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PubMedDate not supplied

[About the effect of the amplitude of fluctuations on the coefficient of rubbing of the brownian oscillator in aqueous media].

The methods of molecular dynamics and correlation functions of coordinates were applied to investigate molecular oscillations in water solution at a variation of the amplitude of fluctuations. It was established, that the amplitude of fluctuation exceeds the threshold value (in water of the order of 0.3-0.45 A), of the force of friction sharply increases, and the vibration mode changes to the mode of limited diffusion. The consistency of the results with the theory of Mössbauer effect and the model of Brownian oscillator for conformational mobility in biopolymers is discussed.

Animals↗
PubMedDate not supplied

Periodic electroencephalographic compleses and subacute sclerosing panencephalitis. A case report and a survey of literature.

The authors report the case of a child with a presumable diagnosis of subacute sclerosing panencephalitis (SSPE). But pathological examination of the brain showed only a moderate neuronal loss in the cortical and subcortical grey matter. From this case and from the data in the literature it is concluded that the electroencephalogram in stage II of SSPE is characterized by the appearance of paroxysmal wave complexes which recur pseudo-periodically against a disorganized basic activity. The criteria for these complexes are not as strict and the periodicity is not as regular as formerly believed. Such a tracing is not pathognomonic of SSPE and may also occur-although rarely-in other brain diseases. The theory that the periodic discharges originate from a diffuse alteration of cortical and subcortical grey matter seems the most plausible.

Child, Preschool↗
PubMed2001

Diffusion of gamma globulin into the arterial wall identifies localized entry of lipid and cells in atherosclerosis.

BACKGROUND: The localization of atheromatous lesions in vulnerable arteries and their relatively rare occurrence in other arteries of the same subject cannot be explained by current theories of the aetiology of atherosclerosis. OBJECTIVE: To determine whether abnormal diffusion of gamma globulin into the arterial wall from the lumen will identify defects of barrier function allowing localized entry of lipid and cells in atherosclerosis. METHODS: Paraffin sections of left anterior descending coronary arteries and corresponding internal thoracic arteries from 80 human subjects aged 1-65 years were stained for gamma globulin by the immunoperoxidase technique. Duplicate sections were stained with orcein to demonstrate the elastin structure. RESULTS: The barrier function of the luminal surface of the thickened intima was associated with the presence of an elastin lamina beneath the endothelial cells. With advancing age, the coronary arteries exhibited breakdown of this barrier function in localized areas with entry into the arterial wall of gamma globulin, lipid and cells. This was rare in the internal thoracic artery. CONCLUSION: Lack of continuity or incomplete formation of this sub-endothelial lamina, which was seen particularly in the coronary artery, was associated with localized entry into the arterial wall of gamma globulin, lipid and cells from the circulating blood and with the development of atheromatous lesions.

Adolescent↗
PubMed2003

Ostwald ripening of water-in-hydrocarbon emulsions.

The purposes of this study are to determine Ostwald ripening rates in water-in-oil (W/O) emulsions and evaluate the potential application of the LSW theory to W/O emulsions. Water-in-oil emulsions were prepared by mixing water and hydrocarbon phases containing Span 83 and homogenizing. n-Heptane, n-decane, n-dodecane, and n-tetradecane were used to obtain a range of solubilities. A linear increase in the cube of the droplet size with time was observed (within the initial period of 1-2 h after emulsion preparation), supporting the LSW theory. Based on this linear relationship, Ostwald ripening rates were determined to be 3.0 x 10(-24), 2.3 x 10(-24), 1.8 x 10(-24), and 5.8 x 10(-25) m3 s(-1) for water-in-heptane, water-in-decane, water-in-dodecane, and water-in-tetradecane emulsions, respectively. These values are in agreement with theoretical predictions calculated using the LSW equation. It was observed that the ripening process gradually slowed, resulting in deviations from the LSW theory. This was attributed to the effect of the interfacial surfactant film through which the dispersed material has to diffuse during Ostwald ripening. This effect is not taken into consideration by the LSW theory. The results showed that Ostwald ripening of W/O emulsions was less sensitive to the nature of oil used and slower compared to O/W emulsions consisting of the same hydrocarbons.

