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Exchange Kinetics (exchange + kinetics)
Selected AbstractsPulsed saturation of the standard two-pool model for magnetization transfer.CONCEPTS IN MAGNETIC RESONANCE, Issue 1 2004Part II: The transition to steady state Abstract The transition of the tissue signal to steady state under periodic selective saturation of macromolecular magnetization can be observed by single-shot echo-planar imaging. The general solution for a two-pool system with linear exchange kinetics contains two transient components. The rapid minor transient causes an initial delay of the transition for fast pulse repetition (PR) and weak saturation. The slow major transient combines progressive direct saturation and transferred saturation. Its PR -dependence provides similar information as the steady state, but is less sensitive to direct saturation and fitting errors. Sampling at different PR allows to quantify all system parameters. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part A 21A: 50,62, 2004 [source] Characterization of polycyclic aromatic hydrocarbon bioavailability in estuarine sediments using thin-film extractionENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2007Christopher J. Golding Abstract It is well documented that the bioavailability of hydrophobic organic chemicals (HOCs) can vary substantially among sediments. This makes risk assessments based on total sediment concentrations problematic. The present study investigates the application of thin-film solid-phase extraction to measure bioavailable concentrations of phenanthrene in estuarine sediment by comparing concentrations of phenanthrene in the amphipod Corophium colo and in thin ethylene/vinyl acetate films at different concentrations in three geochemically different sediments. For all sediment types, concentrations of phenanthrene in sediments and thin films followed linear relationships, indicating first-order exchange kinetics. Organism/thin-film concentration ratios did not vary systematically among sediment types but dropped significantly with increasing phenanthrene concentration in the sediments. While at low phenanthrene concentrations in the sediment fugacities of phenanthrene in the amphipods approached the fugacities in the thin films, they were significantly lower than those in the sediments at higher concentrations. While phenanthrene concentrations in the three sediment types were identical, biota sediment accumulation factors and concentrations in amphipods and thin films were consistently lower in sediments enriched with black carbon than in sediments with sedimentary organic matter bearing a more diagenetic organic signature. It is concluded that, for the range of concentrations tested, thin-film solid-phase extraction can be a useful tool in the characterization of differences in bioavailability of HOCs among sediment types. [source] Atorvastatin therapy improves exercise oxygen uptake kinetics in post-myocardial infarction patientsEUROPEAN JOURNAL OF CLINICAL INVESTIGATION, Issue 6 2007M. Guazzi Abstract Background Statins represent a modern mainstay of the drug treatment of coronary artery disease and acute coronary syndromes. Reduced aerobic work performance and slowed VO2 kinetics are established features of the clinical picture of post-myocardial infarction (MI) patients. We tested the hypothesis that statin therapy improves VO2 exercise performance in normocholesterolaemic post-MI patients. Materials and methods, According to a double-blinded, randomized, crossover and placebo-controlled study design, in 18 patients with uncomplicated recent (3 days) MI we investigated the effects of atorvastatin (20 mg day,1) on gas exchange kinetics by calculating VO2 effective time constant (tau) during a 50-watt constant workload exercise, brachial artery flow-mediated dilatation (FMD) as an index of endothelial function, left ventricular function (echocardiography) and C-reactive protein (CRP, as an index of inflammation). Atorvastatin or placebo was given for 3 months each. Results, Atorvastatin therapy significantly improved exercise VO2 tau and FMD, and reduced CRP levels. We did not observe changes in cardiac contractile function and relaxation properties during all study periods in either group. Conclusions, In post-MI patients exercise performance is a potential additional target of benefits related to statin therapy. Endothelial function improvement is very likely implicated in this newly described therapeutic property. [source] Hydrogen/deuterium exchange kinetics of cytochrome C: An electrospray ionization fast flow experimentISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2003Orit Geller New experiments are described in which the gas phase hydrogen/deuterium (H/D) exchange kinetics is studied for multiply-protonated cytochrome c ions with +10 to +17 charges. The experimental technique involves electrospray ionization (ESI) combined with a fast-flow method. Experimental results are presented including (1) average rate constants for H/D exchange, (2) overall decay kinetics of the reactant ion, and (3) sets of profiles for consecutive deuterium exchanges as a function of the flow rate of ND3 as the deuterating agent. The maximum number of exchanged hydrogen atoms and the exchange rate are observed to increase with increasing charge. The +13 state demonstrates special reactivity with a reactant ion decay constant of 2.5 × 10,9 cc/molecule's. Further insight into the H/D exchange mechanism is anticipated upon analysis