Chemical Exchange Rates Research Articles

Nanoconfinement's Dramatic Impact on Proton Exchange between Glucose and Water.

Glucose nanoconfined by solubilization in water-containing AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles has been investigated using 1H NMR. NMR spectra reveal well-defined signals for the glucose hydroxyl groups that suggest slow chemical exchange between them and the water hydroxyl groups. Using the EXSY (ZZ-exchange) method, the chemical exchange rate from water to glucose hydroxyl groups was measured for glucose in reverse micelles as a function of size (water pool diameter of ∼1-5 nm) at 25 °C. The chemical exchange rates observed in the nanoconfined interior are dramatically slower (5-20 times) than those observed for glucose in bulk aqueous solution at the same concentration as the micelle interior. Exchange rate constants are calculated via a mechanism that accounts for these observations, and implications of these results are presented and discussed.

Towards p-Type Conductivity in SnO2 Nanocrystals through Li Doping and Its Applications in Porous Diodes for Room Temperature Chlorine Gas Detection

This work examined electrical transport properties and Li doping in SnO2 synthesized by the sol–gel method. Solid-state 7Li-NMR line shapes revealed that Li ions occupy two distinct sites with differing dynamic mobilities. The chemical exchange rate between the two sites is, however, too slow for detection on the NMR timescale. Compressed nanoparticulate films of this doped semiconductor exhibited a positive Seebeck coefficient implying a p-type conductivity. A variable-temperature direct current conductivity, over a 25–350 °C temperature range, followed an Efros–Shklovskii variable range hopping (ES-VRH) conduction mechanism (ln(ρ) versus T −1/2) at temperatures below 100 °C with a crossover to 2D Mott variable range hopping (M-VRH) (ln(ρ) versus T −1/3) conduction at temperatures above 250 °C. In a transition region between these two limiting behaviors, direct current resistivity exhibited an anomalous temperature-independent plateau. We suggest that its origin may lie in a carrier inversion phenomenon wherein the majority carriers switch from holes to electrons due to Li ion expulsion from the crystalline core and creation of oxygen vacancies generated by loss of oxygen at elevated temperatures. Using a compression technique, porous diodes consisting of n-type (antimony doped SnO2) and the p-type (lithium-doped) nano-particulate films were prepared. Typical current-voltage curves of such devices resembled the behavior of a typical diode but for one key difference; owing to the porosity of the film, areas near the p-n interface and the p- and n-regions are accessible to gaseous analytes. This access is not possible in conventional diodes fabricated through contemporary planar technology. The gas sensing properties of the diodes were investigated by using Cl2, Br2, HCl, NO, NO2, CHCl3, NH3 and H2. Of all the gases tested, the sensor had very good sensitivity to Cl2 and had a response of 80 at 400 ppb of Cl2. The response time of the sensor was ~30 s at room temperature but the recovery time was temperature dependent. At room temperature the recovery time was 80s after 400 ppb chlorine exposure. The sensors were also found to recover quickly (~ 30 s) when the junctions were biased. Reverse biasing the diode provides a controllable band-bending at a p-n junction that could influence the electron-transfer rates from the adsorbates which create intra-band localized states. As a result reverse biasing can provide a means of desorbing the analyzed species. This approach eliminates the requirement to remove adsorbates by elevating the sensor temperature and hence improves on the energy efficiency of these types of sensors.

Multislice CEST MRI improves the spatial assessment of tumor pH.

Multislice maps of extracellular pH (pHe) are needed to interrogate the heterogeneities of tumors and normal organs. To address this need, we have developed a multislice chemical exchange saturation transfer (CEST) MRI acquisition method with a CEST spectrum-fitting method that measures in vivo pHe over a range of 6.3 to 7.4. The phase offset multiplanar (POMP) method was adapted for CEST fast imaging with steady-state free precession (FISP) MRI to acquire multiple image slices with a single CEST saturation pulse. The Bloch-McConnell equations were modified to include pH based on a calibration of pH and chemical exchange rate for the contrast agent iopamidol. These equations were used to estimate the pixel-wise pHe values throughout the multislice acidoCEST MR images of the tumor, kidney, bladder, and other tissues of a MDA-MB-231 tumor model. Multislice acidoCEST MRI successfully mapped a gradient of pHe from 6.73 to 6.81 units from the tumor core to rim, and also mapped a gradient of pHe 6.56 to 6.97 across the mouse kidney. The bladder was found to be pHe 6.3. AcidoCEST MRI with POMP acquisition and Bloch-McConnel analysis can map pHe in multiple imaging slices through the tumor, kidney, and bladder. This multislice evaluation facilitates assessments of spatial heterogeneity of tissue pHe. Magn Reson Med 78:97-106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Kinetics and reactive stripping modelling of hydrogen isotopic exchange of deuterated waters