Journal Article↗
PubMed2004

Ionic diffusion and space charge polarization in structural characterization of biological tissues.

In this study, a new approach to the analysis of the low-frequency (1-10(7) Hz) dielectric spectra of biological tissue, has been described. The experimental results are interpreted in terms of ionic diffusion and space charge polarization according to Sawada's theory. The new presentation of dielectric spectra, i.e. ([Formula: see text]) [Formula: see text] has been used. This method results in peaks which are narrower and better resolved than both the measured loss peaks and an alternative loss quantity [Formula: see text]. The presented method and Sawada's expression have been applied to the analysis of changes in the spatial molecular structure of a collagen fibril network in pericardium tissue exposed to glutaraldehyde (GA), with respect to the native tissue. The diffusion coefficient of ions was estimated on the basis of a dielectric dispersion measurement for an aqueous NaCl solution with a well-calibrated distance between the electrodes. The fitting procedure of a theoretical function to the experimental data allowed us to determine three diffusive relaxation regions with three structural distance parameters d(s), describing the spatial arrangement of collagen fibrils in pericardium tissue. It has been found that a significant decrease in the structural distance d(s) from 87 nm to 45 nm may correspond to a reduction in the interfibrillar distance within GA cross-linked tissue.

Algorithms↗
PubMed2002

Prediction of steady-state skin permeabilities of polar and nonpolar permeants across excised pig skin based on measurements of transient diffusion: characterization of hydration effects on the skin porous pathway.

The applicability of a two-parameter Fickian diffusion model for predicting the skin steady-state permeability based on measurements of the transient transport of permeants across the skin was tested. Using five model permeants possessing different physicochemical properties and pig skin as the model membrane, the skin permeabilities predicted by the two-parameter Fickian diffusion model were compared with the measured skin permeabilities. Results show that the transient skin permeation profiles of the hydrophobic permeants, estradiol, testosterone, and dolichol, across split-thickness pig skin can be modeled adequately by the two-parameter Fickian diffusion model (with constant parameter values), and therefore, that this model can be utilized to shorten the experimental time required to determine the skin permeabilities of these compounds. However, the skin permeabilities of the highly hydrophilic permeants, mannitol and sucrose, predicted by the two-parameter Fickian diffusion model (with constant parameter values) were significantly lower than the experimentally determined values, indicating that the dominant skin pathway of polar permeants within the excised pig skin undergoes significant structural changes during the in vitro diffusion cell studies. Although the skin permeability values determined experimentally using the traditional steady-state method normally correspond to a highly hydrated skin sample, the two-parameter Fickian diffusion model enables an estimation of the skin permeability of the skin membrane at its less-hydrated state (a condition more representative of in vivo and clinical situations). Using the two-parameter Fickian diffusion model and a recently developed skin porous-pathway theory, the effects of skin hydration on the skin porous pathway within the excised pig skin were characterized. Specifically, we found that hydration leads to induction of new pores/reduction of the tortuosity of existing pores within the excised pig skin during the 48 h diffusion cell studies conducted, while the skin average pore radii remain relatively constant (approximately 26 A) for up to 48 h.

Algorithms↗
PubMed2002

Retarding subdiffusion and accelerating superdiffusion governed by distributed-order fractional diffusion equations.

