The language of nursing science: if not now, when?
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Creativity↗
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Results for “diffusion theory”
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Theory of light propagation incorporating scattering and absorption in turbid media.
A general theoretical approach to the description of light propagating through turbid media is proposed. The theory is a modification of the two-flux model of Kubelka-Munk (KM), extending its applicability to media systems containing an absorptive component. The modified KM model takes into account the influence of internal scattering on the total path length and accommodates a wide range of absorption influences. Experimental results obtained for dyed-paper systems illuminated by diffuse light are demonstrated to be qualitatively and quantitatively reproduced by the theory.
Absorption↗
Does the binding of clusters of basic residues to acidic lipids induce domain formation in membranes?
Several proteins that are important components of the calcium/phospholipid second messenger system (e.g. phospholipase C, protein kinase C, myristoylated alanine-rich C kinase substrate (MARCKS) and pp60src) contain clusters of basic residues that can interact with acidic lipids on the cytoplasmic surface of plasma membranes. We have studied the membrane binding of MARCKS and pp60src, peptides that mimic the basic regions of these proteins, and simple model peptides. Specifically, we determined how the binding of these model peptides depends on (1) the number of basic residues in the peptide (2) the fraction of acidic lipids in the membrane (3) the ionic strength of the solution (4) the chemical nature of the basic residues (Arg versus Lys) and the acidic phospholipids [phosphatidylglycerol (PG) versus phosphatidylserine (PS)] (5) the pressure and (6) the temperature. The results are consistent with a simple theoretical model: each basic residue in a peptide binds independently to an acidic lipid with an intrinsic microscopic association constant of 1-10 M-1 (binding energy congruent to 1 kcal/mol). The binding is described with a mass action formalism and the non-specific electrostatic accumulation of the peptides in the aqueous diffuse double layer is described with the Gouy-Chapman theory. This Gouy-Chapman/mass action model accounts surprisingly well for the sigmoidal dependence of binding on the percentage of acidic lipids in the membrane (apparent co-operativity or Hill coefficient > 1); the model assumes that the multivalent basic peptides bind > 1 acidic lipids and thus induce or stabilize domain formation.
Amino Acid Sequence↗
[Organization and evolution of animal colonies].
Based on the theory of organization and evolution of colonies in an extinct group of hemichordates graptolites (Urbanek, 1960, 1973) the relationship between the events in the late astogeny of bryozoan colonies and their somatic and reproductive cycle. The bryozoan colonies with simple organization are compared with graptoloid colonies and their structure is interpreted within the framework of the morphogen gradient theory. A morphogen produced by the founder-zooid (oozooid) diffuses along the long axis of the colonies and regulates the phenotypic expression of the size and structure of zooids. Evolutionary changes in the graptoloid colonies involve introduction of new characters and their spreading is also accompanied by gradient changes of their manifestation. Evolutionary mechanisms in bryozoan colonies are considered in terms of penetrance and expressivity of genes. In contrast to graptolites, many bryozoan colonies display multiple zones of astogeny changes and repetitions.
Animals↗
Systems thinking and managing complexity, Part two.
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Diffusion of Innovation↗
[Generation of a nerve impulse as a process in plasma. I. Volt-ampere characteristics of membrane contact].
A theory of the nervous impulse generation is proposed. It is based on the plasma (electrolyte) properties near the half-penetrated contact of the regions with different ions concentration. The main processes considered in this theory are following: a change of K+ ions concentration and diffusion of these ions near the outward membrane surface. A correlation between the electric current flux through the contact and the potential difference on this contact is obtained from the solution of the steady-state diffusion equation.
Electricity↗
Molecular dynamics simulations of gas separations using faujasite-type zeolite membranes.
