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PubMed2005

Diffusion MR imaging characteristics of the developing primate brain.

Diffusion-based magnetic resonance imaging holds the potential to non-invasively demonstrate cellular-scale structural properties of brain. This method was applied to fixed baboon brains ranging from 90 to 185 days gestational age to characterize the changes in diffusion properties associated with brain development. Within each image voxel, a probability-theory-based approach was employed to choose, from a group of analytic equations, the one that best expressed water displacements. The resulting expressions contain eight or fewer adjustable parameters, indicating that relatively simple expressions are sufficient to obtain a complete description of the diffusion MRI signal in developing brain. The measured diffusion parameters changed systematically with gestational age, reflecting the rich underlying microstructural changes that take place during this developmental period. These changes closely parallel those of live, developing human brain. The information obtained from this primate model of cerebral microstructure is directly applicable to studies of human development.

Algorithms↗
PubMed2004

Modeling molecular transport in slit pores.

We examine the transport of methane in microporous carbon by performing equilibrium and nonequilibrium molecular dynamics simulations over a range of pore sizes, densities, and temperatures. We interpret these simulation results using two models of the transport process. At low densities, we consider a molecular flow model, in which intermolecular interactions are neglected, and find excellent agreement between transport diffusion coefficients determined from simulation, and those predicted by the model. Simulation results indicate that the model can be applied up to fluid densities of the order to 0.1-1 nm(-3). Above these densities, we consider a slip flow model, combining hydrodynamic theory with a slip condition at the solid-fluid interface. As the diffusion coefficient at low densities can be accurately determined by the molecular flow model, we also consider a model where the slip condition is supplied by the molecular flow model. We find that both density-dependent models provide a useful means of estimating the transport coefficient that compares well with simulation.

Journal Article↗
PubMed2000

Multifocal electroretinogram abnormalities persist following resolution of central serous chorioretinopathy.

OBJECTIVE: To examine results of the multifocal electroretinogram (MERG) after spontaneous resolution of central serous chorioretinopathy (CSC) detachments. METHODS: Multifocal electroretinograms were recorded from both eyes of 5 recovered patients with CSC and 10 age-matched healthy subjects. All patients with CSC had bilaterally subnormal MERG amplitudes during a first attack of CSC occuring 7 to 23 months earlier. RESULTS: After recovery from CSC, MERG A-wave and B-wave amplitudes increased markedly where the detachment resolved, and moderately elsewhere in the posterior pole of both eyes. However, the signals from both eyes remained either subnormal or low-normal relative to controls. Multifocal electroretinogram B-wave latencies improved from prolonged to mid-normal values in both eyes. CONCLUSIONS: Both eyes of patients with active unilateral CSC exhibit diminished MERG amplitudes. Although MERG response amplitudes increased modestly after recovery from CSC, they remained statistically subnormal throughout the posterior pole of both eyes. These findings support the theory that subretinal fluid retention in CSC is secondary to diffuse pathologic changes in the choroid and/or retinal pigment epithelium. They also suggest that the underlying or predisposing abnormalities of CSC resolved only partially in our patients. Components of the MERG may have value as a prognostic tool for judging the risk of developing symptomatic CSC. Arch Ophthalmol. 2000;118:1211-1215

Adult↗
PubMed1999

Venous blood effects in spin-echo fMRI of human brain.

The spin-echo response to visual activation was studied as a function of spatial resolution at a field of 1.5 T. The results showed that the increase in absolute T(2) upon activation was as large as 22.8 +/- 3.1% (P < 0.05) at the highest resolution (5.3 mm(3)), while it was as small as 3.5 +/- 0.2% (P < 0.05) at the lowest resolution (42.2 mm(3)). In addition, upon increasing resolution, the spin-echo signal decay as a function of echo time changed from monoexponential to nonexponential. These data indicate that, when using the standard resolution for fMRI studies at 1.5 T, the effects of spin-echo changes in the draining veins are of major contribution to the total blood oxygenation level-dependent (BOLD) signal changes measured in voxels encompassing the activated brain areas. The data can be quantitatively accounted for using a model based on the intravascular origin of the spin-echo effect including both macrovascular and microvascular effects. Existing theories for the spin-echo BOLD effect based on diffusion through field gradients predict negligible spin-echo effects inside the large vessels and are therefore incompatible with the data. Magn Reson Med 42:617-626, 1999.

Brain↗
PubMed1976

Effect of solvent flow Reynolds number on dissolution rate of a nondisintegrating solid (potassium chloride).