We propose diffusionlike equations with time and space fractional derivatives of the distributed order for the kinetic description of anomalous diffusion and relaxation phenomena, whose diffusion exponent varies with time and which, correspondingly, cannot be viewed as self-affine random processes possessing a unique Hurst exponent. We prove the positivity of the solutions of the proposed equations and establish their relation to the continuous-time random walk theory. We show that the distributed-order time fractional diffusion equation describes the subdiffusion random process that is subordinated to the Wiener process and whose diffusion exponent decreases in time (retarding subdiffusion). This process may lead to superslow diffusion, with the mean square displacement growing logarithmically in time. We also demonstrate that the distributed-order space fractional diffusion equation describes superdiffusion phenomena with the diffusion exponent increasing in time (accelerating superdiffusion).

Journal Article↗
PubMed1977

Cation diffusion selectivity in a pore model. The phosphatidylcholine/water lamellar phase.

The diffusion coefficients D (cm2/s), of four monovalent cations K+, Na+, Rb+ and Cs+ and of Ca2+ have been measured in phosphatidylcholine/water lamellar phase as a function of phase hydration and temperature and in the presence of divalent cations. Diffusion rates vary strongly with phase hydration, between 10(-7) and 10(-6) cm2/s for monovalent and 10(-8) and 10(-7) for Ca2+. The activation energies obtained are relatively small (5--10 kcal/mol). As the phase water content increases, a series of diffusion sequences is obtained, corresponding to the sequences predicted by Eisenman's theory of alkali ion equilibrium selectivity. This diffusional selectivity, which depends exclusively upon non-equilibrium parameters (mobility) within the hydrophilic path is discussed in respect to current theories of pore selectivity.

Cations↗
PubMed2004

The optimized Rouse-Zimm theory of excluded volume effects on chain dynamics.

Based on the optimized Rouse-Zimm (ORZ) approximation to the Kirkwood diffusion equation, we investigate the effects of excluded volume interactions on the single chain dynamics. By incorporating the nonuniformly expanded moments of interbead distances into the expressions for the diffusion and structure matrices appearing in the ORZ diffusion equation, we obtain the general relaxation spectrum for flexible chains that is valid over the range from theta; solvents to good solvents. The present theory involves four parameters: the Kuhn statistical length b(0), the bead number N, the excluded volume parameter z, and the hydrodynamic interaction parameter h(*). These model parameters are determined from structural data of polymers with the aid of the quasi-two-parameter theory. The set of relaxation times of ORZ normal modes calculated with these bead-and-spring model parameters enables the theoretical prediction of various frictional and dynamical properties of polymers within a unified framework. The present ORZ theory generalizes the Ptitsyn-Eizner-type approaches by incorporating the nonuniform chain expansion effect into the structure matrix as well as the diffusion matrix.

Journal Article↗
PubMed2005

Complexity theory and geographies of health: a critical assessment.

The interest of social scientists in complexity theory has developed rapidly in recent years. Here, I consider briefly the primary characteristics of complexity theory, with particular emphasis given to relations and networks, non-linearity, emergence, and hybrids. I assess the 'added value' compared with other, existing perspectives that emphasise relationality and connectedness. I also consider the philosophical underpinnings of complexity theory and its reliance on metaphor. As a vehicle for moving away from reductionist accounts, complexity theory potentially has much to say to those interested in research on health inequalities, spatial diffusion, emerging and resurgent infections, and risk. These and other applications in health geography that have invoked complexity theory are examined in the paper. Finally, I consider some of the missing elements in complexity theory and argue that while it is refreshing to see a fruitful line of theoretical debate in health geography, we need good empirical work to illuminate it.

Geography↗
PubMed2005

The importance of being earnest when crafting definitions: science and scientism are not the same thing.

The APA Division 30 definition of hypnosis is laudable in some respects. For instance, the committee rightly defines the "induction" as nothing more or less than the first suggestion after the introduction. However, the definition stumbles over its nonposition on whether the word hypnosis must be uttered during the procedure. This equivocation invites research designs that preemptively define a hypnotic group and a control group in terms of whether or not the word hypnosis is used in the protocol. These designs represent a backslide into naive operationism; they reveal little new about human nature or hypnosis. The field deserves an optimally heuristic definition that preserves pluralism and is relatively resistant to the teflon shield of preemptive definition. Researchers and practioners require a definition that recognizes the incompleteness of our concepts, generates a level epistemological playing field, and enables hypnosis theories to "reach."