of the data with a newly developed algorithm for extracting site-specific rate constants from profiles for H/D exchange in gas phase protonated amino acids, their clusters, and peptides. The algorithm minimizes the mutual entropy or the Kullback-Leibler information divergence between the observed concentrations and a chosen model. [source] Preparation and characterization of UV-grafted ion-exchange textiles in continuous electrodeionizationJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 12 2004Kyeong-Ho Yeon Abstract Ion-exchange textiles (IETs) suitable for use in continuous electrodeionization (CEDI) stacks were prepared using the ultraviolet (UV)-induced grafting of acrylic acid and sodium styrene sulfonate for cation-exchange textiles, or 2-hydroxyethyl methacrylate and vinylbenzyl trimethyl ammonium chloride for anion-exchange textiles, onto nonwoven polypropylene fabric using benzophenone as photoinitiator. Although the ion-exchange capacity (2.2 meq g,1) of the prepared strong acid cation-exchange textile was lower than that of IRN77 strong acid cation-exchange resin (4.2 meq g,1), the overall rate constant of IET was very high due to its low crosslinking and high specific surface area. There was no significant difference between the two different media in terms of the Co(II) removal rate. Furthermore, the current efficiency for IETs was higher than that of IRN77 cation-exchange resin during a CEDI operation, with efficiencies of 60% and 20%, respectively. The IET also showed the faster exchange kinetics. Therefore, IETs prepared in this study proved to have desirable ion-conducting characteristics within the CEDI systems. Also this study revealed that the primary removal mechanism in CEDI is the transport of ions through a medium and not the ionic capacity of a medium. Copyright © 2004 Society of Chemical Industry [source] Evaluation of mixed-conducting lanthanum-strontium-cobaltite ceramic membrane for oxygen separationAICHE JOURNAL, Issue 10 2009Lei Ge Abstract In this study, La0.4Sr0.6CoO3-, (LSC) oxide was synthesized via an EDTA-citrate complexing process and its application as a mixed-conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four-probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (Dv) and surface exchange coefficient (Kex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6-mm-thick La0.4Sr0.6CoO3-, membrane was ,4.0 × 10,7 mol cm,2 s,1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10,5 mol cm,2 s,1 through a 5-,m-thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Lack of discontinuous gas exchange in a tracheate arthropod, Leiobunum townsendi (Arachnida, Opiliones)PHYSIOLOGICAL ENTOMOLOGY, Issue 2 2002John R. B. Lighton Abstract The discontinuous gas exchange cycle, characterized by stringent spiracular control and periods of near-zero external CO2 emission separated by ,bursts' of CO2 emission, has evolved independently in several taxa of tracheate arthropods. These include the hexapoda, diplopoda, and several arachnid taxa; ticks, pseudoscorpions and solphugids. This paper presents the first data on gas exchange kinetics in a harvestman (Arachnida; Opiliones). The experimental animal, Leiobunum townsendi Weed, from an arid area of the south-western United States, displayed a metabolic rate similar to those of other arthropods at 25 °C (129 ± 22 µW). Their CO2 emission kinetics showed, when the animals were motionless, only minor variations about a mean value of 0.0217 ± 0.0037 mL/h (n = 6, mean body mass 86 mg). Expressed on an intra-recording basis, the coefficient of variation of CO2 emission (= SD/MEAN), which is an index of short-term gas emission fluctuations and thus of spiracular control, had a mean value of only 0.082. In contrast, the coefficient of variation of animals employing a discontinuous gas exchange cycle is >,1.5. Gas exchange in opilionids, unlike the case with most other tracheate arthropods, may therefore be dominated by simple diffusion without a prominent role for wide modulations of spiracular conductance. Contributory to this conservative spiracular control strategy may be the weak degree of tracheation in opilionids, combined with circulating haemocyanin, which acts as both a transport medium and a buffering reservoir for respiratory gas exchange. [source] Hydrogen-deuterium exchange in membrane proteins monitored by IR spectroscopy: A new tool to resolve protein structure and dynamicsBIOPOLYMERS, Issue 1-2 2004C. Vigano Abstract As more and more high-resolution structures of proteins become available, the new challenge is the understanding of these small conformational changes that are responsible for protein activity. Specialized difference Fourier transform infrared (FTIR) techniques allow the recording of side-chain modifications or minute secondary structure changes. Yet, large domain movements remain usually unnoticed. FTIR spectroscopy provides a unique opportunity to record 1H/2H exchange kinetics at the level of the amide proton. This approach is extremely sensitive to tertiary structure changes and yields quantitative data on domain/domain interactions. An experimental setup designed for attenuated total reflection and a specific approach for the analysis of the results is described. The study of one membrane protein, the gastric H+,K+ -ATPase, demonstrates the usefulness of 1H/2H exchange kinetics for the understanding of the molecular movement related to the catalytic activity. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004 [source] | ||||||||||||||||