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PubMed1989

Effect of Ca2+ diffusion on the time course of neurotransmitter release.

The three-dimensional (3D) diffusion model of Fogelson, A. L., and R. S. Zucker (1985. Biophys. J. 48: 1003-1017) has been employed as the basis of a refined version of the "Ca theory" for neurotransmitter release. As such, it has been studied here as to its ability to predict the time course of release under various conditions. In particular, conditions were chosen in which the temporal variations in intracellular Ca2+ concentration, the sole factor controlling the release according to the Ca theory, were modified and tested experimentally. The predictions of this model were compared with the experimental results. It is shown that the 3D diffusion model, similarly to earlier simpler versions of the Ca theory, predicts that the time course of release is highly sensitive to both the level of depolarization and the level of the resting concentration of intracellular Ca2+ Moreover, the 3D diffusion model predicts that the time course of release is insensitive to changes in temperature. In contrast, the experimental results show that the time course of release is invariant to the level of depolarization and to the resting level in intracellular Ca2+, but highly sensitive to variations in temperature.

Animals↗
PubMed1995

Gas exchange across the middle ear mucosa in monkeys. Estimation of exchange rate.

OBJECTIVE: To estimate the rate of exchange of selected gases across the middle ear (ME) mucosa and define the exchange limitations. DESIGN: At separate sessions, the ME was inflated via the eustachian tube with a bolus of pure nitrogen, carbon dioxide, oxygen, or nitrous oxide, and ME pressures were recorded by tympanometry at selected intervals for up to 4 hours. The slope of the function relating pressure change to pressure was calculated by least squares regression and used as an estimate of the rate constant for exchange of that gas (experiment 1). Because of the slow rate of nitrogen exchange, a second experiment was performed in which the tensor veli palatini muscle was unilaterally paralyzed. The ME was inflated with nitrogen, and the slope of the rate-pressure function for measurements at 24-hour intervals was used to estimate the rate constant. SUBJECTS: Ten juvenile cynomolgus monkeys, six for experiment 1 and four for experiment 2. RESULTS: The relative, average rate constants for carbon dioxide, nitrous oxide, oxygen, and nitrogen were 1, 10.7, 18.6, and greater than 700, respectively. Comparisons of these rates with those predicted by theory show that oxygen and carbon dioxide exchange is diffusion limited, and nitrous oxide and nitrogen is perfusion limited. CONCLUSIONS: The perfusion limitation for nitrogen suggests that its exchange rate is notably increased by inflammation from increased mucosal blood flow. Targeting inflammation for therapy of persistent ME effusions may decrease the rate of nitrogen exchange and reestablish normal ME pressure regulation.

Acoustic Impedance Tests↗
PubMed1977

Systems simulation: theory of monosynaptic transfer between neuron populations.

This paper presents a theory for the input-output transformation involved in diffuse monosynaptic activation in a neural tissue or organ system of one population of neurons by another. Main findings are: (1) highly diffuse monosynaptic linkages act very much like filters, selectively sensitive to synchronized clusters of action potentials among the fibers of the input population; (2) partially diffuse monosynaptic linkages are capable of effecting either an amplification or diminution of the number of pulses involved in a single synchronized cluster, depending on parameters of the system; and (3) partially diffuse and spatially organized monosynaptic linkages are capable of effecting a spatial inversion of fine-grained spatial patterns. Theoretical predictions are clarified by mathematical analysis and computer simulation.

Action Potentials↗
PubMed1976

Dissolution rate studies of cholesterol monohydrate in bile acid-lecithin solutions using the rotatingdisk method.

