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Diffusion Constant (diffusion + constant)
Selected AbstractsExciton Diffusion Measurements in Poly(3-hexylthiophene),ADVANCED MATERIALS, Issue 18 2008Paul E. Shaw The problem of making reliable measurements of exciton diffusion lengths in organic semiconductors is addressed. The exciton diffusion length is an extremely important quantity in the operation of organic solar cells. We focus on the polymer P3HT because of its widespread use in solar cells and are able to fit the exciton diffusion in a range of films with a single diffusion constant, showing that our approach is particularly robust. [source] A Kirkwood-Buff derived force field for amidesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2006Myungshim Kang Abstract A force field for the computer simulation of aqueous solutions of amides is presented. The force field is designed to reproduce the experimentally observed density and Kirkwood,Buff integrals for N -methylacetamide (NMA), allowing for an accurate description of the NMA activity. Other properties such as the translational diffusion constant and heat of mixing are also well reproduced. The force field is then extended to include N,N,-dimethylacetamide and acetamide with good success. Analysis of the simulations of low concentrations of NMA in water indicates a high degree of solvation with only 15% of the NMA molecules involved in solute,solute hydrogen bonding. There is only a weak angular dependence of the solute,solute hydrogen bonding interaction with a minimum at an angle of 65° for the NH and CO dipole vectors. The models presented here provide a basis for an accurate force field for peptides and proteins. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1477,1485, 2006 [source] Structure and gas transmission characteristics of microperforations in plastic filmsPACKAGING TECHNOLOGY AND SCIENCE, Issue 4 2008P. Allan-Wojtas Abstract Bright field transmitted light microscopy (BFTLM), differential interference contrast light microscopy (DICLM), conventional scanning electron microscopy (SEM) and low-vacuum scanning electron microscopy (LV-SEM) were used to observe microperforations in plastic film used for modified atmosphere packaging. Characteristics of the microperforations, including size, shape and obstructions are indicators of consistency and reproducibility of the process used to produce the microperforations. In addition, the microperforations appeared differently on the upper and lower surfaces of the plastic films, including a local thickening at the site of the microperforation and size of the opening. Microperforations in the range of 30 to 100,µm in diameter exhibited a linear increase of both O2 and CO2 transmission rates with hole area, for diffusion under calm conditions. Further testing indicated that microperforations larger than 55,µm in diameter can lose their diffusion constant if convection is present. Holes with a diameter less than 55,µm should therefore be used to achieve the required oxygen transmission rates (OTR). Copyright © 2007 Crown in the right of Canada. Published by John Wiley & Sons, Ltd. [source] Transport properties of hydrogen-doped (Zr803d20)1,xHx (3d = Co, Ni) metallic glassesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2004I. Kokanovi Abstract The electrical resistivities of hydrogen-doped (Zr803d20)1,xHx (3d = Ni, Co; x , 0.11) metallic glasses have been measured at temperatures between 2 K and 110 K and in magnetic fields up to 1 T for various dopant concentrations. These systems have a high room-temperature resistivity (, > 160 ,, cm) and become superconducting below 4 K. The increase of the room-temperature resistivity and its temperature coefficient with hydrogen dopant concentration is explained as due to an increase of disorder with hydrogen-doping. The temperature and magnetic field dependence of the resistivity has been analysed using theoretical models of weak-localisation and electron,electron interaction in disordered conductors. The hydrogen dopant is found to reduce the effective electron diffusion constant, D, the spin-orbit scattering rate, ,,1so, the superconducting transition temperature, Tc, and broadens the superconducting transition region. The contribution of the Maki-Thompson interaction to the magnetoresistivity is also reduced. