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PubMedDate not supplied

Translational diffusion of liquids at surface of microporous materials: new theoretical analysis of field cycling magnetic relaxation measurements.

1H spin-lattice relaxation rates of several aprotic polar liquids on calibrated microporous chromatographic glass beads that have paramagnetic ion impurities are recorded over magnetic field strengths using a field-switched magnetic relaxation dispersion spectrometer. The typical bilogarithmic magnetic field dependence of these rates supports quantitatively our theory of nuclear paramagnetic relaxation and gives the translational diffusion at the surface of nanopores. Our results demonstrate that magnetic relaxation dispersion at low magnetic field strengths in high surface area heterogeneous systems may be quantitatively understood in terms of the parameters of the spatial confinement and the local translational dynamics.

Diffusion↗
PubMed1983

Ion-specific diffusion rates through transmembrane protein channels. A molecular dynamics study.

The migration of different alkali metal cations through a transmembrane model channel is simulated by means of the molecular dynamics technique. The parameters of the model are chosen in close relation to the gramicidin A channel. Coulomb- and van der Waals-type potentials between the ions and flexible carbonyl groups of the pore-forming molecule are used to describe the ion channel interaction. The diffusion properties of the ions are obtained from three-dimensional trajectory calculations. The diffusion rates for the different ions Li+, Na+, K+ and Rb+ are affected not only by the mass of the particles but also very strongly by their size. The latter effect is more pronounced for rigid channels, i.e., for binding vibrational frequencies of the CO groups with v greater than 400 cm-1. In this range the selectivity sequence for the diffusion rates is the inverse of that expected from normal rate theory but agrees with that found in experiments for gramicidin A.

Cations, Monovalent↗
PubMed2005

A diffusion model for the fluids confined in micropores.

The self-diffusion coefficients were calculated by molecular dynamics simulations and the effects of pore width, temperature, and fluid density on diffusion behavior of simple fluid argon and polar fluid water confined in micropores were analyzed and studied. A mathematical model describing diffusion behavior of fluids confined in micropores was proposed from the theories of molecular dynamics and molecular kinematics, and validated on the basis of the simulation results at various conditions. The model indicates that the diffusion coefficient is proportional to the square root of the pore width and to the temperature divided by the density squared. It is applicable to either liquid or gas states and only two parameters are required.

Journal Article↗
PubMed2005

Evolving concepts on the pathophysiology of absence seizures: the cortical focus theory.

Four main theories on the pathophysiology of generalized absence seizures have been proposed. The "centrencephalic" theory, proposed in 1954, suggested that discharges originate from a deep-seated diffusely projecting subcortical pacemaker in the midline thalamus. This concept was refined in 1991 with the "thalamic clock" theory, implying that the reticular thalamic nucleus contains the pacemaker cells for the thalamic clock, imposing its rhythm to the cortex. According to other investigators, however, the cortex seems to play a leading role. They suggested that spike-wave discharges have a focal onset in the cortex and are generalized through a rapid propagation. In the "corticoreticular" theory, postulated in 1968, spike-wave discharges are linked to the thalamocortical mechanisms that generate spindles. Rhythmic spindle oscillations generated in the thalamus are transformed into spike-wave discharges when the cortex is hyperexcitable. A 2002 study confirmed in epileptic rats that a functionally intact thalamocortical network is required for the generation of spike-wave discharges. The corticothalamic interrelationships were investigated by means of nonlinear association signal analyses of multiple spike-wave discharges. This showed a consistent focus within the perioral region of the somatosensory cortex. From this focus, seizure activity generalizes rapidly over the cortex. During the first cycles of the seizure the cortex drives the thalamus, while thereafter cortex and thalamus drive each other, thus amplifying and maintaining the rhythmic discharge. In this way the "cortical focus" theory for generalized absence epilepsy bridges cortical and thalamic theories.

Animals↗
PubMed1998

Protein folding dynamics: quantitative comparison between theory and experiment.