A physical model approach was used to investigate cholesterol gallstone dissolution kinetics in simulated bile. Critical experimental and theoretical investigations simulating in vivo conditions showed that, in the bile acid-lecithin solutions, there is a significant interfacial barrier for both cholesterol gallstone and cholesterol monohydrate pellet dissolution. In the present study, the rotating-disk dissolution method and the accompanying Levich theory were applied to assess the contributions of the diffusion convection mass transfer resistance and of the interfacial barrier to the overall kinetics. Cholesterol dissolution rates in bile acid solutions were about 2-20 times slower than diffusion-controlled rates depending upon the degree of agitation. As found in previous studies, these rates in the presence of sufficient concentrations of dissolution accelerators approached the theoretical diffusion-convection-controlled rates. To account for the much slower dissolution rates in bile acid-lecithin solutions, two possible kinetic interpretations were investigated. The first is based upon slow crystal-micellar solution interfacial kinetics, and the second is based upon a slow rate of cholesterol solubilization in the aqueous diffusion layer. For the latter, an analytical mathematical solution was obtained.

Benzoates↗
PubMed1990

Photoacoustic studies of non-radiative relaxation of excited states in melanin.

Photoacoustic measurements made at various chopping frequencies on dense acqueous melanin suspensions have revealed a significant dependence upon pH and redox state. It is shown that such behaviour, differing from the simple predictions of the Rosencwaig-Gersho theory, can be explained in terms of finite carrier diffusion and recombination times. The implications of these findings with respect to the amorphous semiconductor model proposed to describe the dynamic role of epidermal melanin are discussed. From the experimental data, values of physical parameters were calculated which allow a qualitative correlation between chemical states and electronic behaviour and, consequently, some aspects of the molecular biology of the melanosome, founded on a rigorous base.

Acoustic Stimulation↗
PubMed1999

'NC100150', a preparation of iron oxide nanoparticles ideal for positive-contrast MR angiography.

A laboratory-scale synthesis of NC100150 (iron oxide particles with an oxidized starch coating) was characterized by magnetization measurements (vibrating sample magnetometry, VSM), relaxometry (1/T1 NMRD profiles and 1/T2 at 10 and 20 MHz), and dynamic light scattering (photon correlation spectroscopy, PCS). The results were related to give a self-consistent physical description of the particles: a water-impenetrable part making up 12% of the total particle volume, 82% of this volume consisting of an iron oxide core and the remaining 18% consisting of an oxidized starch rind; and, a water-penetrable part making up 88% of the total particle volume, consisting of oxidized starch polymers and entrained water molecules. Relating the magnetization to the relaxometry results required that the oxidized starch coating slows the diffusivity of solvent water molecules in the vicinity of the iron oxide cores. The effect of the organic coating on water diffusivity, not previously considered in the application of relaxation theory to iron oxide nanoparticles, is supported by the much greater (factor of about 2) diameter obtained from the dynamic light scattering measurements in comparison to that obtained from the magnetization measurements. The present work shows that three physical techniques--VSM, relaxometry, and PCS--are needed for properly assessing iron oxide nanoparticles for use as contrast agents for magnetic resonance angiography (MRA). It is also shown that NC100150 has a narrow range of diameters and the smallest value of r2/r1 reported to date, an asset for MRA.

Anisotropy↗
PubMed1985

Electron spin resonance and electron-spin-echo study of oriented multilayers of L alpha-dipalmitoylphosphatidylcholine water systems.

