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PubMed2004

Transport coefficients of aqueous dodecyltrimethylammonium bromide solutions: comparison between experiments, analytical calculations and numerical simulations.

We study dynamical properties of ionic species in aqueous solutions of dodecyltrimethylammonium bromide, for several concentrations below and above the critical micellar concentration (cmc). New experimental determinations of the electrical conductivity are given which are compared to results obtained from an analytical transport theory; transport coefficients of ions in these solutions above the cmc are also computed from Brownian dynamics simulations. Analytical calculations as well as the simulation treat the solution within the framework of the continuous solvent model. Above the cmc, three ionic species are considered: the monomer surfactant, the micelle and the counterion. The analytical transport theory describes the structural properties of the electrolyte solution within the mean spherical approximation and assumes that the dominant forces which determine the deviations of transport processes from the ideal behavior (i.e., without any interactions between ions) are hydrodynamic interactions and electrostatic relaxation forces. In the simulations, both direct interactions and hydrodynamic interactions between solutes are taken into account. The interaction potential is modeled by pairwise repulsive 1/r(12) interactions and Coulomb interactions. The input parameters of the simulation (radii and self-diffusion coefficients of ions at infinite dilution) are partially obtained from the analytical transport theory which fits the experimental determinations of the electrical conductivity. Both the electrical conductivity of the solution and the self-diffusion coefficients of each species computed from Brownian dynamics are compared to available experimental data. In every case, the influence of hydrodynamic interactions (HIs) on the transport coefficients is investigated. It is shown that HIs are crucial to obtain agreement with experiments. In particular, the self-diffusion coefficient of the micelle, which is the largest and most charged species in the present system, is enhanced when HIs are included whereas the diffusion coefficients of the monomer and the counterion are roughly not influenced by HIs.

Journal Article↗
PubMed1997

Light dosimetry in vivo.

This paper starts with definitions of radiance, fluence (rate) and other quantities that are important with regard to in vivo light dosimetry. The light distribution in mammalian tissues can be estimated from model calculations using measured optical properties or from direct measurements of fluence rate using a suitable detector. A historical introduction is therefore followed by a brief discussion of tissue optical properties and of calculations using diffusion theory, the P3-approximation or Monte Carlo simulations. In particular the form of the scattering function is considered in relation to the fluence rate close to the tissue boundary, where light is incident. Non-invasive measurements of optical properties yield the absorption coefficient mu a and mu s(1 - g), where mu s is the scattering coefficient and g is the mean cosine of the scattering angle. An important question is whether this combination is sufficient, or whether g itself must be known. It appears that for strongly forward scattering, as in mammalian tissues, rather detailed knowledge of the scattering function is needed to reliably calculate the fluence rate close to the surface. Deeper in the tissue mu s (1 - g) is sufficient. The construction, calibration and use of fibre-optic probes for measurements of fluence rate in tissues or optical phantoms is discussed. At present, minimally invasive absolute fluence (rate) measurements seem to be possible with an accuracy of 10-20%. Examples are given of in vivo measurements in animal experiments and in humans during clinical treatments. Measurements in mammalian tissues, plant leaves and marine sediments are compared and similarities and differences pointed out. Most in vivo light fluence rate measurements have been concerned with photodynamic therapy (PDT): Optical properties of the same normal tissue may differ between patients. Tumours of the same histological type may even show different optical properties in a single patient. Treatment-induced changes of optical properties may also occur. Scattered light appears to contribute substantially to the light dose. All these phenomena emphasize the importance of in situ light measurements. Another important dosimetric parameter in PDT is the concentration and distribution of the photosensitizer. Apart from in vivo fluorescence monitoring, the photosensitizer part of in vivo PDT dosimetry is still in its infancy.

Animals↗
PubMed2000

Instantaneous normal mode analysis of supercooled water

We use the instantaneous normal mode approach to provide a description of the local curvature of the potential energy surface of a model for water. We focus on the region of the phase diagram in which the dynamics may be described by mode-coupling theory. We find that the diffusion constant depends on the fraction of directions in configuration space connecting different local minima, supporting the hypothesis that the dynamics are controlled by the geometric properties of configuration space. Furthermore, we find a relation between the number of basins accessed in equilibrium and the connectivity between them.

Journal Article↗
PubMed2003

Destabilization of Turing structures by electric fields.

The dynamic behavior of hexagonal Turing patterns in the chlorine dioxide-iodine-malonic acid reaction was studied in a newly developed open reactor under the influence of externally applied weak directed current up to 17.5 mA. A transition from stationary hexagonal patterns to spots moving parallel to the direction of the applied electric field could be observed. The velocity of the drift increases monotonously with the applied current. Experimental results could be qualitatively reproduced in numerical simulations with a reaction-diffusion-advection model, based on a realistic kinetic mechanism.