The development of a quantitative kinetic scheme is a central goal in mechanistic studies of biological phenomena. For fast-folding proteins, which lack experimentally observable kinetic intermediates, a quantitative kinetic scheme describing the order and rate of events during folding has yet to be developed. In the present study, the folding mechanism of monomeric lambda repressor is described using the diffusion-collision model and estimates of intrinsic alpha-helix propensities. The model accurately predicts the folding rates of the wild-type protein and five of eight previously studied Ala left and right arrow Gly variants and suggests that the folding mechanism is distributed among multiple pathways that are highly sensitive to the amino acid sequence. For example, the model predicts that the wild-type protein folds through a small number of pathways with a folding time of 260 micros. However, the folding of a variant (G46A/G48A) is predicted to fold through a large number of pathways with a folding time of 12 micros. Both folding times quantitatively agree with the experimental values at 37 degrees C extrapolated to 0 M denaturant. The quantitative nature of the diffusion-collision model allows for rigorous experimental tests of the theory.

Bacteriophage lambda↗
PubMed2000

Restricted diffusion model for surface diffusion in reversed-phase liquid chromatography

The analysis of experimental results in reversed-phase liquid chromatography (RPLC) allows further discussion of the restricted diffusion model of surface diffusion formulated on the basis of the absolute rate theory. Chromatographic data were acquired on different RPLC systems with two series of homologous compounds, several stationary phases having different alkyl ligand densities and ligands of various lengths, and methanol/water mobile phases of different compositions. The enthalpy-entropy compensation observed and the linear free energy relationships found for surface diffusion suggest that the surface diffusion mechanism remains probably the same in all RPLC conditions studied. Whereas the isosteric heat of adsorption approaches zero with decreasing retention, the activation energy of surface diffusion tends toward a finite limit and the surface diffusion coefficient tends toward a value near the corresponding molecular diffusivity. These results support the validity of the restricted diffusion model. The influence of different factors on the validity of this model (i.e., the activation energy and the frequency factor of surface diffusion, and the surface tortuosity) was also considered.

Journal Article↗
PubMed1970

The role of diffusion in enzyme kinetics.

A discussion is given of the general role of diffusion in enzyme kinetics based upon a rigorous theory for bimolecular association and dissociation steps that was presented previously, and criteria are formulated for the dependence of the over-all rate on medium viscosity. With these criteria it is possible to conclude that a number of enzymes will exhibit no appreciable dependence of over-all rate on the medium viscosity, quite irrespective of the as yet unmeasured rate constants for association and dissociation of enzyme and substrate. The effect of adsorbing the enzyme onto the surface of a much larger spherical colloidal particle is considered with the conclusion that the rate will either remain the same or decrease, and that its sensitivity to medium viscosity will remain the same or increase.

Diffusion↗
PubMed2004

Rotational diffusivity of fractal clusters.

The rotational diffusion behavior of fractal clusters generated through an off-lattice cluster-cluster aggregation algorithm in both diffusion-limited cluster aggregation and reaction-limited cluster aggregation conditions is investigated. The extended Kirkwood-Riseman theory (Garcia de la Torre et al., Macromolecules, 1987) is used to estimate the cluster rotational diffusion tensor. The three eigenvalues of this tensor, which correspond to the three main rotational diffusivity values of the cluster, have been computed for each generated cluster. Once the eigenvalues have been sorted in ascending order, each of them has been averaged over several thousands of clusters. It is found that one of the three main average rotational diffusivities is substantially larger than the other two, indicating significant anisotropy of fractal clusters. Moreover, a rotational hydrodynamic radius Rh,r has been determined on the basis of the mean value of the three average rotational diffusivities, which is about 25% larger than the mean translational hydrodynamic radius Rh calculated through the same Kirkwood-Riseman theory. Finally, the obtained Rh,r values have been applied to interpret dynamic light scattering data from aggregating colloidal systems and to investigate the reliability of the assumption, Rh = Rh,r, typically made in the literature.

Journal Article↗
PubMed2006

Dielectric properties of aqueous zwitterionic liposome suspensions.

