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PubMed1988

Use of weak acids to determine the bulk diffusion limitation of H+ ion conductance through the gramicidin channel.

The addition of 2 M formic acid at pH 3.75 increased the single channel H+ ion conductance of gramicidin channels 12-fold at 200 mV. Other weak acids (acetic, lactic, oxalic) produce a similar, but smaller increase. Formic acid (and other weak acids) also blocks the K+ conductance at pH 3.75, but not at pH 6.0 when the anion form predominates. This increased H+ conductance and K+ block can be explained by formic acid (HF) binding to the mouth of the gramicidin channel (Km = 1 M) and providing a source of H+ ions. A kinetic model is derived, based on the equilibrium binding of formic acid to the channel mouth, that quantitatively predicts the conductance for different mixtures of H+, K+, and formic acid. The binding of the neutral formic acid to the mouth of the gramicidin channel is directly supported by the observation that a neutral molecule with a similar structure, formamide (and malonamide and acrylamide), blocks the K+ conductance at pH 6.0. The H+ conductance in the presence of formic acid provides a lower bound for the intrinsic conductance of the gramicidin channel when there is no diffusion limitation at the channel mouth. The 12-fold increase in conductance produced by formic acid suggests that greater than 90% of the total resistance to H+ results from diffusion limitation in the bulk solution.

Amides↗
PubMed2005

Multivariate analysis of infrared spectra for monitoring and understanding the kinetics and mechanisms of adsorption processes.

Water adsorption onto thin zeolite 3A wafers has been investigated as a function of time, water vapor concentration, and zeolite sample mass using mid-infrared spectroscopy coupled with multivariate data analysis. Principal component analysis (PCA) of the spectral region of the water combination band was used for quantitative characterization of water adsorption onto the zeolite. The kinetics of the adsorption of water are found to be very reproducible and nearly linear with time. The kinetics of water adsorption based on data from different masses of zeolite are consistent with a diffusion/immobilization model for which the interparticle diffusion rate is comparable to the rate of adsorption. The infrared zeolite bands (1340-1550 cm(-1)) change during the adsorption process and yield more detail about the adsorption sites of the material. PCA applied to the zeolite bands was not directly interpretable. However, multivariate curve resolution applied to the spectral region containing the zeolite bands readily demonstrates that zeolite 3A has three water adsorption sites or environments that are sequentially occupied. Potential explanations for the observations of the multivariate curve resolution (MCR) analysis of these infrared (IR) kinetic adsorption experiments are presented. The explanation most consistent with our data suggests that water adsorbs sequentially on the zeolite to form single, double, and triple water adsorption on single zeolite adsorption sites. The combination of infrared spectroscopy and multivariate analysis is therefore demonstrated to be a powerful method to study detailed adsorption kinetics and mechanisms of the adsorption of molecules onto surfaces.

Adsorption↗
PubMed1998

HIV prevention among Asian and Pacific Islander American men who have sex with men: a critical review of theoretical models and directions for future research.

Nationally, the incidence of AIDS is increasing at a higher rate among Asian and Pacific Islander American men who have sex with men (API MSM) than among white MSM. Furthermore, current HIV prevention efforts are inadequate to slow the rapidly rising HIV epidemic in the gay API community, and little attention has been paid to the applicability of existing behavior change models to APIMSM. This paper reviews the five major models of health behavior change used in HIV prevention for the MSM population: the health belief model, theory of reasoned action, social learning theory, diffusion theory, and the AIDS risk reduction model. Although some of these models have been useful in designing risk reduction programs for API MSM, recent empirical data suggest that the models do not adequately address environmental influences affecting API MSM and limit our choices in prevention strategies to the level of the individual. We propose an ecological model for health promotion as a potentially useful theoretical framework, and suggest prevention strategies directed at the individual, the family, the general API community, and the mainstream gay community to reduce HIV risk among API MSM.

Asian↗
PubMed2001

Current concepts: diffuse axonal injury-associated traumatic brain injury.