Journal Article↗
PubMed2004

Librational motion of spin-labeled lipids in high-cholesterol containing membranes from echo-detected EPR spectra.

Two-pulse, echo-detected (ED) electron paramagnetic resonance (EPR) spectroscopy was used to study the librational motions of spin-labeled lipids in membranes of dipalmitoylphosphatidylcholine + 50 mol % cholesterol. The temperature dependence, over the range 77-240 K, and the dependence on position of spin-labeling in the sn-2 chain (n=5, 7, 10, 12, and 14) of the phospholipid, were characterized in detail. The experimental ED-spectra were corrected for instantaneous spin diffusion arising from static spin-spin interactions, by using spectra recorded at 77 K, where motional contributions are negligible. Simulations according to a model of rapid, small-amplitude librations about an axis whose direction is randomly distributed are able to describe the experimental spectra. Calibrations, in terms of the amplitude-correlation time product, alpha2tauc, were constructed for diagnostic spectral line-height ratios at different echo delay times, and for relaxation spectra obtained from the ratio of ED-spectra recorded at two different echo delays. The librational amplitude, alpha2, was determined for a spin label at the 14-C position of the lipid chain from the partially motionally averaged hyperfine splitting in the conventional EPR spectra. The librational correlation time, tauc, which is deduced from combination of the conventional and ED-EPR results, lies in the subnanosecond regime and depends only weakly on temperature. The temperature dependence of the ED-EPR spectra arises mainly from an increase in librational amplitude with increasing temperature, and position down the lipid chain. A gradual transition takes place at higher temperatures, from a situation in which segmental torsional librations are cumulative, i.e., the contributions of the individual segments add up progressively upon going down the chain, to one of concerted motion only weakly dependent on chain position. Such librational motions are important for glass-like states and are generally relevant to high lipid packing densities, e.g., in cholesterol-containing raft domains and condensed complexes.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1999

[Physiopathology of tumor-induced osteolysis].

Osteolysis, the most common expression of bone tumor, can cause pain, pathological fracture, epidural spinal cord compression and hypercalcemia. Multinucleated osteoclast-like cells, the main agents in bone resorption, are numerous in benign giant cell tumor of bone and can be recruited and activated by various carcinoma cell lines in vitro in animal models. Polykarion macrophages are also able to resorb bone matrix in a favourable tumoral environment. Direct bone resorption by tumor cells has recently been described in vitro and in vivo in animals. The presence of diffusible substances such as hormones, cytokines and growth factors creates a favourable microenvironment for stimulation of osteoclast-like cells and polykarion macrophages functional ability to resorb bone matrix. These mediators act within a complex but still unelucidated network involving high cell production (tumor cells, normal and reactional stroma as well as hematopoietic cells) and many targets (tumor production (tumor cells, normal and reactional stroma as well as hematopoietic cells) and many targets (tumor cells, monocyte/macrophage lineage cells, and osteoclast-like cells). The presence in the same environment of all these stimulating factors for tumor cell growth and resorbing ability could explain the vicious circle of tumoral and osteolytic progression. A better understanding of the complex mechanism of tumor induced osteolysis is essential for improving the conventional surgical approach to this pathology.

Animals↗
PubMed1988

Quantitative determination of tumor blood flow and perfusion via deuterium nuclear magnetic resonance spectroscopy in mice.

Murine RIF-1 tumor blood flow and perfusion were quantified by deuterium NMR using D2O as a freely diffusible tracer. After direct intratumor injection of D2O saline solution, the tracer (HOD) residue from the tumor was detected by deuterium NMR and the deuterium residue washout time course was then analyzed employing multicompartment flow models (S-G. Kim and J.J.H. Ackerman, manuscript submitted for publication). The mean tumor blood flow and perfusion rate was 18.5 +/- 8.5 SD ml/(100 g.min) (n = 46) when analyzed by a two-compartment in-series flow model. A number of tumors (n = 15 out of 61 total) showed a biexponential deuterium tracer washout curve. Application of a three-compartment flow model (S-G. Kim and J.J.H. Ackerman, manuscript submitted for publication) fitted the biexponential residue decay data well and yielded a mean tumor blood flow of 15.7 +/- 9.7 SD, fast- and slow-flow components of 36.8 +/- 19.8 SD and 9.7 +/- 5.8 SD ml/(100 g.min), and a fast-flow component fraction of 21 +/- 10 SD%. Small tumors of less than 0.5 cm3 had faster blood flow, 21.1 +/- 8.4 SD ml/(100 g.min) (n = 27), than large tumors of greater than 1.0 cm3, 9.4 +/- 2.9 SD ml/(100 g.min) (n = 13). The NMR measurement of tumor blood flow and perfusion was not dependent on the number of direct intratumor injection sites and was found reproducible upon repeated measurements of individual tumors. Good agreement with previous in situ photon activation H215O flow determinations was observed.