A detailed electron spin resonance (ESR) study of spin-labeled-oriented multilayers of L alpha-dipalmitoylphosphatidylcholine (DPPC) water systems for low water content (2-10% by weight) is reported with the purpose of characterizing the dynamical and structural properties of model membrane systems. Emphasis is placed on the value of combining such experiments with detailed simulations based on current slow-motional theories. Information is obtained regarding ordering and anisotropic rotational diffusion rates via ESR lineshape analysis over the entire motional range, from the fast motional region through the moderately slow and slow to the rigid limit. This includes the low-temperature gel phase, the liquid crystalline L alpha (1) phase and what appears to be a third high-temperature phase above the L alpha phase. Cholestane (CSL) and spin-labeled DPPC (5-PC, 8-PC, and 16-PC) have been used to probe different depths of the bilayer. While CSL and 5-PC both reflect the high ordering of the bilayer close to the lipid-water interface, CSL appears to be located close enough to the water for the nitroxide to be involved in hydrogen bonding with water molecules. 16-PC reflects the relatively low ordering near the tail of the hydrocarbon chain in the bilayer. Quantitative estimates of ordering and motion are obtained for these cases. The results from CSL indicate that close to the lipid-water interface the DPPC molecule is oriented approximately perpendicular to the bilayer in these low water-content systems. However, all three labeled lipid probes indicate that the hydrocarbon chain of DPPC may be bent away from the bilayer normal by as much as 30 degrees and this evidence is stronger at low temperatures. When cholesterol is added to the DPPC-water system at a concentration greater than or equal to 2.5 mol %, the ordering is greatly increased although the rotational diffusion rate remains almost unaffected in the gel phase. Electron spin echoes (ESE) are observed for the first time from oriented lipid-water multilayers. Results obtained from cw ESR lineshape analysis are correlated with data from ESE experiments, which give a more direct measurement of relaxation times. These results indicate that for detection of very slow motions (close to the rigid limit) ESE experiments are more sensitive to dynamics than continuous wave ESR for which inhomogeneous broadening becomes a major problem.

Electron Spin Resonance Spectroscopy↗
PubMed1991

General continuum theory for multiion channel. II. Application to acetylcholine channel.

The general theory (Levitt, D. G. 1990. Biophys. J. 59:271-277) is applied to a model channel that resembles the acetylcholine receptor channel (ACH). The model incorporates the known features of the ACH geometry and fixed charge locations. The channel has a wide mouth facing the outer solution, tapering to a narrow region facing the interior of the cell. Rings of fixed negative charge are placed at the two surfaces where the bilayer begins, corresponding to the known charges at the ends of the M2 segment. It is assumed that the forces acting on the ion are electrostatic: ion-channel wall, ion-ion, Born image and applied voltage. Analytical expressions for these forces are derived that take account of the low dielectric lipid region. In addition, there is a local hard sphere repulsive force that prevents ions from piling up on each other in regions of the channel with a high fixed charge density. A classical continuum theory is used to obtain an expression for the diffusion coefficient in the channel. The model can mimic the major qualitative and, in many cases, quantitative experimental features of the ACH channel: current-voltage relation, conductance versus concentration and interaction between monovalent and divalent ions. The model calculations were also compared with the site directed mutagenesis experiments of Imoto, K., C. Busch, B. Sakmann, M. Mishina, T. Konno, J. Nakai, H. Bujo, Y. Mori, K. Fukuda, and S. Numa. (1988. Nature (Lond.). 335:645-648) in which the charge at the ends of the channel was systematically varied.

Animals↗
PubMed1992

Theory of fluorescence recovery after photobleaching measurements on cylindrical surfaces.

The theory of fluorescence recovery after photobleaching measurements of isotropic diffusion on a cylindrical surface is developed for Gaussian beam illumination centered perpendicular to an infinitely-long cylinder. A general analytical solution is obtained which is a function of the ratio of the cylindrical radius (r) to the beam exp [-2] radius omega. Numerical analysis of this solution demonstrates that significant deviations from one dimensional recovery are observed for omega less than 3r and from two-dimensional recovery for omega greater than or equal to 0.5r. Numerical data and an algorithm for analysis of recovery data where 0.5r less than or equal to omega less than or equal to 3r is presented.

Biophysical Phenomena↗
PubMed2003

Characterization of pigment particle absorption efficiencies using frequency domain photon migration.

Time-dependent measurements of multiply scattered light were made using frequency domain photon migration (FDPM) techniques in polystyrene latex as a function of ppm pigment concentration (by weight) in order to determine the wavelength-dependent absorption efficiencies for three different pigment particles. The results demonstrate that the absorption spectra of pigment particles within their dispersing vehicles concur with the complementary color chart. FDPM offers a first-principles method for assessing optical characteristics of pigments within their dispersing vehicles and without the need to resort to conventional measurement of diffuse reflectance from coatings and data analysis using phenomenological theory.