OBJECTIVES: To review the probable physical, physiologic mechanisms that result in the medical and neuropsychologic complications of diffuse axonal injury (DAI)-associated traumatic brain injury (TBI). DATA SOURCES: Various materials were accessed: MEDLINE, textbooks, scientific presentations, and current ongoing research that has been recently reported. STUDY SELECTION: Included were scientific studies involving TBI, particularly direct injury to the axons and glia of the central nervous system (CNS) in both in vitro and in vivo models. These studies include pathologic findings in humans as well as the medical complications and behavioral outcomes of DAI. Studies that addressed animal models of DAI as well as cellular and/or tissue models of neuronal injury were emphasized. The review also covered work on the physical properties of materials involved in the transmission of energy associated with prolonged acceleration-deceleration injuries. DATA EXTRACTION: Studies were selected with regard to those that addressed the mechanism of TBI associated with DAI and direct injury to the axon within the CNS. The material was generally the emphasis of the article and was extracted by multiple observers. Studies that correlate the above findings with the clinical picture of DAI were included. DATA SYNTHESIS: Concepts were developed by the authors based on the current scientific findings and theories of DAI. The synthesis of these concepts involves expertise in physical science, basic science concepts of cellular injury to the CNS, acute medical indicators of DAI, neuropsychologic indicators of DAI, and rehabilitation outcomes from TBI. CONCLUSIONS: The term DAI is a misnomer. It is not a diffuse injury to the whole brain, rather it is predominant in discrete regions of the brain following high-speed, long-duration deceleration injuries. DAI is a consistent feature of TBI from transportation-related injuries as well as some sports injuries. The pathology of DAI in humans is characterized histologically by widespread damage to the axons of the brainstem, parasagittal white matter of the cerebral cortex, corpus callosum, and the gray-white matter junctions of the cerebral cortex. Computed tomography and magnetic resonance imaging scans taken initially after injury are often normal. The deformation of the brain due to plastic flow of the neural structures associated with DAI explains the micropathologic findings, radiologic findings, and medical and neuropsychologic complications from this type of injury mechanism. There is evidence that the types of cellular injury in TBI (DAI, anoxic, contusion, hemorrhagic, perfusion-reperfusion) should be differentiated, as all may involve different receptors and biochemical pathways that impact recovery. These differing mechanisms of cellular injury involving specific biochemical pathways and locations of injury may, in part, explain the lack of success in drug trials to ameliorate TBI.

Animals↗
PubMed2001

Effects of excluded surface area and adsorbate clustering on surface adsorption of proteins. II. Kinetic models.

Models for equilibrium surface adsorption of proteins have been recently proposed (Minton, A. P., 2000. Biophys. Chem. 86:239-247) in which negative cooperativity due to area exclusion by adsorbate molecules is compensated to a variable extent by the formation of a heterogeneous population of monolayer surface clusters of adsorbed protein molecules. In the present work this concept is extended to treat the kinetics of protein adsorption. It is postulated that clusters may grow via two distinct kinetic pathways. The first pathway is the diffusion of adsorbed monomer to the edge of a preexisting cluster and subsequent accretion. The second pathway consists of direct deposition of a monomer in solution onto the upper (solution-facing) surface of a preexisting cluster ("piggyback" deposition) and subsequent incorporation into the cluster. Results of calculations of the time course of adsorption, carried out for two different limiting models of cluster structure and energetics, show that in the absence of piggyback deposition, enhancement of the tendency of adsorbate to cluster can reduce, but not eliminate, the negative kinetic cooperativity due to surface area exclusion by adsorbate. Apparently noncooperative (Langmuir-like) and positively cooperative adsorption progress curves, qualitatively similar to those reported in several published experimental studies, require a significant fraction of total adsorption flux through the piggyback deposition pathway. According to the model developed here and in the above-mentioned reference, the formation of surface clusters should be a common concomitant of non-site-specific surface adsorption of proteins, and may provide an important mechanism for assembly of organized "protein machines" in vivo.

Adsorption↗
PubMed2005

Photooxidation of diglycine in confined media. Application of the microreactor model for spin-correlated radical pairs in reverse micelles and water-in-oil microemulsions.

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles and microemulsions are presented, simulated, and discussed using the microreactor model. The radicals are formed inside the water pool using photooxidation of diglycine by the excited triplet states of two different anthraquinone sulfonate salts. Water pool size and temperature effects on the spectra are reported, and the simulations allow for extraction of the diffusion coefficient in the interior, which monotonically increases with water pool size. The data directly correlate with the diffusional properties of correlated radical pairs in regular aqueous micelle solutions studied previously by similar methods. Competition between H-atom abstraction and electron transfer is observed with anthraquinone sulfonate, but electron transfer is the only reaction pathway observed when anthraquinone disulfonate triplet state is the sensitizing species.

