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At least 1,261 records · Page 70Linked to original sources
PubMed1996

Rapid repetition of the "burst" sequence: the role of diffusion and consequences for imaging.

When the Burst pulse sequence (Hennig et al. MAGMA 1, 39-48 (1993)) is applied with elementary pulses of finite flip angle, a non-uniform longitudinal magnetization is created, in the form of a series of fine bands of saturated spins. Both during and after the pulse train, the molecules contained within these bands diffuse. In this paper a full description is given of the evolution of the non-uniform longitudinal magnetization subsequent to the pulse train and the theory is verified experimentally. The value of the diffusion coefficient influences strongly the signal obtained when the Burst sequence is repeated rapidly. The consequences of this for imaging are discussed and, in particular, for the so-called "frequency-shifted" Burst technique.

Humans↗
PubMed2001

Electrolyte diffusion of cesium bromide in water at 25 degrees C.

Electrolyte diffusion coefficients of CsBr in water have been measured using 137Cs isotope with an open ended capillary technique without stirring. The measurements were done at 25 degrees C over the concentration range of 10(-6) -0.25 M and the measured values are compared with the theoretical values of diffusion coefficients estimated on the basis of Onsager-Fuoss theory. At low concentrations, there is a good agreement between the experimental and theoretical values of diffusion coefficients. The deviations at higher concentrations are explained on the basis of the occurrence of ion-ion interactions.

Journal Article↗
PubMed1992

Correlations of atomic movements in lysozyme crystals.

Diffuse scattering data have been collected on two crystal forms of lysozyme, tetragonal and triclinic, using synchrotron radiation. The observed diffraction patterns were simulated using an exact theory for simple model crystals which relates the diffuse scattering intensity distribution to the amplitudes and correlations of atomic movements. Although the mean square displacements in the tetragonal form are twice that in the triclinic crystal, the predominant component of atomic movement in both crystals is accounted for by short-range coupled motions where displacement correlations decay exponentially as a function of atomic separation, with a relaxation distance of approximately 6 A. Lattice coupled movements with a correlation distance approximately 50 A account for only about 5-10% of the total atomic mean square displacements in the protein crystals. The results contradict various presumptions that the disorder in protein crystals can be modeled predominantly by elastic vibrations or rigid body movements.

Crystallization↗
PubMed1982

Lateral diffusion in an archipelago. Effects of impermeable patches on diffusion in a cell membrane.

Lateral diffusion of molecules in lipid bilayer membranes can be hindered by the presence of impermeable domains of gel-phase lipid or of proteins. Effective-medium theory and percolation theory are used to evaluate the effective lateral diffusion constant as a function of the area fraction of fluid-phase lipid and the permeability of the obstructions to the diffusing species. Applications include the estimation of the minimum fraction of fluid lipid needed for bacterial growth, and the enhancement of diffusion-controlled reactions by the channeling effect of solid patches of lipid.

Cell Membrane↗
PubMed2005

Comparison of and investigation into the size effects on the rotational dynamics of two spherical molecules: CCl4 and C60.

13C spin-lattice relaxation measurements were performed on CCl4 and C60, two spherical molecules, to investigate the role of molecular size on reorientational dynamics. Measurements were taken at common temperatures and in a common environment; deuterated chlorobenzene-d5. Our data indicate that both CCl4 and C6) reorient in the small-step diffusion limit. Correlation times for CCl4 were found to be smaller in CBZ than in the neat and much shorter than predicted by viscosity arguments. We have attributed the higher rotational freedom to higher "free volume" or lower solute-solvent contact probability, in CBZ than in the neat. Experimental tau C values for CCl4 and C60 in CBZ were found to be much different than those predicted by hydrodynamics. Pure inertia differences did not explain the disparities in the correlation times. However, a very reasonable explanation for our experimental findings was found by considering the solvent-to-probe molecular volume ratio and its effect on rotational diffusion. We applied four hydrodynamic-based theories to our data and found that the model by Gierer-Wirtz best duplicated our experimental observations. The success of this model suggests that the relative size of solute and solvent molecules, as well as events at the microscopic level, rather than bulk properties, are excellent descriptors of the factors affecting rotational diffusion.