Animals↗
PubMed1998

A membrane bioreactor for biotransformations of hydrophobic molecules

The Membrane Bioreactor for Biotransformations (MBB) is based on the aqueous/organic two-phase system, and uses a tubular silicone rubber membrane to separate the two liquid phases. This avoids the key problem associated with direct contact two-phase processes, specifically, product emulsification. The baker's yeast mediated reduction of geraniol to citronellol was used as a model biotransformation to demonstrate MBB operation. Values for the overall mass transfer coefficient were determined for geraniol, (2.0 x 10(-5) ms-1), and for citronellol, (2.1 x 10(-5) ms-1) diffusion across the silicone rubber membrane. Using these values, and the specific activity of the biocatalyst (5 nmols-1g biomass-1), a suitable membrane surface area: biomass ratio was determined as 2.4 x 10(-3) m2g biomass-1. The bioreactor was operated at this surface area: biomass ratio and achieved a product accumulation rate 90-95% that of a conventional direct contact two-phase system. The slight reduction in product accumulation rate was shown not to be due to mass transfer limitations with respect to reactant delivery or product extraction. Copyright 1998 John Wiley & Sons, Inc.

Journal Article↗
PubMedDate not supplied

The effects of random directional distributed flow in nuclear magnetic resonance imaging.

Capillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even-echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6-T nuclear magnetic resonance imager, are presented.

Algorithms↗
PubMed1996

An analytical solution to solute transport in continuous arterio-venous hemodiafiltration (CAVHD).

In conventional intermittent hemodialysis, the overall mass transfer coefficient (Kd) of a dialyser is mostly calculated at zero ultrafiltration and at relatively high dialysate flow rates. In continuous arterio-venous hemodiafiltration (CAVHD), the dialysate flow rates are low as comparable to the rates of ultrafiltration flows, making the dialysis treatment as slow as possible. Therefore the overall mass transfer coefficient (Kd) of a CAVHD hemofilter has to be calculated in the presence of ultrafiltration. A mathematical model of CAVHD is presented in order to calculate the diffusive mass transfer coefficient (Kd) for a solute when blood, filtrate and dialysate flow rates and solute concentrations are known. The ultrafiltration volume flux (Jv) is assumed to vary linearly along the axial direction of the hemofilter. The calculated mass transfer coefficient Kd shows that at high values of dialysate flow and low values of ultrafiltration, the overall mass transfer coefficient (Kd) of a CAVHD hemofilter equals mass transfer coefficient (Kd) of a dialyser in conventional intermittent hemodialysis. Also, the calculated mass transfer coefficient Kd shows no significant differences when the ultrafiltration volume flux is assumed to be constant along the length of the hemofilter if no backfiltration occurs in the hemofilter.

Biological Transport, Active↗
PubMed1983

Isolated intestinal cells of guinea pig: a suitable model for assessing direct toxic effects of ethanol on the mucosa of the upper small intestine.

The effects of acute ethanol (1-10%) on membrane transport and respiration in isolated jejunal cells of guinea pig were investigated. Three substrates with different transport mechanisms were employed: uridine (active, Na+-dependent transport), choline (saturable, Na+-independent transport), and chenodeoxycholate (simple diffusion). Uridine and choline transport were half-maximally inhibited by 3% ethanol; this inhibition was paralleled by a similar decrease in mitochondrial respiration and ATP production. Chenodeoxycholate uptake in contrast was virtually unaffected by up to 10% ethanol. It is concluded that acute ethanol inhibits active carrier-mediated absorption, but has no effect on transcellular diffusion of lipophilic compounds.

Adenosine Triphosphate↗
PubMed2003

Comparison between the predictions of diffusion-reaction models and localized Ca2+ transients in amphibian skeletal muscle fibers.