An apparatus for measuring dissolution rates of solids in the form of disks was designed to possess the following features. The solvent flowed continuously and reproducibly past the disk at various rates associated with calculable Reynolds numbers, Re. The effluent solution was adequately mixed before analysis. The concentration of dissolved solute was much less than the solubility. The surface area of the disk in contact with the solvent was constant during measurements. The dissolution rate of the disk was reproducible, and the disk and its surface could be readily characterized. The apparatus was tested at 37 degrees with compressed potassium chloride and water. The intrinsic dissolution rate, G, was a linear function of Re from Re=360 to greater than 6000. This relationship enabled one unknown constant in each dissolution theory to be expressed in terms of Re. For the diffusion layer model, the thickness of this layer, calculated from the experimental value of G, agreed well with that calculated from the various physical properties, provided that natural convection did not predominate. The dissolution of potassium chloride in this system was, therefore, controlled by diffusion.

Chemistry, Pharmaceutical↗
PubMed1977

Dissolution kinetics of cholesterol in simulated bile I: Influence of bile acid type and concentration, bile acid-lecithin ration, and added electrolyte.

A physical model approach was utilized to investigate cholesterol monohydrate dissolution kinetics in simulated bile. The static pellet method and the Berthoud theory were employed to assess the contributions of the diffusion-convection mass transfer resistance and those of the interfacial resistance to the overall kinetics. For almost all situations studied, the interfacial resistance was the dominant rate-determining factor. The effects of four bile acids and their concentrations, the bile acid-lecithin ratio, and the added electrolytes and their concentrations on the interfacial resistance were examined. The results were correlated with those obtained with human bile samples, and the indications were that the kinetics of cholesterol dissolution in bile may be explainable on the basis of the principal bile acids, lecithin, and the electrolytes in the bile.

Bile↗
PubMed1997

The Methyl-Rotor Electron-Spin Dynamics in the Smoluchowski Drift-Diffusional Framework

The EPR behavior of the internal motion of the methyl fragment in radicals is simulated using the Smoluchowski drift diffusion model. EPR stochastic Liouville lineshape calculations using this model are presented, allowing the exploration of the whole span of conditions between the discrete-site-exchange and the stepless-free-diffusion limits and offering a unification of these two approximate theories. An accurate value of the correlation time for the isotropic hyperfine interaction is calculated for the full ranges of the parameters describing the system and is compared to those given by the approximate models in the two limiting cases, allowing for a discussion of their quality. Long and short correlation time limit spectra are reported and interpreted using analytic models. For the isotropic hyperfine interaction studied here, no broadening effects are observed under Redfield (strong narrowing) conditions. This suggests absence of lifetime broadening effects in agreement with site-model results. The addition of a cos varphi term in the hyperfine interaction is essential for pyramidal radical centres, as is demonstrated by simulation of experimental reports in the literature.

Journal Article↗
PubMed1980

Biased random walk models for chemotaxis and related diffusion approximations.

Stochastic models of biased random walk are discussed, which describe the behavior of chemosensitive cells like bacteria or leukocytes in the gradient of a chemotactic factor. In particular the turning frequency and turn angle distribution are derived from certain biological hypotheses on the background of related experimental observations. Under suitable assumptions it is shown that solutions of the underlying differential-integral equation approximately satisfy the well-known Patlak-Keller-Segel diffusion equation, whose coefficients can be expressed in terms of the microscopic parameters. By an appropriate energy functional a precise error estimation of the diffusion approximation is given within the framework of singular perturbation theory.

Animals↗
PubMed1991

Dynamic light scattering by kappa- and lambda-carrageenan solutions.

The intensity correlation functions of kappa- and lambda-carrageenan in various salt solutions and at different concentrations have been determined with the help of dynamic light scattering. From the first cumulant of these correlation functions the values of the translational diffusion coefficients D have been derived. They increase with macromolecular concentration. The extrapolated values to infinite dilution of the diffusion coefficients increase with increasing salt concentration as expected from the salt concentration dependence of the r.m.s. radii of gyration determined previously by static light scattering. The translational diffusion coefficient of lambda-carrageenan in 0.1 M NaCl is smaller than the corresponding value for the kappa species. This is consistent with the difference in contour length and linear charge density of the two samples used. No satisfactory interpretation for the concentration dependence of the diffusion coefficient seems to be possible at present. Although current theories for the macromolecular and salt concentration dependence of D, taking into account charge effects, seem to be applicable, they do not allow for a consistent interpretation of the data. No specific difference between the solution behaviour of kappa- and lambda-carrageenan has been detected.