Journal Article↗
PubMed1997

Liesegang Ring Type Structures and Bifurcation in Solid-Vapor and Liquid Phase Reactions between Cobalt Nitrate and Ammonium Hydroxide

New results on the reaction between (i) cobalt nitrate (A) and ammonium hydroxide (B) in gelatin and (ii) cobalt nitrate (solid) and ammonia vapor are reported. Spatial bifurcation of the blue wave into a number of normal Liesegang type bands is observed in a one-dimensional tube in gel media depending on the concentration of electrolytes and gel. Similar bifurcation is also observed when the reaction was carried out in gel media between two microslides. Results show that Ksp (solubility product), as envisaged in recent theories, plays a significant role in initiating bifurcation depending on K, the magnitude of the product of concentration of A and B, which determines the degree of supersaturation. The value of K at the instant of bifurcation, when (i) concentration of Co(NO3)2 is alone changed or when (ii) concentration of NH4OH alone is changed, is similar when uncertainties are taken into account. The bifurcation behavior when gel concentration is changed suggests that in case of high gel concentration pore size is affected, thereby influencing the diffusion of ions. The results satisfy the spacing law. The results further satisfy the relation d2 = m't + C' at any time t, where d is the distance between the lower front of the precipitate and the junction and m' and C' are constants, thereby suggesting that the role of diffusion is vital, as envisaged in various theories of the phenomenon. A colored banded structure separated by air gaps (vacant spaces) is formed when the solid cobalt nitrate reacts with ammonia vapor. This type of behavior has not been reported earlier. Initially, a single band is formed in a one-dimensional experiment, but in the course of time ( approximately 1 month) several bands are formed simultaneously. The diffusion kinetics in the initial stages of the solid-vapor reaction has been studied in a one-dimensional tube. The data obey the relation xi2 = kt, where xi is the thickness of the product layer at any time t and k is some constant. Similar studies were made on microslides which showed that during the reaction, a colored product is formed which undergoes change in the sequence pink --> blue --> brown. Results in the one-dimensional case support recent theories which postulate nucleation followed by aggregation. Copyright 1997Academic Press

Journal Article↗
PubMed2003

Hybrid Monte Carlo-diffusion method for light propagation in tissue with a low-scattering region.

The heterogeneity of the tissues in a head, especially the low-scattering cerebrospinal fluid (CSF) layer surrounding the brain has previously been shown to strongly affect light propagation in the brain. The radiosity-diffusion method, in which the light propagation in the CSF layer is assumed to obey the radiosity theory, has been employed to predict the light propagation in head models. Although the CSF layer is assumed to be a nonscattering region in the radiosity-diffusion method, fine arachnoid trabeculae cause faint scattering in the CSF layer in real heads. A novel approach, the hybrid Monte Carlo-diffusion method, is proposed to calculate the head models, including the low-scattering region in which the light propagation does not obey neither the diffusion approximation nor the radiosity theory. The light propagation in the high-scattering region is calculated by means of the diffusion approximation solved by the finite-element method and that in the low-scattering region is predicted by the Monte Carlo method. The intensity and mean time of flight of the detected light for the head model with a low-scattering CSF layer calculated by the hybrid method agreed well with those by the Monte Carlo method, whereas the results calculated by means of the diffusion approximation included considerable error caused by the effect of the CSF layer. In the hybrid method, the time-consuming Monte Carlo calculation is employed only for the thin CSF layer, and hence, the computation time of the hybrid method is dramatically shorter than that of the Monte Carlo method.

Adult↗
PubMed1994

Diffusive and convective solute transport through hemodialysis membranes: a hydrodynamic analysis.