Journal Article↗
PubMed2006

Ab initio molecular dynamics simulation of the structure and proton transport dynamics of methanol-water solutions.

Ab initio molecular dynamics simulations are employed to study the structural and proton transport properties of methanol-water mixtures. Structural characteristics analyzed at two different methanol mole fractions (X(M) = 0.25 and X(M) = 0.5) reveal enhanced structuring of water as the methanol mole fraction increases in agreement with recent neutron diffraction experiments. The simulations reveal the existence of separate hydrogen-bonded water and methanol networks, also in agreement with the neutron diffraction data. The addition of a single proton to the X(M) = 0.5 mixture leads to an anomalous structural or Grotthuss-type diffusion mechanism of the charge defect in which water-to-water, methanol-to-water, and water-to-methanol proton transfer reactions play the dominant role with methanol-to-methanol transfers being much less significant. Unlike in bulk water, where coordination number fluctuations drive the proton transport process, suppression of the coordination number of waters in the first solvation shell of the defect diminish the importance of coordination number fluctuations as a driving force in the structural diffusion process. The charge defect is found to reside preferentially at the interface between water and methanol networks. The length of the ab initio molecular dynamics run (approximately 120 ps), allowed the diffusion constant of the charge defect to be computed, yielding a value of D = 4.2 x 10(-5) cm2/s when deuterium masses are assigned to all protons in the system. The relation of this value to excess proton diffusion in bulk water is discussed. Finally, a kinetic theory is introduced to identify the relevant time scales in the proton transfer/transport process.

Journal Article↗
PubMed1992

The influence of liquid crystalline phases on drug percutaneous absorption. II. Permeation studies through excised human skin.

The influence of liquid crystalline (LC) phases on the percutaneous absorption of a model compound (ploxicromil; PXC) was studied with the use of the phase diagram for the surfactant, oil, and water comprising the vehicles. Two separate sets of vehicles, representing two different tie lines lying in the L1 + LC phase region, were prepared in which the concentration of LC was varied over the range 0 to 100% along each tie line. In vitro permeation studies of PXC from these systems were conducted using excised human skin and the flux values determined as a function of the percentage LC present in the vehicles. In virtually all cases, the flux reached a peak at 5-10% LC and then decreased significantly as the fraction of LC present increased further. The pattern of behavior observed is discussed in terms of current theories describing membrane-controlled and vehicle-controlled diffusion, none of which adequately model the results obtained.

Administration, Cutaneous↗
PubMed1985

The concentration-dependence of macromolecular parameters.

Theories concerning the concentration-dependence of sedimentation and diffusion coefficients for macro-molecules in dilute solution are compared and discussed, together with their experimental basis. An attempt has been made to clarify an important uncertainty still present in the literature as to whether sedimentation coefficients should be corrected for solvent or solution density. It is pointed out that the two processes yield the same extrapolation limit but different concentration-dependencies, which have, however, been related. A general expression is derived thermodynamically for the concentration-dependence of diffusion that includes the coefficient of the concentration term involved in sedimentation (on the basis of sedimentation coefficients corrected from solution density). For rigid spherical particles the expression is shown to be exactly equivalent to one given by Batchelor [(1976) J. Fluid Mech. 74, 1-29], which was derived on the basis of sedimentation coefficients corrected from solvent density. Finally, we discuss the concentration-dependence of apparent weight-average relative molecular masses ('molecular weights') (from, e.g., sedimentation equilibrium) and note an important omission in some earlier representations.

Centrifugation↗
PubMed2006

End-to-end distance distributions and intrachain diffusion constants in unfolded polypeptide chains indicate intramolecular hydrogen bond formation.