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Using diffusion MRI for measuring the temperature of cerebrospinal fluid within the lateral ventriclesACTA PAEDIATRICA, Issue 2 2010LR Kozak Abstract Aim:, Hypothermia is often induced to reduce brain injury in newborns, following perinatal hypoxic,ischaemic events, and in adults following traumatic brain injury, stroke or cardiac arrest. We aimed to devise a method, based on diffusion-weighted MRI, to measure non-invasively the temperature of the cerebrospinal fluid in the lateral ventricles. Methods:, The well-known temperature dependence of the water diffusion constant was used for the estimation of temperature. We carried out diffusion MRI measurements on a 3T Philips Achieva Scanner involving phantoms (filled with water or artificial cerebrospinal fluid while slowly cooling from 41 to 32°C) and healthy adult volunteers. Results:, The estimated temperature of water phantoms followed that measured using a mercury thermometer, but the estimates for artificial cerebrospinal fluid were 1.04°C lower. After correcting for this systematic difference, the estimated temperature within the lateral ventricles of volunteers was 39.9°C. Using diffusion directions less sensitive to cerebrospinal fluid flow, it was 37.7°C, which was in agreement with the literature. Conclusion:, Although further improvements are needed, measuring the temperature within the lateral ventricles using diffusion MRI is a viable method that may be useful for clinical applications. We introduced the method, identified sources of error and offered remedies for each. [source] Effect Of Plasma Cholesterol On Red Blood Cell Oxygen TransportCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 12 2000Henry Buchwald SUMMARY 1. Oxygen (O2) transfer from the blood to tissues is a function of the red blood cell (RBC) O2 saturation (SO2), the plasma O2 content being negligible. Under conditions of increased tissue O2 demand, the SO2 of arterial blood does not change appreciably (97%); however, the SO2 of mixed venous blood, equal to that of the perfused tissues, can go as low as 20%. 2. Tissue O2 availability is limited by the exposure time to a RBC, which decreases under conditions of maximum stress (< 1 s). If the O2 unloading time was to increase significantly, because of a decrease in the RBC diffusion constant or an increase in the RBC membrane thickness, the RBC O2 unloading time would exceed tissue (e.g. cardiac) transit time and O2 transfer would be impaired. 3. Cholesterol constitutes the non-polar, hydrophobic lipid of the enveloping layer of the RBC membrane. As the cholesterol content of the RBC increases, the fluidity of the membrane decreases and the lipid shell stiffens. 4. Early studies demonstrated that high blood cholesterol concentrations were associated with reduced blood O2 transport; in essence, the haemoglobin dissociation curve was shifted to the left. 5. Current investigations have shown that the cholesterol RBC membrane barrier to O2 diffusion delayed O2 entry into the RBC during saturation and delayed O2 release from the RBC during desaturation. In an analysis of 93 patients divided by their cholesterol concentration into five groups, the percentage change in blood O2 diffusion was inversely proportional to the cholesterol concentration. 6. The RBC membrane cholesterol is in equilibrium with the plasma cholesterol concentration. It stands to reason that as the plasma cholesterol increases, the RBC membrane becomes impaired and O2 transport is reduced. 7. The implications of this new perspective on O2 transport include the ability to increase tissue oxygenation by lowering plasma cholesterol. [source] The Andersen thermostat in molecular dynamicsCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 1 2008Weinan E We carry out a mathematical study of the Andersen thermostat [1], which is a frequently used tool in molecular dynamics. After reformulating the continuous- and discrete-time Andersen dynamics, we prove that in both cases the Andersen dynamics is uniformly ergodic. A detailed numerical analysis is presented, establishing the rate of convergence of most commonly used numerical algorithms for the Andersen thermostat. Transport properties such as the diffusion constant are also investigated. It is proved for the Lorentz gas model where there is intrinsic diffusion that the diffusion coefficient calculated using the Andersen thermostat converges to the true diffusion coefficient in the limit of vanishing collision frequency in the Andersen thermostat. © 2007 Wiley Periodicals, Inc. [source] Singlet states open the way to longer time-scales in the measurement of diffusion by NMR spectroscopyCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2008Simone Cavadini Abstract Nuclear magnetic resonance is a powerful nonintrusive technique for measuring diffusion coefficients through the use of pulsed field gradients. The main limitation to the application range of this method is imposed by the relaxation time constants of the magnetization. The recently introduced singlet-state spectroscopy affords obtaining relaxation time constants for pairs of coupled spins which can be longer by more than an order of magnitude than the spin-lattice relaxation time constants. We review in this paper the advantages that are offered by these long relaxation time constants for diffusion measurements. Using experiments that combine singlet-state and diffusion spectroscopy, slower diffusion constants can be determined. The coupling of the two methods constitutes an alternative to the use of special probes equipped with strong gradients for the study of large molecules that diffuse slowly in solution. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 68,78, 2008. [source] The normal and cancerous living cellINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2006Janos Ladik Abstract We do not have a definition of the living and cancerous states; we can give only their main characteristics at the different levels of organization: cell, organ, and organism. A simple model is proposed for a normal eukaryotic cell based on Prigogine's equation of chemical kinetics with diffusion. In this model, possibly only a few hundred key biochemical reactions should be selected together with their rate and diffusion constants. To solve these coupled nonlinear partial differential equation systems, it is proposed that the model cell be subdivided into compartments and that the problem be worked out always for one compartment (finite element method). This is possible, since the most important biochemical reactions and reaction cycles occur in different parts of the cell. The solutions (concentrations) obtained in one compartment can be used as input to the other compartments (together with the components entering from the environment). As an example, the problem of 10 reactions and 3 compartments has been solved by discretizing the space coordinates and choosing time steps. The solutions obtained by solving the 10 differential equations directly and by the compartmentalization agree very well. The main obstacles to further progress lie in the right choice of reactions and compartments, as well as in the correct estimation of the rate and diffusion constants, which were measured in only a few cases. If such a model cell can be obtained, the solutions should be investigated to determine (i) for their stability (homeostasis); (ii) whether changing the input concentrations to a larger degree one would obtain a new stationary state showing the characteristics of a precancerous state; and (iii) a method of extracting those input concentrations, or functions of them, which are the most important regulatory parameters. If successful, this would provide a scientific definition of the living state in the normal and cancerous states, respectively, at least at the cell level. Finally, outline is provided showing how the model might be extended to multicellular cases, as well as the main difficulties of such a process. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Broadband proton-decoupled proton spectra,MAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2007Andrew J. Pell Abstract We present a new method for recording broadband proton-decoupled proton spectra with absorption mode lineshapes and substantially correct integrals; in both these respects, the new method has significant advantages over conventional J -spectroscopy. In our approach, the decoupled spectrum is simply obtained from the 45° projection of the diagonal-peak multiplets of an anti z -COSY spectrum. This method is straightforward to apply, and does not require any unusual data processing. However, there is a significant reduction in sensitivity when compared to a conventional proton spectrum. The method is demonstrated for typical medium-sized molecules, and it is also shown how such a decoupled spectrum can be used to advantage in measurements of diffusion constants (DOSY), the measurement of relaxation parameters, and the analysis of complex mixtures. Copyright © 2007 John Wiley & Sons, Ltd. [source] Qualitative model of concrete acidification due to cathodic protection,MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 2 2008W. H. A. Peelen In this paper a mathematical description and numerical implementation for ion transport in concrete due to current passage is developed, in which the heterogeneous equilibrium between Ca2+, OH, and the solid Ca(OH)2 is incorporated. The description is based on the Nernst,Planck equation for ion transport, and reaction terms for the dissolution/precipitation of Ca(OH)2. This description was implemented in the finite element package Comsol Multiphysics. In this way Ca(OH)2 depletion in a zone at a CP anode adjacent to a bulk of concrete with Ca(OH)2 could be modelled in one calculation. Drawback