Recent clinical studies have shown that the overall effectiveness of hemodialysis is determined by both the convective and diffusive transport of a wide range of different molecular weight solutes. In this study, transport data were obtained for vitamin B12 and for polydisperse dextrans with a wide range of molecular weights using flat sheet Cuprophan and AN69 polyacrylonitrile membranes. The flux dependence of the actual sieving coefficient was described using classical membrane transport theory, allowing accurate measurements of both the diffusive and convective contributions to the overall solute transport through the porous structure of these dialysis membranes. Asymptotic membrane sieving coefficients and hindered diffusivities were in good agreement with a hydrodynamic model that accounts for the membrane pore size distribution through an expression for the solute partition coefficient in a random porous medium. This model provides an accurate quantitative description of both solute diffusion and convection through hemodialysis membranes, which is critical for the effective design and operation of hemodialyzers.

Acrylic Resins↗
PubMed2000

[Putting it into practice is difficult. A qualitative study on the theory-practice transfer in the case of kinesthetics].

The theory-practice-gap in nursing is a well-known phenomenon. However, to date there is only very little well-founded evidence in nursing research available on this topic, especially in the context of the FRG. Therefore, a qualitative nursing research project, oriented by the methods of Grounded Theory, on theory transfer into practice using the example of a continuing in-service education program on kinesthetics in nursing was carried out that aimed at examining factors that influence the implementation of kinesthetics into nursing practice. After the program, the researcher conducted mainly semi-structured interviews with the participants about their experiences with using what they had learned in their daily work on the ward. As part of the results, the impact of the category "attributes of kinesthetics" on the theory-practice-transfer of the participants in the continuing education program on the topic will be shown. That is to say something about whether the fact that kinesthetics is perceived as practical, strange and exotic, connected with closeness, or as difficult or risky, has an influence on the use of newly learned knowledge in practice. Finally, the relation of this category to the key-category "working in knowledge and new things" will be shown. The findings of this study suggest that putting emphasis on transfer of knowledge alone is not sufficient to account for the complex structure of the process of theory-practice-transfer.

Diffusion of Innovation↗
PubMed2001

Time-dependent fluctuations in single molecule spectroscopy: a generalized Wiener-Khintchine approach.

We consider the theory of time-dependent fluctuations in single molecule spectroscopy. In particular, the relation between stationary spectral diffusion processes and photon counting statistics is investigated. Within linear response theory, a general relation between Mandel's Q parameter and a three time dipole correlation function is obtained. For a prototype spectral diffusion process, an exact solution of Q is found which exhibits rich types of physical behaviors.

Journal Article↗
PubMed1989

Fractal scaling mechanisms in biomembranes. Oscillations in the lateral diffusion coefficient.

A modified model of the Cohen-Turnbull free volume theory for lateral transport processes in biomembranes is presented. The model which is based on renormalization group theoretical concepts incorporates fractal rather than Markovian diffusion kinetics. It predicts harmonic oscillations in the lateral diffusion coefficient around a dominant power-law trend and clarifies, in addition, recently observed deviations from the Cohen-Turnbull exponential law.

Cell Membrane↗
PubMed2000

Structure of a ligand-binding intermediate in wild-type carbonmonoxy myoglobin.

Small molecules such as NO, O2, CO or H2 are important biological ligands that bind to metalloproteins to function crucially in processes such as signal transduction, respiration and catalysis. A key issue for understanding the regulation of reaction mechanisms in these systems is whether ligands gain access to the binding sites through specific channels and docking sites, or by random diffusion through the protein matrix. A model system for studying this issue is myoglobin, a simple haem protein. Myoglobin has been studied extensively by spectroscopy, crystallography, computation and theory. It serves as an aid to oxygen diffusion but also binds carbon monoxide, a byproduct of endogenous haem catabolism. Molecular dynamics simulations, random mutagenesis and flash photolysis studies indicate that ligand migration occurs through a limited number of pathways involving docking sites. Here we report the 1.4 A resolution crystal structure of a ligand-binding intermediate in carbonmonoxy myoglobin that may have far-reaching implications for understanding the dynamics of ligand binding and catalysis.

Animals↗
PubMed2001

High-order perturbation theory for light scattering from a rough metal surface.