We developed a three-dimensional cylindrical diffusion-reaction model of a single amphibian myofibril in which Ca(2+) release occurred only at the Z-line. The model incorporated diffusion of Ca(2+), Mg(2+), and all relevant buffer species, as well as the kinetic binding reactions between the buffers and appropriate ions. Model data was blurred according to a Gaussian approximation of the point spread function of the microscope and directly compared with experimental data obtained using the confocal spot methodology. The flux parameters were adjusted until the simulated Z-line transient matched the experimental one. This model could not simultaneously predict key parameters of the experimental M- and Z-line transients, even when model parameters were adjusted to unreasonably extreme values. Even though the model was accurate in predicting the Z-line transient under conditions of high [EGTA], it predicted a significantly narrower Ca(2+) domain than observed experimentally. We modified the model to incorporate a broader band of release centered at the Z-line. This extended release model was superior both in simultaneously predicting critical features of the Z- and M-line transients as well as the domain profile under conditions of high [EGTA]. We conclude that a model of release occurring exclusively at the Z-line cannot explain our experimental data and suggest that Ca(2+) may be released from a broader region of the sarcoplasmic reticulum than just the T-tubule-sarcoplasmic reticulum junction.

Amphibians↗
PubMed2000

Tethered polymer-supported planar lipid bilayers for reconstitution of integral membrane proteins: silane-polyethyleneglycol-lipid as a cushion and covalent linker.

There is increasing interest in supported membranes as models of biological membranes and as a physiological matrix for studying the structure and function of membrane proteins and receptors. A common problem of protein-lipid bilayers that are directly supported on a hydrophilic substrate is nonphysiological interactions of integral membrane proteins with the solid support to the extent that they will not diffuse in the plane of the membrane. To alleviate some of these problems we have developed a new tethered polymer-supported planar lipid bilayer system, which permitted us to reconstitute integral membrane proteins in a laterally mobile form. We have supported lipid bilayers on a newly designed polyethyleneglycol cushion, which provided a soft support and, for increased stability, covalent linkage of the membranes to the supporting quartz or glass substrates. The formation and morphology of the bilayers were followed by total internal reflection and epifluorescence microscopy, and the lateral diffusion of the lipids and proteins in the bilayer was monitored by fluorescence recovery after photobleaching. Uniform bilayers with high lateral lipid diffusion coefficients (0.8-1.2 x 10(-8) cm(2)/s) were observed when the polymer concentration was kept slightly below the mushroom-to-brush transition. Cytochrome b(5) and annexin V were used as first test proteins in this system. When reconstituted in supported bilayers that were directly supported on quartz, both proteins were largely immobile with mobile fractions < 25%. However, two populations of laterally mobile proteins were observed in the polymer-supported bilayers. Approximately 25% of cytochrome b(5) diffused with a diffusion coefficient of approximately 1 x 10(-8) cm(2)/s, and 50-60% diffused with a diffusion coefficient of approximately 2 x 10(-10) cm(2)/s. Similarly, one-third of annexin V diffused with a diffusion coefficient of approximately 3 x 10(-9) cm(2)/s, and two-thirds diffused with a diffusion coefficient of approximately 4 x 10(-10) cm(2)/s. A model for the interaction of these proteins with the underlying polymer is discussed.

Annexin A5↗
PubMed1999

Polymer additive migration to foods--a direct comparison of experimental data and values calculated from migration models for high density polyethylene (HDPE).

To reduce the amount of compliance testing for food contact polymers the use of migration modelling has been proposed. This study was conducted to provide valid data for the independent evaluation of two such diffusion-based models using a range of different high density polyethylene (HDPE) polymers and plastics additives. Seventy-two experimental migration data have been obtained in triplicate and used to evaluate two Fickian-based migration models in the prediction of specific migration of four HDPE additives into olive oil. All tests were conducted using olive oil, representing the most severe case for fatty foods with test conditions of 2 h at 70 degrees C, 6 h at 70 degrees C, 10 days at 40 degrees C representing short term exposures at high temperatures and room temperature storage. Predicted migration values were calculated by inserting the measured initial concentration of additive in the polymers (Cp,0) into the equations together with known variables such as additive molecular weight, temperature and exposure time. The results indicate that both models predict migration values into olive oil close to, or in excess of, the experimental results. The Piringer migration model, using the 'exact' calculations of the Migratest Lite program, gave an overestimation for 83% of the migration values generated in this study. The highest overestimation was 3.7 times the measured value. For all measurements, the predicted migration from the Migratest Lite program was greater than 50% of the observed value. The FDA model was found more accurately to predict migration in most situations but underestimated migration more frequently. Differences in the polymer specification had little effect on specific migration of the additives investigated.

Chemical Phenomena↗
PubMed1995

Analytical expressions for the NMR apparent diffusion coefficients in an anisotropic system and a simplified method for determining fiber orientation.