Anthraquinones↗
PubMed1977

Diffusion of oxygen at the endothelial surface of the rabbit cornea.

1. An in vitro investigation was made to determine the oxygen tension level required at the endothelial surface of rabbit cornea to produce a net oxygen flux into the cornea across this surface when the epithelial surface was exposed to air. 2. The experimental design was based on a mathematical model which showed that the direction of oxygen flux measured in an agar layer adjacent to the endothelium was the same as the direction of oxygen flux across the endothelial surface. 3. From micro-electrode measurements of oxygen tension in the agar layer, it was found that an oxygen tension greater than 102 mmHg at the endothelial surface was required tocause a net flux of oxygen into the cornea. 4. Comparing this result to the in vivo situation,it was concluded that all layers of the rabbit cornea receive oxygen from the atmosphere under open eye conditions.

Animals↗
PubMed1977

Effect of the rate of oxygen consumption on muscle respiration.

An important role of myoglobin in red muscle is to facilitate the diffusion of oxygen for metabolism. We consider a model for muscle respiration in which the oxygen consumption is of a Michaelis-Menten form. The resulting mathematical model is solved in two different ways with two different boundary conditions. The first uses the singular perturbation method of Murray (1974), while the second, which gives another justication of the simpler procedure, is a direct numerical computation of the full problem. The oxygen tension and saturation are often small. For realistic values of the Michaelis-MENTEN CONSTANT the oxygen tension, the saturation and the radius of the region in which the oxygen tension is negligibly small can be calculated using the constant consumption model of Murray (1974), with corrected boundary conditions (those for a Stefan problem), which in certain circumstances mardedly affect the solution.

Diffusion↗
PubMed2001

endo-Fullerene and doped diamond nanocrystallite-based models of qubits for solid-state quantum computers.

Models of encapsulated nuclear spin 1/2 1H and 31P atoms in fullerene and diamond nanocrystallite, respectively, are proposed and examined with an ab initio local density functional method for possible applications as single quantum bits (qubits) in solid-state quantum computers. A 1H atom encapsulated in a fully deuterated fullerene, C20D20, forms the first model system and ab initio calculation shows that the 1H atom is stable in its atomic state at the center of the fullerene with a barrier of about 1 eV to escape. A 31P atom positioned at the center of a diamond nanocrystallite is the second model system, and 31P atom is found to be stable at the substitutional site relative to interstitial sites by 15 eV. Vacancy formation energy is 6 eV in diamond, so the substitutional 31P atom will be stable against diffusion during the formation mechanisms within the nanocrystallite. The coupling between the nuclear spin and the weakly bound (valance) donor electron in both systems is found to be suitable for single qubit applications, whereas the spatial distributions of (valance) donor electron wave functions are found to be preferentially spread along certain lattice directions, facilitating two or more qubit applications. The feasibility of the fabrication pathways for both model solid-state qubit systems within practical quantum computers is discussed within the context of our proposed solid-state qubits.

Computer Simulation↗
PubMed2005

Mechanisms of morphogen movement.

Morphogens are defined as signaling molecules that are produced locally, yet act directly at a distance to pattern the surrounding field of cells in a concentration-dependent manner. In recent years many laboratories have devoted their attention to how morphogens actually reach distant cells. Several models have been proposed, including diffusion in the extracellular space and planar transcytosis. A combination of genetic, developmental, and cell-biological approaches have been taken to tackle this issue. I will present the models and discuss the types of experiments that have been designed to test them. It stands out that most of the work has been carried out in Drosophila. Morphogens contribute to patterning of the vertebrate nervous system, and the same signaling molecules have recently been shown to play important, possibly instructive, roles in axon guidance. Little, if anything, is known about the movement of morphogens in the context of nervous system development. The long-standing tradition of biophysical studies on diffusion in the brain extracellular space, along with the sophisticated in vitro culture systems developed in neurobiology laboratories, may provide new tools and ideas to test these models in a new context.

Animals↗
PubMed2005

Voltammetric study of extracellular dopamine near microdialysis probes acutely implanted in the striatum of the anesthetized rat.