The angular distribution of the mean diffuse intensity scattered from a metal surface with one-dimensional roughness is studied with perturbation theory. From an approach based on the reduced Rayleigh equations in p polarization, exact perturbation terms up to eighth order in the height parameter are developed for surface roughness consistent with a stationary Gaussian process. The theory is evaluated for a number of cases in which surface plasmon polariton excitation is significant and produces effects such as backscattering enhancement. For surface roughness having a wide Gaussian power spectrum, it is found that the high-order terms lead to roughness-induced broadening of the backscattering peak. For rectangular spectra, two cases are studied in which backscattering effects are due to sixth- and eighth-order terms; both cases provide good comparisons with previously unexplained experimental results. Further, because of an eighth-order term, the diffuse intensity is shown to contain a specular peak that also relies on polariton excitation. This new effect is studied in detail and is found to arise from the constructive interference of contributions produced by multiple-scattering processes, although the time-reversed paths that produce backscattering enhancement are not essential to the specular effect.

Journal Article↗
PubMed1996

Nonatomic games on Loeb spaces.

In the setting of noncooperative game theory, strategic negligibility of individual agents, or diffuseness of information, has been modeled as a nonatomic measure space, typically the unit interval endowed with Lebesgue measure. However, recent work has shown that with uncountable action sets, for example the unit interval, there do not exist pure-strategy Nash equilibria in such nonatomic games. In this brief announcement, we show that there is a perfectly satisfactory existence theory for nonatomic games provided this nonatomicity is formulated on the basis of a particular class of measure spaces, hyperfinite Loeb spaces. We also emphasize other desirable properties of games on hyperfinite Loeb spaces, and present a synthetic treatment, embracing both large games as well as those with incomplete information.

Journal Article↗
PubMedDate not supplied

Dissemination and utilization of health promotion and disease prevention knowledge: theory, research and experience.

We review the theoretical and research foundations of knowledge dissemination and utilization. We examine first the features that describe the typical pattern of the spread and adoption of ideas in populations. A second level of theory examines the forces that explain and predict the diffusion of health promotion and disease prevention knowledge and innovations in two spheres, the public sphere and te professional sphere. A third level of theory produces models to guide the planning and evaluation of dissemination efforts and advocacy for policies to support the application of knowledge. We trace major research traditions through the literature of various disciplines of professions that have studied diffusion. We then outline some key issues related to knowledge dissemination and utilization in the fields of health promotion and disease prevention.

Ethics, Professional↗
PubMed2005

A theory of transport properties in molten salts.

Expressions for diffusion constants in molten salts have been obtained in terms of the inter-ionic pair potentials and the pair distribution functions. Numerical attempts for diffusion constants in molten alkali halides are carried out and results agreed fairly with those obtained by molecular-dynamics simulation and with some experimental data. Based on the coupling of generalized Langevin equation and damped Einstein oscillator equation, ions' velocity autocorrelation functions have also been described and are numerically applied for molten potassium fluoride. The deviation from the Nernst-Einstein relation was also discussed in detail. In Appendixes A x B x C, the short-time expansion of velocity correlation functions in relation to the partial conductivities and the diffusion constants were obtained up to the term of t(4) and these were compared with a model function described by the form of cos(omegat)sech(ttau).

Journal Article↗
PubMed2006

Interfacial segregation and electro-diffusion of dopants in superlattices.

A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.

Journal Article↗
PubMed1975

Effect of rotation on the diffusion-controlled rate of ligand-protein association.

The rate of binding a fairly large ligand molecule to a protein is reduced below the usual diffusion-controlled rate by the requirement of a certain rotational orientation. A simple, approximate treatment of this effect is given for special cases of spherical and ellipsoidal ligands. As the center of an ellipsoidal ligand approaches a protein surface, there is an effective repulsive potential between ligand and surface owning to restricted rotation of the ligand. The frequency factor kT/h of the Eyring rate theory is replaced in these reactions involving diffusion in solution by D/Rlambda, where D = diffusion coefficient of ligand, lambda = thermal deBroglie wavelength of ligand, and R = "capture" distance around the binding site on the protein.

Adsorption↗