Gas separations with faujasite zeolite membranes have been examined using the method of molecular dynamics. Two binary mixtures are investigated, oxygen/nitrogen and nitrogen/carbon dioxide. These mixtures have been found experimentally to exhibit contrasting behavior. In O(2)/N(2) mixtures the ideal selectivity (pure systems) is higher than the mixture selectivity, while in N(2)/CO(2) the mixture selectivity is higher than the ideal selectivity. One of the key goals of this work was to seek a fundamental molecular level understanding of such divergent behavior. Our simulation results (using previously developed intermolecular models for both the gases and zeolites investigated) were found to replicate this experimental behavior. By examining the loading of the membranes and the diffusion rates inside the zeolites, we have been able to explain such contrasting behavior of O(2)/N(2) and N(2)/CO(2) mixtures. In the case of O(2)/N(2) mixtures, the adsorption and loading of both O(2) and N(2) in the membrane are quite competitive, and thus the drop in the selectivity in the mixture is primarily the result of oxygen slowing the diffusion of nitrogen and nitrogen somewhat increasing the diffusion of oxygen when they pass through the zeolite pores. In N(2)/CO(2) systems, CO(2) is rather selectively adsorbed and loaded in the zeolite, leaving very little room for N(2) adsorption. Thus although N(2) continues to have a higher diffusion rate than CO(2) even in the mixture, there are so few N(2) molecules in the zeolite in mixtures that the selectivity of the mixture increases significantly compared to the ideal (pure system) values. We have also compared simulation results with hydrodynamic theories that classify the permeance of membranes to be either due to surface diffusion, viscous flow, or Knudsen diffusion. Our results show surface diffusion to be the dominant mode, except in the case of N(2)/CO(2) binary mixtures where Knudsen diffusion also makes a contribution to N(2) transport.
Journal Article↗
Relaxation in a glassy binary mixture: comparison of the mode-coupling theory to a Brownian dynamics simulation.
We solved the mode-coupling equations for the Kob-Andersen binary mixture using structure factors calculated from Brownian dynamics simulations of the same system. We found, as was previously observed, that the mode-coupling temperature T(c) inferred from simulations is about two times greater than that predicted by the theory. However, we find that many time-dependent quantities agree reasonably well with the predictions of the mode-coupling theory if they are compared at the same reduced temperature epsilon = (T - T(c))/T(c), and if epsilon is not too small. Specifically, the simulation results for the incoherent intermediate scattering function, the mean square displacement, the relaxation time, and the self-diffusion coefficient agree reasonably well with the predictions of the mode-coupling theory. We find that there are substantial differences for the non-Gaussian parameter. At small reduced temperatures the probabilities of the logarithm of single particle displacements demonstrate that there is hopping-like motion present in the simulations, and this motion is not predicted by the mode-coupling theory. The wave-vector-dependent relaxation time is shown to be qualitatively different from the predictions of the mode-coupling theory for temperatures where hopping-like motion is present.
Journal Article↗
Modeling the role of incisures in vertebrate phototransduction.
Phototransduction is mediated by a G-protein-coupled receptor-mediated cascade, activated by light and localized to rod outer segment (ROS) disk membranes, which, in turn, drives a diffusion process of the second messengers cGMP and Ca2+ in the ROS cytosol. This process is hindered by disks-which, however, bear physical cracks, known as incisures, believed to favor the longitudinal diffusion of cGMP and Ca2+. This article is aimed at highlighting the biophysical functional role and significance of incisures, and their effect on the local and global response of the photocurrent. Previous work on this topic regarded the ROS as well stirred in the radial variables, lumped the diffusion mechanism on the longitudinal axis of the ROS, and replaced the cytosolic diffusion coefficients by effective ones, accounting for incisures through their total patent area only. The fully spatially resolved model recently published by our group is a natural tool to take into account other significant details of incisures, including their geometry and distribution. Using mathematical theories of homogenization and concentrated capacity, it is shown here that the complex diffusion process undergone by the second messengers cGMP and Ca2+ in the ROS bearing incisures can be modeled by a family of two-dimensional diffusion processes on the ROS cross sections, glued together by other two-dimensional diffusion processes, accounting for diffusion in the ROS outer shell and in the bladelike regions comprised by the stack of incisures. Based on this mathematical model, a code has been written, capable of incorporating an arbitrary number of incisures and activation sites, with any given arbitrary distribution within the ROS. The code is aimed at being an operational tool to perform numerical experiments of phototransduction, in rods with incisures of different geometry and structure, under a wide spectrum of operating conditions. The simulation results show that incisures have a dual biophysical function. On the one hand, since incisures line up from disk to disk, they create vertical cytoplasmic channels crossing the disks, thus facilitating diffusion of second messengers; on the other hand, at least in those species bearing multiple incisures, they divide the disks into lobes like the petals of a flower, thus confining the diffusion of activated phosphodiesterase and localizing the photon response. Accordingly, not only the total area of incisures, but their geometrical shape and distribution as well, significantly influence the global photoresponse.