Establishing in vivo microdialysis methods for the quantitative determination of dopamine concentrations in the extracellular space of the brain is an important yet challenging objective. The source of the challenge is the difficulty in directly measuring the microdialysis recovery of dopamine during an in vivo experiment. The recovery value is needed for quantitative microdialysis, regardless of whether conventional or no-net-flux methods are used. Numerical models of microdialysis that incorporate both diffusion and active transport processes suggest that dopamine recovery is strongly affected by processes occurring in the tissue closest to the probe. Some evidence suggests that the tissue adjacent to the probe becomes disrupted during probe implantation. Hence, the objective of the present study was to further identify whether the tissue adjacent to the probe is disrupted and, if so, whether that disruption might affect dopamine recovery. The experiments were conducted with microdialysis probes implanted acutely in the striatum of rats anesthetized with chloral hydrate. Carbon fiber voltammetric microelectrodes were used to monitor extracellular dopamine at three sites near the probes; immediately adjacent to the probe, 220-250 microm from the probe, and 1 mm from the probe. Probes were lowered slowly over a 30 min period, so that dialysate dopamine levels were stable, in the low nanomolar range, and partially TTX-sensitive by the time experiments began. Starting 2h after probe implantation, dopamine was monitored by fast-scan cyclic voltammetry during electrical stimulation of the medial forebrain bundle and during administration of the dopamine uptake inhibitor, nomifensine. The findings of this study show that a gradient of dopamine release and uptake activity extends at least 220 microm from microdialysis probes implanted acutely in the striatum of the anesthetized rat.

Anesthetics, Intravenous↗
PubMed2004

Building reliable lattice Monte Carlo models for real drift and diffusion problems.

We revisit the well-known issue of representing an overdamped drift-and-diffusion system by an equivalent lattice random-walk model. We demonstrate that commonly used Monte Carlo algorithms do not conserve the diffusion coefficient when a driving field of arbitrary amplitude is present, and that such algorithms would actually require fluctuating jumping times and one clock per Cartesian direction to work properly. Although it is in principle possible to construct valid algorithms with fixed time steps, we show that no such algorithm can be used in more than two dimensions if the jumps are made along only one axis at each time step.

Journal Article↗
PubMed2005

Surface relaxation and chemical exchange in hydrating cement pastes: a two-dimensional NMR relaxation study.

We report the first nuclear magnetic resonance (NMR) two-dimensional correlation T(1) - T(2) and T(2) - T(2) measurements of hydrating cement pastes. A small but distinct cross peak in the two-dimensional relaxation spectrum provides the first direct evidence of chemical exchange of water between gel and capillary pores occurring over the first 14 days of hydration. A correlation of features along the line T(1) = 4T(2) provides strong supportive evidence for the surface diffusion model of (1)H nuclear spin relaxation in cements and for a multimodal discrete pore size distribution. Differences in detail of the results are reported for white cement paste and white cement paste with added silica fume. Both the method and the theory presented can be applied more widely to other high surface area materials with other reactive surface areas.

Journal Article↗
PubMed2004

A downstream algorithm based on extended gradient vector flow field for object segmentation.

For object segmentation, traditional snake algorithms often require human interaction; region growing methods are considerably dependent on the selected homogeneity criterion and initial seeds; watershed algorithms, however, have the drawback of over segmentation. A new downstream algorithm based on a proposed extended gradient vector flow (E-GVF) field model is presented in this paper for multiobject segmentation. The proposed flow field, on one hand, diffuses and propagates gradients near object boundaries to provide an effective guiding force and, on the other hand, presents a higher resolution of direction than traditional GVF field. The downstream process starts with a set of seeds scored and selected by considering local gradient direction information around each pixel. This step is automatic and requires no human interaction, making our algorithm more suitable for practical applications. Experiments show that our algorithm is noise resistant and has the advantage of segmenting objects that are separated from the background, while ignoring the internal structures of them. We have tested the proposed algorithm with several realistic images (e.g., medical and complex background images) and gained good results.

Algorithms↗
PubMed1993

Directional hearing aid based on array technology.