Characterization of the unfolded state is essential for the understanding of the protein folding reaction. We performed time-resolved FRET measurements to gain information on the dimensions and the internal dynamics of unfolded polypeptide chains. Using an approach based on global analysis of data obtained from two different donor-acceptor pairs allowed for the determination of distance distribution functions and diffusion constants between the chromophores. Results on a polypeptide chain consisting of 16 Gly-Ser repeats between the FRET chromophores reveal an increase in the average end-to-end distance from 18.9 to 39.2 Angstrom between 0 and 8 M GdmCl. The increase in chain dimensions is accompanied by an increase in the end-to-end diffusion constant from (3.6 +/- 1.0) x 10(-7) cm(2) s(-1) in water to (14.8 +/- 2.5) x 10(-7) cm(2) s(-1) in 8 M GdmCl. This finding suggests that intrachain interactions in water exist even in very flexible chains lacking hydrophobic groups, which indicates intramolecular hydrogen bond formation. The interactions are broken upon denaturant binding, which leads to increased chain flexibility and longer average end-to-end distances. This finding implies that rapid collapse of polypeptide chains during refolding of denaturant-unfolded proteins is an intrinsic property of polypeptide chains and can, at least in part, be ascribed to nonspecific intramolecular hydrogen bonding. Despite decreased intrachain diffusion constants, the conformational search is accelerated in the collapsed state because of shorter diffusion distances. The measured distance distribution functions and diffusion constants in combination with Szabo-Schulten-Schulten theory were able to reproduce experimentally determined rate constants for end-to-end loop formation.

Fluorescence Resonance Energy Transfer↗
PubMed2005

Does a fast mixer really exist?

In this paper, we are concerned about the limit behavior of the decay rate of variance of a passive and diffusive scalar in a flow field as the diffusivity of the scalar goes to zero. Motivated by the concept of the fast dynamo in the dynamo theory, we term a flow as fast mixer if the decay rate remains away from zero as the diffusivity goes to zero. We first repeat numerical simulations with flow maps and velocity fields used in the existing literature, including the lattice map, the 1D baker's map, and the sinusoidal shear flow. Our simulations shows that, in all cases, the decay rate tends to zero as the diffusivity goes to zero. For the closed flows in a bounded domain, we then theoretically proved this result under certain plausible conditions on the flows. For the open flows in the whole space, we show that the effective diffusivity matrix tends to zero in the limit without the conditions for the closed flow. In conclusion, although a fast mixer might exist, it could be very difficult to find one.

Journal Article↗
PubMed2005

Observation of surface Andreev bound States of superfluid 3He by transverse acoustic impedance measurements.

Complex transverse acoustic impedance of the superfluid (3)He-B was measured at the frequencies of 10 to 80 MHz at 17.0 bar by a cw bridge method. The observed temperature dependence was well explained by the quasiclassical theory with random S-matrix model for a diffusive surface. The temperature dependence was influenced by pair breaking and by quasiparticle density of states at the surface, which was drastically modified from the bulk one by the presence of surface Andreev bound states.

Journal Article↗
PubMed2005

Deterministic weak localization in periodic structures.

In some perfect periodic structures classical motion exhibits deterministic diffusion. For such systems we present the weak localization theory. As a manifestation for the velocity autocorrelation function a universal power law decay is predicted to appear at four Ehrenfest times. This deterministic weak localization is robust against weak quenched disorders, which may be confirmed by coherent backscattering measurements of periodic photonic crystals.

Journal Article↗
PubMed2006

Phase diagram and electrical conductivity of high energy-density water from density functional theory.

The electrical conductivity and structure of water between 2000-70,000 K and 0.1-3.7 g/cm3 is studied by finite temperature density functional theory (DFT). Proton conduction is investigated quantitatively by analyzing diffusion, the pair-correlation function, and Wannier center locations, while the electronic conduction is calculated in the Kubo-Greenwood formalism. The conductivity formulation is valid across three phase transitions (molecular liquid, ionic liquid, superionic, electronic liquid). Above 100 GPa the superionic phase directly borders an electronically conducting fluid, not an insulating ionic fluid, as previously concluded. For simulations of high energy-density systems to be quantitative, we conclude that finite temperature DFT should be employed.

Computer Simulation↗