Animals↗
Adsorption of an amphiphilic penicillin onto human serum albumin: characterisation of the complex.
The complex formed by the interaction of the amphiphilic penicillin drug nafcillin and human serum albumin (HSA) in water at 25 degrees C has been characterised using a range of physicochemical techniques. Measurements of the solution conductivity and the electrophoretic mobility of the complexes have shown an ionic adsorption of the drug on the protein surface leading to a surface saturation at a nafcillin concentration of 0.012 mmol kg(-1) and subsequent formation of drug micelles in solutions of higher nafcillin concentration. Measurements of the size of the complex and the thickness of the adsorbed layer by static and dynamic light scattering have shown a gradual change in hydrodynamic radius of the complex with increasing drug concentration typical of a saturation rather than a denaturation process, the magnitude of the change being insufficient to account for any appreciable extension or unfolding of the HSA molecule. The interaction potential between the HSA/nafcillin complexes, and the stability of the complexes were determined from the dependence of diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug/protein complexes with an increase in the concentration of added drug.
Adsorption↗
Spectrophotometric analysis of molecular transport in gels.
An automated spectrophotometric method has been developed for analyzing molecular transport out from and into gels. A Beckman DU7500 diode-array UV-visible spectrophotometer with gel scanner was modified to accept and longitudinally scan a quartz diffusion cell, 0.3x10x40 mm. Molecules of interest are identified and concentrations quantitated via analysis of spectrophotometric absorbance peaks relative to background absorbance of the gel. Thus, concentration profiles are obtained as functions of both position and time. Test data are fitted to a diffusion model via nonlinear least-squares regression. Precision and accuracy of the method were assessed via analysis of several test molecules and gels: (1) 30 mg/ml nonoxynol-9 (N9), contained in 1% sodium alginate gel cross-linked with 2.5 mM calcium chloride, permeating standardized, reconstituted bovine cervical mucus (BCM); (2) 2.5 mg/ml sodium fluorescein, contained in and permeating 10 mg/ml and 100 mg/ml gelatin gels; and (3) 1.0 mg/ml sodium ganciclovir, contained in and permeating 10 mg/ml sodium hyaluronate gel. Diffusion coefficients for (1) and (3) were 3.8x10-7 and 54.1x10-7 cm2/s, respectively. All measurements of diffusion coefficients, partition coefficients, and solute loads obtained in this study were highly repeatable (most C.V.'s<8%). The mean diffusion coefficient for (2) was within 3% of values predicted from theory for the 100 mg/ml gel; the mean partition coefficient for (3) was within 2% of the expected value. This new technique is simpler than many traditional ones in that it does not require labeling of test molecules nor changes in refractive index of target materials. It is particularly well-suited to situations in which the target material is a gel, because no stirring of the target is necessary.
Alginates↗
Self-diffusion of supercooled o-terphenyl near the glass transition temperature.
Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.
Journal Article↗
Steady-state electrophoresis: a technique for measuring physical properties of macro-ions.
The basic theory of a newly discovered physical technique, steady-state electrophoresis, is described followed by analyses of several representative macro-ion systems with the technique. This method measures apparent moments of molar diminished charge (defined below) of solvated macro-ions from an analysis of a concentration gradient of the macro-ions stabilized against a semipermeable membrane by the opposing forces of diffusion and an external electric field. The experimental results confirm that the theory adequately describes the behavior of the macro-ion systems at electrophoretic steady state and suggest applications for the technique in addition to measuring macro-ion-small ion interactions.
Electrochemistry↗
Properties of a diffused photon-pair density wave in a multiple-scattering medium.