Clinical Trials as Topic↗
PubMed1999

An analysis of the size selectivity of solute partitioning, diffusion, and permeation across lipid bilayers.

The lipid bilayers of cell membranes are primarily responsible for the low passive transport of nonelectrolytes across cell membranes, and for the pronounced size selectivity of such transport. The size selectivity of bilayer permeation has been hypothesized to originate from the hindered transport of solutes across the ordered-chain region. In this paper, we develop a theoretical description that provides analytical relationships between the permeation properties of the ordered-chain region of the lipid bilayer (partition and diffusion coefficients) and its structural properties, namely, lipid chain density, free area, and order parameter. Emphasis is placed on calculating the size selectivity of solute partitioning, diffusion, and overall permeability across the ordered-chain region of the lipid bilayer. The size selectivity of solute partitioning is evaluated using scaled-particle theory, which calculates the reversible work required to create a cavity to incorporate a spherical solute into the ordered-chain region of the lipid bilayer. Scaled-particle theory is also used to calculate the work required to create a diffusion path for solutes in the interfacial region of the lipid bilayer. The predicted size dependence of the bilayer permeability is comparable to that observed experimentally. The dependence of solute partition and diffusion coefficients on the bilayer structural parameters is also discussed.

Diffusion↗
PubMed2006

Microdroplet dissolution into a second-phase solvent using a micropipet technique: test of the Epstein-Plesset model for an aniline-water system.

The Epstein-Plesset model was originally derived for the dissolution of a single gas bubble in an infinite aqueous solution (Epstein, P. S.; Plesset, M. S. J. Chem. Phys. 1950, 18, 1505-1509). The micropipet manipulation technique was previously shown to test this theory on air microbubbles and air-filled lipid-coated microparticles accurately and appropriately (Duncan, P. B.; Needham, D. Langmuir 2004, 20, 2567-2578). This same theory is now tested to model liquid microdroplet dissolution in a well-defined solution environment. As presented previously for the gas-bubble system, holding a single microparticle at the end of a micropipet was not shown to affect the dissolution profile and allowed isotropic diffusion significantly, a necessary condition for the validation of the theory. Here, an aniline-water system with an initial droplet diameter of 50 microm was used as a model liquid-liquid system. A microdroplet of aniline in an aqueous solution presatureated with aniline at distinct levels was tested, as was the reverse system of a water droplet in an aniline solution. The dissolution lifetime was shown to increase with increasing medium saturation fraction according to the Epstein-Plesset time-dependent theory (including the time required to establish the stationary layer) neglecting interfacial tension. The droplet lifetime can be increased by an order of magnitude (from about 10 to 100 s) by increasing the saturation fraction from 0 to 0.9 and by another order of magnitude by increasing from 0.9 to 0.99. The technique proved to be an accurate and appropriate method to test the dissolution of single liquid microdroplets in a second liquid solution and establishes a systematic experimental and theoretical approach to the investigation of the formation of polymer and other microparticles.

Aniline Compounds↗
PubMed1978

[Influence of environmental salinity on the diffusion fluxes of water in the Isopod Sphaeroma serratum (Frabricius)].

1.--In Sphaeroma serratum, the diffusion fluxes of water are 3 or 4 times more important in sea water (SW) than in diluted (50%) sea water. In 100% and in 50% SW influx equals the outflux. 2.--When the animal is quickly transferred from 100% SW to 50% SW, and from 50% SW to 100% SW, the outflux is instantaneously and entirely reset. 3.--The main factor of the instantaneous resettlement of the outflux seems to be the variation of the concentration of Na+ or Cl- or both together in the external medium; the Ca2+ and Mg2+ concentrations do not seem to have any effect on the diffusion flux. 4.--The validity of our results is discussed (theory of unstirred layers, blood-circulation, oxygen interchange, ultrastructure). Our results are compared with those obtained in other Crustaceans and in Fishes.