The hearing impaired often have great difficulty understanding speech in surroundings with background noise or reverberation. A directional hearing aid might be beneficial in reducing background noise in relation to the desired speech signal. To this end microphone systems were developed with strongly directional characteristics, using array techniques. Considerable attention was paid to optimization and stability. Free-field simulations of several robust models showed that a directivity index of 9 dB can be obtained. Simulations were verified with a laboratory model. Based on simulations and measurements, two portable prototypes were developed and tested using a KEMAR-manikin. The KEMAR-measurements showed that the two prototypes gave an improvement of the signal to noise ratio of 7 dB in a fully diffuse sound field. The benefit of these microphone arrays for the hearing impaired was tested in a sound insulated room. One loudspeaker was placed in front of the listener simulating the partner in a discussion, and a diffuse background noise was produced by eight loudspeakers placed on the corners of a cube. The hearing impaired subject was seated in the centre of the cube. The speech-reception threshold in noise for simple Dutch sentences was determined with a normal single omni-directional microphone and with one of the prototypes. The results of the listening tests with 45 hearing impaired subjects showed an average improvement of the S/N-ratio of 7.0 dB for monaural fitting.

Acoustic Stimulation↗
PubMed1999

A model for the cytoplasmic trafficking of signalling proteins involving the hsp90-binding immunophilins and p50cdc37.

A number of transcription factors and protein kinases involved in signal transduction exist in heterocomplexes with the ubiquitous and essential protein chaperone hsp90. These signalling protein x hsp90 heterocomplexes are assembled by a multiprotein chaperone system comprising hsp90, hsp70, Hop, hsp40, and p23. In the case of transcription factors, the heterocomplexes with hsp90 also contain a high molecular weight immunophilin with tetratricopeptide repeat (TPR) motifs, such as FKBP52 or CyP-40. In the case of the protein kinases, the heterocomplexes contain p50cdc37. The immunophilins bind to a single TPR acceptor site on hsp90, and p50cdc37 binds to an adjacent site so that binding is exclusive for p50cdc37 or an immunophilin. Direct interaction of immunophilins with the transcription factors or p50cdc37 with the protein kinases leads to selection of different heterocomplexes after their assembly by a common mechanism. Studies with the glucocorticoid receptor, for which translocation from the cytoplasm to the nucleus is under hormonal control, suggest that dynamic assembly of the heterocomplexes is required for rapid movement of the receptor through the cytoplasm along cytoskeletal tracts. As for the similar short-range trafficking of vesicles along microtubules, there must be a mechanism for linking the signalling protein solutes to the molecular motors involved in movement. We present here a model in which the immunophilins and p50cdc37 target, respectively, the retrograde or anterograde direction of signalling protein movement by functioning as connectors that link the signalling proteins to the movement machinery.

Animals↗
PubMed2005

Local signaling with molecular diffusion as a decoder of Ca2+ signals in synaptic plasticity.

Synaptic plasticity is induced by the influx of calcium ions (Ca2+) through N-methyl-D-aspartate receptors (NMDARs), and the direction and strength of the response depend on the frequency of the synaptic inputs. Recent studies have shown that the direction of synaptic plasticity is also governed by two distinct NMDAR subtypes (NR1/NR2A, NR1/NR2B). How are the different types of regulation (frequency-dependent and receptor-specific) processed simultaneously? To clarify the molecular basis of this dual dependence of synaptic plasticity, we have developed a mathematical model of spatial Ca2+ signaling in a dendritic spine. Our simulations revealed that calmodulin (CaM) activation in the vicinity of NMDARs is strongly affected by the diffusion coefficient of CaM itself, and that this 'local CaM diffusion system' works as a dual decoder of both the frequency of Ca2+ influxes and their postsynaptic current shapes, generated by two NMDAR subtypes, implying that spatial factors may underlie the complicated regulation scheme of synaptic plasticity.

Animals↗
PubMedDate not supplied

[A study of water autodiffusion in model biological membranes, oriented lipid bilayers].

The autodiffusion of water in a multibilayer structure formed by dipalmitoyl phosphatidylcholine and oriented on glass plates was studied by the method of NMR with magnetic field pulse gradient. It was shown that water molecules occur in several states differing in the degree of interaction with lipid molecules. A spectrum of the coefficients of water autodiffusion in a direction transversal to bilayers was found. The use of samples with different distances between the plates and an analysis of the dependence of the mode of diffuse decay of spin echo on diffusion time and the orientation of the sample, as well as measurements at temperatures above and below the gel-liquid crystal phase transition in cholesterol-containing samples enabled one to discriminate the diffuse decay component responsible for the transbilayer movement of water. The coefficient of bilayer permeability was estimated using the Tanner model. It was shown that the formation of mechanical defects ("cracks") in plane oriented bilayers is the most probable reason for the presence of the water component with the relatively high coefficient of diffusion.

1,2-Dipalmitoylphosphatidylcholine↗