NMR measurements of anisotropic diffusion were studied using a three-dimensional random-walk model. It was found that the apparent diffusion coefficient can be expressed in a canonical form as the product of a diagonal matrix, an orthonormal rotation matrix, and a vector representing the encoding magnetic field gradient. The diffusion coefficient can be interpreted as the sum of the corresponding coefficients measured along the principal diffusion axes, weighted by the squares of the directional cosines of the encoding direction with respect to the principal axes. The analysis revealed that determining the orientation of anisotropy, in a cylindrically symmetric system, requires a minimum of four diffusion measurements. A special pulse sequence which minimized gradient cross-terms and possible restricted diffusion effects was used to characterize diffusion anisotropy in cut chicken gizzards. Diffusion coefficients parallel to the muscle fibers were found to be approximately two to three times larger than those in the transverse direction. Furthermore, the method was successful in detecting the angular change when the sample was rotated by 30 degrees. Results indicate that the proposed approach to measure fiber orientation is valid and may be used to improve the time efficiency of diffusion anisotropy measurements.

Algorithms↗
PubMed2006

Direct simulation of plastocyanin and cytochrome f interactions in solution.

Most biological functions, including photosynthetic activity, are mediated by protein interactions. The proteins plastocyanin and cytochrome f are reaction partners in a photosynthetic electron transport chain. We designed a 3D computer simulation model of diffusion and interaction of spinach plastocyanin and turnip cytochrome f in solution. It is the first step in simulating the electron transfer from cytochrome f to photosystem 1 in the lumen of thylakoid. The model is multiparticle and it can describe the interaction of several hundreds of proteins. In our model the interacting proteins are represented as rigid bodies with spatial fixed charges. Translational and rotational motion of proteins is the result of the effect of stochastic Brownian force and electrostatic force. The Poisson-Boltzmann formalism is used to determine the electrostatic potential field generated around the proteins. Using this model we studied the kinetic characteristics of plastocyanin-cytochrome f complex formation for plastocyanin mutants at pH 7 and a variety of ionic strength values.

Computer Simulation↗
PubMed1989

Effects of intravoxel incoherent motions (IVIM) in steady-state free precession (SSFP) imaging: application to molecular diffusion imaging.

A theoretical analysis of the effects of diffusion and perfusion in steady-state free precession (SSFP) imaging sequences sensitized to intravoxel incoherent motions by magnetic field gradients is presented and supported by phantom studies. The capability of such sequences to image diffusion and perfusion quickly was recently demonstrated. The possible residual effects of T1 and T2 in diffusion measurements are evaluated, as are the effects of the sequence design and the acquisition parameters (repetition time, flip angle, gradient pulses). It is shown theoretically and confirmed by experiments on phantoms that diffusion coefficients can be directly measured from SSFP images when large enough diffusion gradient pulses are used.

Diffusion↗
PubMed1973

Vertical distributions of pulmonary diffusing capacity and capillary blood flow in man.

In six normal upright subjects, a 100 mol bolus-composed of equal parts of neon, carbon monoxide, and acetylene (Ne, CO, and C(2)H(2))-was inspired from either residual volume (RV) or functional residual capacity (FRC) during a slow inspiration from RV to total lung capacity (TLC). After breath holding and subsequent collection of the exhalate, diffusing capacity and pulmonary capillary blood flow per liter of lung volume (D(L)/V(A) and Q(C)/V(A)) were calculated from the rates of CO and C(2)H(2) disappearances relative to Ne. The means: D(L)/V(A) = 5.26 ml/min x mm Hg per liter (bolus at RV), 6.54 ml/min x mm Hg per liter (at FRC); Q(C)/V(A) 0.537 liters/minute per liter (bolus at RV), 0.992 liters/minute per liter (at FRC). Similar maneuvers using Xenon-133 confirmed that, during inspiration, more of the bolus goes to the upper zone if introduced at RV and more to the lower, if at FRC. A lung model has been constructed which describes how D(L)/V(A) and Q(C)/V(A) must be distributed to satisfy the experimental data. According to this model, there is a steep gradient of Q(C)/V(A), increasing from apex to base, similar to that previously determined by other techniques-and also a gradient in the same direction, although not as steep, for D(L)/V(A). This more uniform distribution of D(L)/V(A) compared with Q(C)/V(A) indicates a vertical unevenness of diffusing capacity with respect to blood flow (D(L)/Q(C)). However, the relative degree of vertical unevenness of D(L)/V(A) compared with Q(C)/V(A) can account only in part for previous observations attributed to the inhomogeneity of D(L)/V(A) and Q(C)/V(A). Thus, a more generalized unevennes of these ratios must exist throughout the lung, independent of gravitation.

Acetylene↗