The dielectric spectra of aqueous suspensions of unilamellar liposomial vesicles built up by zwitterionic phospholipids (dipalmitoylphosphatidyl-choline, DPPC) were measured over the frequency range extending from 1 kHz to 10 MHz, where the interfacial polarization effects, due to the highly heterogeneous properties of the system, prevail. The dielectric parameters, i.e., the permittivity epsilon'(omega) and the electrical conductivity sigma(omega), have been analyzed in terms of dielectric models based on the effective medium approximation theory, considering the contribution associated with the bulk ion diffusion on both sides of the aqueous interfaces. The zwitterionic character of the lipidic bilayer has been modeled by introducing an "apparent" surface charge density at both the inner and outer aqueous interface, which causes a tangential ion diffusion similar to the one occurring in charged colloidal particle suspensions. A good agreement with the experimental results has been found for all the liposomes investigated, with size ranging from 100 to 1000 nm in diameter, and the most relevant parameters have briefly discussed in the light of the effective medium approximation theory.

Colloids↗
PubMed2006

Diffusion in multilayer media: transient behavior of the lateral diffusion coefficient.

A general formalism for treating lateral diffusion in a multilayer medium is developed. The formalism is based on the relation between the lateral diffusion and the distribution of the cumulative residence time, which the diffusing particle spends in different layers. We exploit this fact to derive general expressions which give the global and local time-dependent diffusion coefficients in terms of the average cumulative times spent by the particle in different layers and the probabilities of finding the particle in different layers, respectively. These expressions are used to generalize two recently obtained results: (a) A solution for the short-time behavior of the lateral diffusion coefficient in two layers separated by a permeable membrane obtained by a perturbation theory is extended to the entire range of time. (b) A solution for the time-dependent diffusion coefficient of a ligand, which repeatedly dissociates and rebinds to sites on a planar surface, obtained under the assumption that the medium above the surface is infinite, is generalized to allow for the medium layer of finite thickness. For the latter problem we derive an expression for the Fourier-Laplace transform of the propagator in terms of the double Laplace transform of the probability density of the cumulative residence time spent by the ligand in the medium layer.

Diffusion↗
PubMed1978

Isobaric inert gas supersaturation: observations, theory, and predictions.

An isobaric inert gas supersaturation model incorporating both diffusion and perfusion properties of biological tissue is presented in a form which allows ready comparison with experimental observations. This model requires only measurement of inert gas flux and blood gas solubility in order to evaluate "counterdiffusion potential". Inert gas flux across the skin of Yorkshire piglets anesthetized with pentobarbital was measured for He, Ne, CH4, C2H4, N2O, and SF6. Model predictions based upon these data compare favorably with published reports of isobaric inert gas supersaturation, as well as several previously unpublished observations. The possibility of supersaturation resulting from the use of hydrogen as a breathing gas in a helium environment is also discussed, and extensive animal testing is recommended before potentially dangerous human exposure occurs.

Animals↗
PubMed1995

Parallels and contradictions in the theory and practice of action research and nursing.

This paper draws on the authors' experiences of nursing in general and of action research projects. Parallels will be drawn between, on the one hand, the relationship between the practice and theory of nursing and, on the other band, the relationship between the practice and theory of action research. Difficulties common to nursing and action research will be identified (e.g. between the implementation of the theory of nursing and of action research). Over- simplification, non-specificity of issues and gaps in theory are frequent but inevitable problems that hinder theoretical applications. Similarly, comparisons will be made between the problems associated with the translation of nursing practice into theory and the difficulties in transposing the experience of research into theory. The complexities of practice, whether nursing or research, and the evolutionary nature of experience inescapably give rise to dilemmas when analysing theory and practice. The concepts of theory and practice when conceived as a dichotomy present a useful overview of issues. We argue that, through a discussion of the core themes of action research, including 'knowledge in action' and 'self and group reflection', consideration should also be given to a symbiotic and complementary analysis of theory and practice. We observe that many contradictions exist within the theory and practice of action research and of nursing, and that these need to be addressed to promote theoretical and practical advances. Action research offers theoretical and practical insights that contribute to the current debate on the reciprocal relationship between theory and practice.

Diffusion of Innovation↗
PubMed2000

Depletion theory of protein transport in semi-dilute polymer solutions.

