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Results for “diffusion theory”

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PubMed2005

Elementary steps in excited-state proton transfer.

The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts, bases, and acids on these reactions is analyzed.

Journal Article↗
PubMed1986

DNA and the neutral theory.

The neutral theory claims that the great majority of evolutionary changes at the molecular (DNA) level are caused not by Darwinian selection but by random fixation of selectively neutral or nearly neutral mutants. The theory also asserts that the majority of protein and DNA polymorphisms are selectively neutral and that they are maintained in the species by mutational input balanced by random extinction. In conjunction with diffusion models (the stochastic theory) of gene frequencies in finite populations, it treats these phenomena in quantitative terms based on actual observations. Although the theory has been strongly criticized by the 'selectionists', supporting evidence has accumulated over the years. Particularly, the recent outburst of DNA sequence data lends strong support to the theory both with respect to evolutionary base substitutions and DNA polymorphism, including rapid evolutionary base substitutions in pseudogenes. In addition, the observed pattern of synonymous codon choice can now be readily explained in the framework of this theory. I review these recent findings in the light of the neutral theory.

Animals↗
PubMed2005

Poisson-Nernst-Planck theory approach to the calculation of current through biological ion channels.

The Poisson-Nernst-Planck (PNP) theory of electro-diffusion is reviewed. Techniques for numerical solution of the three-dimensional PNP equations are summarized, and several illustrative applications to ion transport through protein channels are presented. Strengths and weaknesses of the theory are discussed, as well as attempts to improve it via increasingly realistic evaluation of the force acting on each ion due to the protein/membrane environment.

Animals↗
PubMed1989

Self diffusion of interacting membrane proteins.

A two-dimensional version of the generalized Smoluchowski equation is used to analyze the time (or distance) dependent self diffusion of interacting membrane proteins in concentrated membrane systems. This equation provides a well established starting point for descriptions of the diffusion of particles that interact through both direct and hydrodynamic forces; in this initial work only the effects of direct interactions are explicitly considered. Data describing diffusion in the presence of hard-core repulsions, soft repulsions, and soft repulsions with weak attractions are presented. The effect that interactions have on the self-diffusion coefficient of a real protein molecule from mouse liver gap junctions is also calculated. The results indicate that self diffusion is always inhibited by direct interactions; this observation is interpreted in terms of the caging that will exist at finite protein concentration. It is also noted that, over small distance scales, the diffusion coefficient is determined entirely by the very strong Brownian forces; therefore, as a function of displacement the self-diffusion coefficient decays (rapidly) from its value at infinite dilution to its steady-state interaction-averaged value. The steady-state self-diffusion coefficient describes motion over distance scales that range from approximately 10 nm to cellular dimensions and is the quantity measured in fluorescence recovery after photobleaching experiments. The short-ranged behavior of the diffusion coefficient is important on the interparticle-distance scale and may therefore influence the rate at which nearest-neighbor collisional processes take place. The hard-disk theoretical results presented here are in excellent agreement with lattice Monte-Carlo results obtained by other workers. The concentration dependence of experimentally measured diffusion coefficients of antibody-hapten complexes bound to the membrane surface is consistent with that predicted by the theory. The variation in experimental diffusion coefficients of integral membrane proteins is greater than that predicted by the theory, and may also reflect protein-induced perturbations in membrane viscosity.

Animals↗
PubMed2006

Structure-specific magnetic field inhomogeneities and its effect on the correlation time.

We describe the relationship between the correlation time and microscopic spatial inhomogeneities in the static magnetic field. The theory takes into account diffusion of nuclear spins in the inhomogeneous field created by magnetized objects. A simple general expression for the correlation time is obtained. It is shown that the correlation time is dependent on a characteristic length, the diffusion coefficient of surrounding medium, the permeability of the surface and the volume fraction of the magnetized objects. For specific geometries (spheres and cylinders), exact analytical expressions for the correlation time are given. The theory can be applied to contrast agents (magnetically labeled cells), capillary network, BOLD effect and so forth.

Magnetic Resonance Imaging↗
PubMed2001

Mathematical modeling of bioerodible, polymeric drug delivery systems.