of this model is that the kinetic parameters in the reaction terms are not known, and must be chosen high to ensure the dissolution of Ca(OH)2 to be in equilibrium. This proved numerically challenging and sometimes caused long calculation times. The growth rate of the zone without solid depends on the current density applied, concrete cover, the pore liquid composition and the diffusion constants of Ca2+ and OH,. This rate must be evaluated numerically. This qualitative model of anode acidification shows no participation of Na+; therefore transport properties of this ion do not affect the acidification rate of concrete. The same would hold for any other ion included in the model, which is not involved in electrochemical or chemical reactions. [source] Fluorescence recovery after photobleaching and photoconversion in multiple arbitrary regions of interest using a programmable array microscope,MICROSCOPY RESEARCH AND TECHNIQUE, Issue 6 2009Guy M. Hagen Abstract Photomanipulation (photobleaching, photoactivation, or photoconversion) is an essential tool in fluorescence microscopy. Fluorescence recovery after photobleaching (FRAP) is commonly used for the determination of lateral diffusion constants of membrane proteins, and can be conveniently implemented in confocal laser scanning microscopy (CLSM). Such determinations provide important information on molecular dynamics in live cells. However, the CLSM platform is inherently limited for FRAP because of its inflexible raster (spot) scanning format. We have implemented FRAP and photoactivation protocols using structured illumination and detection in a programmable array microscope (PAM). The patterns are arbitrary in number and shape, dynamic and adjustable to and by the sample characteristics. We have used multispot PAM,FRAP to measure the lateral diffusion of the erbB3 (HER3) receptor tyrosine kinase labeled by fusion with mCitrine on untreated cells and after treatment with reagents that perturb the cytoskeleton or plasma membrane or activate coexpressed erbB1 (HER1, the EGF receptor EGFR). We also show the versatility of the PAM for photoactivation in arbitrary regions of interest, in cells expressing erbB3 fused with the photoconvertible fluorescent protein dronpa. dronpa. Microsc. Res. Tech., 2009. © 2009 Wiley-Liss, Inc. [source] Application of NMR, molecular simulation, and hydrodynamics to conformational analysis of trisaccharides,BIOPOLYMERS, Issue 4 2003Ann M. Dixon Abstract The preferred conformations and conformational flexibilities of the trisaccharides ,-D-Glcp -(1,2)-,-D-Glcp -(1,3)-,-D-Glcp -OMe (I) and ,-D-Glcp -(1,3)[,-D-Glcp -(1,4)]-,-D-Glcp -OMe (II) in aqueous solution were determined using nuclear magnetic resonance (NMR) spectroscopy, molecular dynamics (MD) and Langevin dynamics (LD) simulations, and hydrodynamics calculations. Both trisaccharides have a vicinal substitution pattern in which long range (nonsequential) interactions may play an important role. LD simulation at 600 K indicated that the all- syn conformation predominated, though other conformations were apparent. NOE data and MD and LD simulations at 298 K all indicated that trisaccharide I is a single all- syn conformer in solution. Given that previous studies showed evidence of anti -conformers in ,-D-Glcp -(1,2)-,-D-Glcp -(1,3)-,-D-Glcp -OMe, this result provides an example of how changing the anomeric configuration of one residue from , to , can make an oligosaccharide more rigid. Discrepancies in inter-ring distances obtained by experiment and by simulation of the all- syn conformer suggest the presence of an anti -, conformation at the ,-(1,4)-linkage for II. A combined analysis of measured and calculated translational diffusion constants and 13C T1 relaxation times yield order parameters of 0.9 for each trisaccharide. This implies that any interconversion among conformations is significantly slower than tumbling. Anisotropies of approximately 1.6 and 1.3 calculated for I and II, respectively, are consistent with the observed relatively flat T1 profiles because the tumbling is not in the motional narrowing regime. Published 2003 Wiley Periodicals, Inc. Biopolymers 69: 448,460, 2003 [source] Quantum Diffusion of Hydrogen and Isotopes in MetalsCHEMPHYSCHEM, Issue 9 2005Matthew Dyer Quantum diffusion constants of H and its isotopes in Pd and Nb are computed using Kubo theory applied on potential-energy surfaces obtained from density functional theory. The figure shows an excited state of H in Pd. This state is among several states which are below the classical barrier but are also delocalised between the octahedral and tetrahedral sites that contribute to quantum diffusion at low temperatures. [source] | ||||||||||||||||