A novel diffused photon-pair density wave (DPPDW) composed of correlated polarized photon pairs at different temporal frequencies and orthogonal linearly polarized states is proposed. A theory of DPPDWs is developed. A DPPDW selected by coherence gating and polarization gating that satisfies the diffusion equation has been verified experimentally. The sensitivity of amplitude and phase detection of the heterodyne signal has been improved by the properties of synchronized detection and common-path propagation of polarized pair photons in a multiple-scattering medium. Both reduced scattering coefficient micro2s' and absorption coefficient micro2alpha of the scattering medium in terms of the measured phase and amplitude of the heterodyne signal have been obtained. The detection sensitivity of micro2s' and micro2alpha and the properties of a DPPDW in a multiple-scattering medium are discussed and analyzed.
Diffusion↗
Diffusion of monochromatic classical waves.
We study the diffusion of monochromatic classical waves in a disordered acoustic medium by scattering theory. In order to avoid artifacts associated with mathematical point scatterers, we model the randomness by small but finite insertions. We derive expressions for the configuration-averaged energy flux, energy density, and intensity for one-, two-, and three-dimensional (3D) systems with an embedded monochromatic source using the ladder approximation to the Bethe-Salpeter equation. We study the transition from ballistic to diffusive wave propagation and obtain results for the frequency dependence of the medium properties such as mean free path and diffusion coefficient as a function of the scattering parameters. We discover characteristic differences of the diffusion in 2D as compared to the conventional 3D case, such as an explicit dependence of the energy flux on the mean free path and quite different expressions for the effective transport velocity.
Journal Article↗
Review article: altered states of consciousness, theories of recovery, and assessment following a severe traumatic brain injury.
The causes and consequences of severe traumatic brain injury are reviewed, including mechanical injury and neurochemical changes following focal and diffuse injuries. A discussion of terminology seeks to clarify current nomenclature and descriptions of behaviours during emergence from coma. Theories of recovery following severe brain injury are reviewed including diaschisis, plasticity and compensation, redundancy, environmental regulation, and excess disability. Several assessment tools that have been recommended for use with this population of individuals in an altered state of consciousness following coma are discussed and critiqued. They include the Glasgow Coma Scale, Ranchos Los Amigos Levels of Cognitive Functioning Assessment Scale, Western Neuro Sensory Stimulation Profile, Coma Near Coma Scale, Freeman Questionnaire, and the Sensory Modality Assessment Rehabilitation Technique.
Brain Injuries↗
Computation of dendritic microstructures using a level set method
We compute time-dependent solutions of the sharp-interface model of dendritic solidification in two dimensions by using a level set method. The steady-state results are in agreement with solvability theory. Solutions obtained from the level set algorithm are compared with dendritic growth simulations performed using a phase-field model and the two methods are found to give equivalent results. Furthermore, we perform simulations with unequal diffusivities in the solid and liquid phases and find reasonable agreement with the available theory.
Journal Article↗
Effect of cell arrangement and interstitial volume fraction on the diffusivity of monoclonal antibodies in tissue.
We present theoretical calculations relating the effective diffusivity of monoclonal antibodies in tissue (Deff) to the actual diffusivity in the interstitium (Dint) and the interstitial volume fraction phi. Measured diffusivity values are effective values, deduced from concentration profiles with the tissue treated as a continuum. By using homogenization theory, the ratio Deff/Dint is calculated for a range of interstitial volume fractions from 10 to 65%. It is assumed that only diffusion in the interstitial spaces between cells contributes to the effective diffusivity. The geometries considered have cuboidal cells arranged periodically, with uniform gaps between cells. Deff/Dint is found to generally be between (2/3) phi and phi for these geometries. In general, the pathways for diffusion between cells are not straight. The effect of winding pathways on Deff/Dint is examined by varying the arrangement of the cells, and found to be slight. Also, the estimates of Deff/Dint are shown to be insensitive to typical nonuniformities in the widths of gaps between cells. From our calculations and from published experimental measurements of the effective diffusivity of an IgG polyclonal antibody both in water and in tumor tissue, we deduce that the diffusivity of this molecule in the interstitium is one-tenth to one-twentieth its diffusivity in water. We also conclude that exclusion of molecules from cells (an effect independent of molecular weight) contributes as much as interstitial hindrance to the reduction of effective diffusivity, for small interstitial volume fractions (around 20%). This suggests that the increase in the rate of delivery to tissues resulting from the use of smaller molecular-weight molecules (such as antibody fragments or bifunctional antibodies) may be less than expected.
Animals↗