We consider the effect of polymer depletion on the transport (diffusion and electrophoresis) of small proteins through semi-dilute solutions of a flexible polymer. A self-consistent field theory may be set up in the important case of quasi-ideal interactions when the protein is small enough. Dynamic depletion, the reorganization of the depletion layer as the protein diffuses, is computed within a free-draining approximation. The transport of the dressed particle (protein + depletion layer) is tackled by extending Ogston's analysis of probe diffusion through fibrous networks to the case of a probe diffusing through a semi-dilute polymer inhomogeneous on the scale of the polymer correlation length. The resulting exponential retardation agrees almost quantitatively with that found in recent electrophoresis experiments of small proteins in polymer solutions that have been ascertained to be semi-dilute (S. P. Radko and A. Chrambach, Electrophoresis, 17:1094-1102, 1996; Biopolymers, 4:183-189, 1997).

Animals↗
PubMed2004

Mixing and reaction fronts in laminar flows.

Autocatalytic reaction fronts between unreacted and reacted mixtures in the absence of fluid flow propagate as solitary waves. In the presence of imposed flow, the interplay between diffusion and advection enhances the mixing, leading to Taylor hydrodynamic dispersion. We present asymptotic theories in the two limits of small and large Thiele modulus (slow and fast reaction kinetics, respectively) that incorporate flow, diffusion, and reaction. For the first case, we show that the problem can be handled to leading order by the introduction of the Taylor dispersion replacing the molecular diffusion coefficient by its Taylor counterpart. In the second case, the leading-order behavior satisfies the eikonal equation. Numerical simulations using a lattice gas model show good agreement with the theory. The Taylor model is relevant to microfluidics applications, whereas the eikonal model applies at larger length scales.

Journal Article↗
PubMed2004

Ab initio calculations to model anomalous fluorine behavior.

Implanted fluorine is observed to behave unusually in silicon, manifesting apparent uphill diffusion and reducing diffusion and enhancing activation of boron. In order to investigate fluorine behavior, we calculate the energy of fluorine defect structures in the framework of density functional theory. In addition to identifying the ground-state configuration and diffusion migration barrier of a single fluorine atom in silicon, a set of energetically favorable fluorine defect structures were found (F(n)V(m)). The decoration of vacancies and dangling silicon bonds by fluorine suggests that fluorine accumulates in vacancy-rich regions, which explains the fluorine redistribution behavior reported experimentally.

Journal Article↗
PubMed2002

Effects of restricted diffusion on MR signal formation.

Numerous functional MRI (fMRI) and diffusion MR studies have recently boosted interest in the theory of MR signal formation in biological systems in the presence of mesoscopic magnetic field in homogeneities. Herein we report an exact solution to the problem of free induction decay (FID) and spin echo (SE) signal formation in the presence of a constant field gradient in three models of one-, two-, and three-dimensional restricted diffusion. We demonstrate the transition with increasing diffusion coefficient from the oscillating FID signal behavior in the static dephasing regime to a monotonic exponential behavior in the motional narrowing regime. Quantitative criteria are presented for applicability of the Gaussian approximation for the description of the MR signal. The spatial distribution of signal density and the edge enhancement effect are analyzed. We also demonstrate that the presence of restrictive barriers in a one-compartment model can lead to a quasi-two-compartment behavior of the MR signal. This result suggests a simple rationale for the experimental findings of biexponential echo attenuation curves in MR diffusion experiments with tissue systems.

Diffusion↗
PubMed1987

The inverse relationship between drug affinity and effectiveness: prediction under rate theory, paradox under occupancy theory.

The two major theories of drug action are occupancy theory and rate theory. Although the two theories make a number of predictions that should serve to clearly distinguish one from the other, data generated over the last 25 years in intact muscle or organ systems have failed in this respect because the interpretation of those data has been confounded by unknown and unmeasurable factors such as the rates and extent of drug diffusion, uptake and metabolism. In the present communication rate theory and occupancy theory are discussed in the light of data obtained from a broken-cell system in which uncontrollable factors are minimized, the beta adrenergic-responsive adenylate cyclase of the S49 mouse lymphoma cell line. It is pointed out that rate theory predicts an inverse relationship between the affinity of isoproterenol (INE) for the beta adrenergic receptor and the magnitude of stimulation of adenylate cyclase. On the other hand, occupancy theory predicts that the affinity of INE and the magnitude of its effect will be directly related. By fitting data obtained in this system to the simple equations derived from the two theories it is demonstrated that the available data are in excellent agreement with the predictions of rate theory and diametrically opposed to the predictions of occupancy theory.

Adenylyl Cyclases↗