The aim of this article is to give an introduction into mathematical modeling approaches of bioerodible controlled drug delivery systems and to present the most important erosion theories reported in the literature. First, important parameters such as degradation and erosion are defined and physicochemical methods for their investigation are briefly presented. Then, phenomenological empirical models as well as models based on diffusion and chemical reaction theory are discussed. Due to the significant chemical and physicochemical differences among individual bioerodible polymers used for controlled drug delivery systems, various mathematical models have been developed to describe the chemical reactions and physical mass transport processes involved in erosion-controlled drug release. Various examples of practical applications of these models to experimental drug release data are given. For those involved in the design and development of biodegradable drug delivery systems this will help to choose the appropriate mathematical model for a specific drug release problem. Important selection criteria such as the desired predictive power and precision, but also the effort required to apply a model to a particular system will be discussed. Furthermore, before models can be used for drug release predictions certain parameters such as drug dissolution or polymer degradation rate constants, have to be known. The number of parameters to be determined significantly differs between the models. The practical benefit of carefully choosing the right model is that effects of composition and device geometry on the drug release kinetics can be predicted which can reduce laborious formulation studies to a minimum.

Drug Delivery Systems↗
PubMed1980

Passage of proteins through membranes--old assumptions and new perspectives.

This article contains a summary of the ideas that led to the development of the equilibrium theory of protein secretion as an alternative to the current cisternal packaging-exocytosis paradigm. The argument has at its base the philosophical view that the simplest hypothesis should be favored until it is explicity excluded by the weight of evidence. For the transport of molecules across biological membranes this simplest hypothesis is diffusion, and the equilibrium theory simply includes proteins under this rubric. The article then goes on to consider a particular group of experiments that bear on one of three tests of a diffusion mechanism--evidence for the equilibrating nature of the transport process. The evidence that is presented is consistent with a diffusion-based process and does not support secretion by exocytosis. The article then speculates on the potential means by which individual proteins may pass through membranes and focuses on the observed interactions between a digestive enzyme and the membrane phospholipid, phosphatidylinositol. The potential significance of such interactions in the membrane transport of proteins is discussed.

Amylases↗
PubMed1997

Innovation theory and its applicability to our understanding of the diffusion of new management practices in health care organizations.

There has been a longstanding interest in understanding how new management practices and organizational structures are diffused through the health care system. This article reviews current literature on innovation and diffusion to provide insight into how new management practices and organizational structures are introduced into the system. Understanding the process may help in accommodating new developments and provide managerial opportunities to take a more active role in encouraging or discouraging their further evolution.

Canada↗
PubMed2006

Dielectrophoresis of DNA: time- and frequency-dependent collections on microelectrodes.

This paper reports measurements that characterize the collection of DNA onto interdigitated microelectrodes by high-frequency dielectrophoresis. Measurements of time-dependent collection of 12 kilobase pair plasmid DNA onto microelectrodes by dielectrophoresis show significant reduction in the response as the frequency increases from 100 kHz to 20 MHz. Collection time profiles are quantitatively measured using fluorescence microscopy over the range 100 kHz to 5 MHz and are represented in terms of two parameters: the initial dielectrophoretic collection rate, and the initial to steady-state collection transition. Measured values for both parameters are consistent with trends in the frequency-dependent real part of the effective polarizability measured for the same plasmid DNA using dielectric spectroscopy. The experimentally measured parameters are qualitatively compared with trends predicted by theory that takes into account dielectrophoretic particle movement and diffusion. The differences between experiment and theory are discussed with suggested improvements to theoretical models, for example, including the effects of electrohydrodynamically driven fluid motion.

Coated Materials, Biocompatible↗
PubMed2000

Inward diffusion of oxygen on a silicon surface

Cluster computations with the density functional theory were performed to clarify the diffusion mechanism of an O atom adsorbed on a Si surface. The activation energy required for the oxygen diffusion into the Si substrate is estimated to be 1.6 eV, which is fairly small compared to oxygen diffusion within a Si crystal. The presence of a surface dangling bond is responsible for the lowering of the activation energy, assisting the generation of an intermediate which is involved in the rearrangement of the Si-O connections. Adsorption of another O atom on the Si surface appears to further enhance the inward oxygen diffusion.

Journal Article↗
PubMed2003

Adatom kinetics on and below the surface: the existence of a new diffusion channel.

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.

Journal Article↗