated that prostaglandin-E(2) also had an effect on the GlyRalpha2 subunit. Exposure to prostaglandin-E(2) decreased the maximum peak current amplitude of glycinergic currents in GlyRalpha2/mEP2 transfected cells in the same manner as phosphorylation of the GlyRalpha3 subunit. It led to a significant increase of the desensitization time constants and thus significantly affected the desensitization behaviour. These results indicate that the GlyRalpha2 and the GlyRalpha3 subunits act as important subunits for the modulation of glycine receptor kinetics and function.

Systems level circuit model of C. elegans undulatory locomotion: mathematical modeling and molecular genetics

To establish the relationship between locomotory behavior and dynamics of neural circuits in the nematode C. elegans we combined molecular and theoretical approaches. In particular, we quantitatively analyzed the motion of C. elegans with defective synaptic GABA and acetylcholine transmission, defective muscle calcium signaling, and defective muscles and cuticle structures, and compared the data with our systems level circuit model. The major experimental findings are: (1) anterior-to-posterior gradients of body bending flex for almost all strains both for forward and backward motion, and for neuronal mutants, also analogous weak gradients of undulatory frequency, (2) existence of some form of neuromuscular (stretch receptor) feedback, (3) invariance of neuromuscular wavelength, (4) biphasic dependence of frequency on synaptic signaling, and (5) decrease of frequency with increase of the muscle time constant. Based on (1) we hypothesize that the Central Pattern Generator (CPG) is located in the head both for forward and backward motion. Points (1) and (2) are the starting assumptions for our theoretical model, whose dynamical patterns are qualitatively insensitive to the details of the CPG design if stretch receptor feedback is sufficiently strong and slow. The model reveals that stretch receptor coupling in the body wall is critical for generation of the neuromuscular wave. Our model agrees with our behavioral data (3), (4), and (5), and with other pertinent published data, e.g., that frequency is an increasing function of muscle gap-junction coupling.

Photoinduced characteristics in La0.67Ca0.33MnO3 film

A systematic investigation of photoinduced properties is carried out in La0.67Ca0.33MnO3 film prepared on LaAlO3 (100) substrate by magnetron sputtering method. At T < 270 K, the resistivity of film induced by laser increases because of the demagnetization effect of manganites. The photoinduced relaxation character of film indicates that the time constant increases with increasing temperature, which is attributed to the growing thermal fluctuation. After laser irradiation, the resistivity returns to the original value and the relaxation time seems to be independent of temperature. In insulating state, laser irradiation induces the reduction in resistivity of film due to the excitation of small polarons.

Retinal influences induce bidirectional changes in the kinetics of N-methyl- d-aspartate receptor-mediated responses in striate cortex cells during postnatal development

Development of the visual callosal projection in rodents goes through an early critical period, from postnatal day (P) 4 to P6, during which retinal input specifies the blueprint for normal topographic connections, and a subsequent period of progressive pathway maturation that is largely complete by the time the eyes open, around P13. This study tests the hypothesis that these developmental stages correlate with age-related changes in the kinetics of synaptic responses mediated by the N-methyl- d-aspartate subclass of glutamate receptors (NMDARs). We used an in vitro slice preparation to perform whole-cell recordings from retrogradely-labeled visual callosal cells, as well from cortical cells with unknown projections. We analyzed age-related changes in the decay time constant of evoked as well as spontaneous excitatory postsynaptic currents mediated by N-methyl- d-aspartate subclass of glutamate receptors (NMDAR-EPSCs) in slices from normal pups and pups enucleated at different postnatal ages. In normal pups we found that the decay time constant of NMDAR-EPSCs increases starting at about P6 and decreases by about P13. In contrast, these changes were not observed in rats enucleated at birth. However, by delaying the age at which enucleation was performed we found that the presence of the eyes until P6, but not until P4, is sufficient for inducing slow NMDAR-EPSC kinetics during the second postnatal week, as observed in normal pups. These results provide evidence that the eyes exert a bidirectional effect on the kinetics of NMDARs: during a P4–P6 critical period, retinal influences induce processes that slow down the kinetics of NMDAR-EPSCs, while, near the age of eye opening, retinal input induces a sudden acceleration of NMDAR-EPSC kinetics. These findings suggest that the retinally-driven processes that specify normal callosal topography during the P4–P6 time window also induce an increase in the decay time constant of NMDAR-EPSCs. This increase in response kinetics may play an important role in the maturation of cortical topographic maps after P6. Using ifenprodil, a noncompetitive NR2B-selective blocker, we obtained evidence that although NR1/NR2B diheteromeric receptors contribute to evoked synaptic responses in both normal and enucleated animals, they are not primarily responsible for either the age-related changes in the kinetics of NMDAR-mediated responses, or the effects that bilateral enucleation has on the kinetics of NMDAR-EPSCs.