Carrageenan↗
PubMed2003

Hydrodynamic instability and coalescence in trains of emulsion drops or gas bubbles moving through a narrow capillary.

We investigate the effect of surfactant on the hydrodynamic stability of a thin liquid film formed between two emulsion drops or gas bubbles, which are moving along a narrow capillary. A ganglion (deformed drop or bubble in a pore) is covered by an adsorption monolayer of surfactant. Due to the hydrodynamic viscous friction, the surfactant is dragged from the front part of a moving ganglion toward its rear part. Consequently, the front and rear parts are, respectively, depleted and enriched in adsorbed surfactant. When such two ganglia move one after another, surfactant molecules desorb from the rear part of the first ganglion and are transferred by diffusion, across the intermediate liquid film, to the front part of the second ganglion. This leads to the appearance of a diffusion-driven hydrodynamic instability, which may cause coalescence of the two neighboring drops or bubbles. The coalescence occurs through a dimple-like perturbation in the film thickness, which is due to a local lowering in the pressure caused by a faster circulation of the liquid inside the film, which in turn is engendered by the accelerated surfactant diffusion across the thinner parts of the film. The developed theory predicts the critical distance between the two ganglia, which corresponds to the onset of coalescence, and its dependence on the radius of the capillary channel, velocity of motion, surfactant concentration and type of the operative surface forces. The results can be useful for a better understanding and quantitative description of the processes accompanying the flow of emulsions and foams though porous media.

Journal Article↗
PubMed1987

Development and validation of the occupational identity scale.

Ego-identity research utilizing Marcia's (1966) identity statuses has been prolific during the past 15 years. The four types of statuses--achievement, moratorium, foreclosure, diffusion--have become part of the ego-identity development theory. The development of a research tool to study further one of the dimensions of ego-identity development (occupational dimension) was perceived as a need. Therefore, items were created utilizing the criteria established by previous research and content validated by a group of experts. These statements were validated by 417 students from six high schools and colleges. Responses were analyzed and measures of construct and concurrent validity were obtained. Also indexes of internal consistency and item discrimination were estimated. Through factor analysis techniques, four factors were identified for the occupational identity statuses. They accounted for 49 per cent of the total variance. Reliability coefficients ranged between 0.70 and 0.87. Concurrent validity coefficients ranged between 0.38 and 0.79, when correlated with a similar instrument. After deletion of those items that did not contribute significantly to the validity of the instrument, a 28-item Occupational Identity Scale was established.

Adolescent↗
PubMed2001

In situ nasal absorption of midazolam in rats.

Intranasal (i.n.) midazolam (MDZ) administrations may be used successfully for preoperative sedation, especially in young patients. However, clinicians have to use the commercial parenteral formulation, the low pH of which (3.3), necessary to solubilize MDZ (pK(a) 6.1), is probably responsible for the signs of local irritation frequently reported. As a starting point to design a formulation suitable for the nasal route, MDZ nasal absorption was investigated in rats. The effects of the MDZ solution concentration (10--100 microg/ml), osmolality (from less than 10 mOsm/kg up to 450 mOsm/kg) and pH (3.3--7.4) were studied using an in situ perfusion technique. MDZ was determined by reversed-phase HPLC in the circulating solution and results were expressed in clearance terms. MDZ absorption was independent of its concentration. The pH of the solutions was the key-parameter and only a pH above 4 allowed significant absorption. These results were consistent with a passive diffusion absorption of MDZ and partly followed the pH partition theory. In conclusion, satisfactory MDZ absorption should be expected with a formulation at a pH suitable for the nasal route in human (5.5--6.5).

Absorption↗
PubMed2006

Steady-state voltammetric response of the nanopore electrode.

The steady-state voltammetric response of the truncated conical-shaped glass nanopore electrode is presented. Analytical theory, finite-element simulations, and experimental measurement of the diffusive flux of a redox molecule through the pore orifice demonstrate that the steady-state current decreases rapidly as the pore depth increases and then asymptotically approaches a constant value when the pore depth is approximately 50x larger than the pore orifice. The asymptotic limit of the steady-state current is only a function of the pore orifice radius and the cone angle of the pore and has a finite value for all cone angles greater than zero. Experimental confirmation of the predicted dependence on pore depth is obtained using nanopore electrodes with 100-1000 nm orifice radii, by measuring the steady-state voltammetric current corresponding to the oxidation of ferrocene in acetonitrile solutions containing an excess of supporting electrolyte.

Computer Security↗
PubMed1984

Rotational dynamics of transfer ribonucleic acid: effect of ionic strength and concentration.