Animals↗
PubMed1987

A kinetic, chromatographic method for studying protein hydrodynamic behavior.

A chromatographic method based on "split-peak" behavior was described for the determination of the coefficient of mass transfer of proteins on small reversed-phase columns. The coefficient of mass transfer was found to be a linear function of the protein translational diffusion coefficient and inversely proportional to the square of the support particle diameter, as predicted by chromatographic theory. As an example of the practical application of this method for the measurement of protein diffusion coefficients, the denaturation of bovine serum albumin with decreasing solution pH was followed by measuring the change in the coefficient of mass transfer. A major advantage of this method was that the results were not affected by the interaction of the protein with the stationary phase.

Animals↗
PubMed2006

Polymer chains in a soft nanotube: a Monte Carlo study.

We study the equilibrium properties of flexible polymer chains confined in a soft tube by means of extensive Monte Carlo simulations. The tube wall is that of a single sheet six-coordinated self-avoiding tethered membrane. Our study assumes that there is no adsorption of the chain on the wall. By varying the length N of the polymer and the tube diameter D we examine the variation of the polymer gyration radius Rg and diffusion coefficient Ddiff in soft and rigid tubes of identical diameter and compare them to scaling theory predictions. We find that the swollen region of the soft tube surrounding the chain exhibits a cigarlike cylindrical shape for sufficiently narrow tubes with D<<Rg. The observed scaling of static conformational properties with chain length N and tube diameter D follows the predictions of scaling theory and displays no significant difference between soft and rigid tubes. The Brownian dynamics of the polymer diffusion in a rigid tube is found to slow down in a tube with soft walls by an amount which depends on the Rg/D ratio albeit the relaxation time tau(parallel) for diffusive motion along the tube still scales as tau(parallel) proportional, N3.

Journal Article↗
PubMed2005

Analytical theories of transport in concentrated electrolyte solutions from the MSA.

Ion transport coefficients in electrolyte solutions (e.g., diffusion coefficients or electric conductivity) have been a subject of extensive studies for a long time. Whereas in the pioneering works of Debye, Hückel, and Onsager the ions were entirely characterized by their charge, recent theories allow specific effects of the ions (such as the ion size dependence or the pair association) to be obtained, both from simulation and from analytical theories. Such an approach, based on a combination of dynamic theories (Smoluchowski equation and mode-coupling theory) and of the mean spherical approximation (MSA) for the equilibrium pair correlation, is presented here. The various predicted equilibrium (osmotic pressure and activity coefficients) and transport coefficients (mutual diffusion, electric conductivity, self-diffusion, and transport numbers) are in good agreement with the experimental values up to high concentrations (1-2 mol L(-1)). Simple analytical expressions are obtained, and for practical use, the formula are given explicitly. We discuss the validity of such an approach which is nothing but a coarse-graining procedure.

Journal Article↗
PubMed2005

Reactions on cell membranes: comparison of continuum theory and Brownian dynamics simulations.

Biochemical transduction of signals received by living cells typically involves molecular interactions and enzyme-mediated reactions at the cell membrane, a problem that is analogous to reacting species on a catalyst surface or interface. We have developed an efficient Brownian dynamics algorithm that is especially suited for such systems and have compared the simulation results with various continuum theories through prediction of effective enzymatic rate constant values. We specifically consider reaction versus diffusion limitation, the effect of increasing enzyme density, and the spontaneous membrane association/dissociation of enzyme molecules. In all cases, we find the theory and simulations to be in quantitative agreement. This algorithm may be readily adapted for the stochastic simulation of more complex cell signaling systems.

Algorithms↗