We have investigated the influence of ionic strength and nucleic acid concentration on the rotational Brownian motion of Escherichia coli tRNA1Val by studying the decay of the fluorescence polarization anisotropy (FPA) of intercalated ethidium on a nanosecond time scale. The rotational relaxation time tau R remains essentially constant as the ionic strength is varied from 2 to 100 mM at a tRNA concentration of 54 mg/mL. tau R also remains practically unchanged as the tRNA concentration is varied from 0.3 to 54 mg/mL at an ionic strength of 130 mM. Present hydrodynamic theories generally predict a more pronounced concentration dependence for rotational diffusion than we observe. This disagreement may result from a nonrandom distribution of the tRNA molecules in solution due to electrostatic interactions. By combining independent data from time-resolved nuclear Overhauser effect (NOE) cross-relaxation experiments and FPA experiments on the same tRNA, we are able to estimate the interproton spacing for the guanine N1-H and the uracil N3-H of the GU-50 base pair in E. coli tRNA1Val. This distance is 0.272 nm.

Escherichia coli↗
PubMed1989

Macro and micro rate zonal analytical centrifugation of polydisperse and slowly diffusing sedimenting systems in isovolumetric density gradients. Application to cartilage proteoglycans.

A method to study the polydispersity of zonally sedimenting and slowly diffusing macromolecules or particles in isokinetic or isovolumetric density gradients is presented. First, a brief theory is given for predicting the zonal profile after a "triangular" (or "inverse") zone is centrifuged. This type of zone is essential to preserve hydrodynamic stability of the very slowly diffusing polydisperse solutes. It is proven, both by semitheoretical considerations and by computer calculations, that the resulting concentration profile of macrosolute is almost identical with that obtainable with a rectangular zone coextensive with the triangular one and carrying the same total mass. Next, practical procedures are described for the convectionless layering of very small triangular zones (50 microL or less). The linearity and stability of the zones are experimentally tested and verified. Finally, the method is applied to cartilage proteoglycan preparations that included either the monomeric molecules only or both the monomeric and the aggregated ones. The zonal results are compared with those obtained by using conventional boundary sedimentation. The two sets of results are seen to coincide fairly well, thus proving that the present technique can add to preparative zonal centrifugation the analytical precision of boundary sedimentation. A multimodal polydisperse system is suggested to describe the aggregated proteoglycan macromolecules.

Animals↗
PubMed2006

Conformational analysis of sulfate species on Ag/Al2O3 by means of theoretical and experimental vibration spectra.

The formation and configuration of sulfate species on Ag/Al2O3 were studied by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. The comparison between theoretical and experimental vibration spectra enable us to draw the following conclusions: Bidentate sulfate species rather than tridentate sulfate species are the predominant surface species on Ag/Al2O3. Moreover, both bidentate and tridentate sulfate species may coexist on the catalyst surface at a lower coverage. The accumulation of surface sulfate species could well explain the blue shift of the sulfate species in IR spectra. In addition, the in situ DRIFTS could distinguish between the sulfate species that linked to Al site and Ag site, which was well supported by temperature-programmed desorption (TPD) results.

Journal Article↗
PubMed2004

Study of nonsteady diffusional growth of a droplet in a supersaturated vapor: treatment of the moving boundary and material balance.

A new mathematical treatment of the problem of droplet growth via diffusion of molecules from a supersaturated vapor is presented. The theory is based on a semiquantitative analysis with good physical arguments and is justified by its reasonable predictions. For example it recovers the time honored growth law in which, to a high degree of approximation, the droplet radius increases with the square root of time. Also, to a high degree of approximation, it preserves material balance such that, at any time, the number of molecules lost from the vapor equals the number in the droplet. Estimates of the remaining approximational error are provided. On another issue, we show that, in contrast, the conventional treatment of droplet growth does not maintain material balance. This issue could be especially important for the nucleation of another droplet in the vicinity of the growing droplet where the rate of nucleation depends exponentially on supersaturation. Suggestions for further improvement of rigor are discussed.

Journal Article↗
PubMed2005

Integral encounter theory of strong electron transfer.

The integral encounter theory (IET) has been extended to the reactions limited by diffusion along the reaction coordinate to the level crossing points where either thermal or hot electron transfer occurs. IET describes the bimolecular ionization of the instantaneously excited electron donor D* followed by the hot geminate backward transfer which precedes the ion pair equilibration and its subsequent thermal recombination. We demonstrate that the fraction of ion pairs which avoids the hot recombination is much smaller than their initial number when the electron tunneling is strong.

Journal Article↗