This work presents results of experimental kinetics and modelling of the isotopic exchange between hydrogen and water in a reactive stripping column for water dedeuteriation. The missing physical properties of deuterium and tritium isotopologues in hydrogen gas and water forms were predicted and validated using existing literature data. The kinetic model relevant to a styrene-divinyl-benzene co-polymer–supported platinum catalyst was used for modelling, by Aspen plus modular package, impact of design parameters including temperature, total pressure, gas to liquid flowrate ratio, pressure drop and flow mixing, on the separation of deuterium and further the separation of tritium. The modelling by the rate-based non-equilibrium, including design correlations of model of mass and heat transfers, chemical kinetic constants, mass transfer coefficients and overall exchange rate constants, allowed access to separation trends in a good agreement with published data. The synergy between the rates of chemical isotopic exchange and gas/liquid mass transfer, and by inference the performance of reactive stripping, was particularly sensitive to high temperatures, low hydrogen flow rates, pressure drops and internals properties. Extension to tritium confirmed a slightly slower mass transport compared with deuterium leading to potentially under-estimated design features for detritiation processing when deuterium is used instead.

QUESPOWR MRI: QUantification of Exchange as a function of Saturation Power On the Water Resonance

QUantification of Exchange as a function of Saturation Power On the Water Resonance (QUESPOWR) MRI is a new method that can estimate chemical exchange rates. This method acquires a series of OPARACHEE MRI acquisitions with a range of RF powers for the WALTZ16∗ pulse train, which are applied on the water resonance. A QUESPOWR plot can be generated from the power dependence of the % water signal, which is similar to a QUESP plot that is generated from CEST MRI acquisition methods with RF saturation applied off-resonance from water. A QUESPOWR plot can be quantitatively analyzed using linear fitting methods to provide estimates of average chemical exchange rates. Analyses of the shapes of QUESPOWR plots can also be used to estimate relative differences in average chemical exchange rates and concentrations of biomolecules. The performance of QUESPOWR MRI was assessed via simulations, an in vitro study with iopamidol, and an in vivo study with a mouse model of mammary carcinoma. The results showed that QUESPOWR MRI is especially sensitive to chemical exchange between water and biomolecules that have intermediate to fast chemical exchange rates and chemical shifts that are close to water, which are notoriously difficult to assess with other CEST MRI methods. In addition, in vivo QUESPOWR MRI detected acidic tumor tissues relative to normal tissues that are pH-neutral, and therefore may be a new paradigm for tumor detection with MRI.

A theoretical analysis of chemical exchange saturation transfer echo planar imaging (CEST-EPI) steady state solution and the CEST sensitivity efficiency-based optimization approach.

Chemical exchange saturation transfer (CEST) MRI is sensitive to dilute labile protons and microenvironmental properties, augmenting routine relaxation-based MRI. Recent developments of quantitative CEST (qCEST) analysis such as omega plots and RF-power based ratiometric calculation have extended our ability to elucidate the underlying CEST system beyond the simplistic apparent CEST measurement. CEST MRI strongly varies with experimental factors, including the RF irradiation level and duration as well as repetition time and flip angle. In addition, the CEST MRI effect is typically small, and experimental optimization strategies have to be carefully evaluated in order to enhance the CEST imaging sensitivity. Although routine CEST MRI has been optimized largely based on maximizing the magnitude of the CEST effect, the CEST signal-to-noise (SNR) efficiency provides a more suitable optimization index, particularly when the scan time is constrained. Herein, we derive an analytical solution of the CEST effect that takes into account key experimental parameters including repetition time, imaging flip angle and RF irradiation level, and solve its SNR efficiency. The solution expedites CEST imaging sensitivity calculation, substantially faster than the Bloch-McConnell equation-based numerical simulation approach. In addition, the analytical solution-based SNR formula enables the exhaustive optimization of CEST MRI, which simultaneously predicts multiple optimal parameters such as repetition time, flip angle and RF saturation level based on the chemical shift and exchange rate. The sensitivity efficiency-based optimization approach could simplify and guide imaging of CEST agents, including glycogen, glucose, creatine, gamma-aminobutyric acid and glutamate. Copyright © 2016 John Wiley & Sons, Ltd.

A biomarker-responsive T2ex MRI contrast agent.

This study investigated a fundamentally new type of responsive MRI contrast agent for molecular imaging that alters T2 exchange (T2ex ) properties after interacting with a molecular biomarker. The contrast agent Tm-DO3A-oAA was treated with nitric oxide (NO) and O2 . The R1 and R2 relaxation rates of the reactant and product were measured with respect to concentration, temperature, and pH. Chemical exchange saturation transfer (CEST) spectra of the reactant and product were acquired using a 7 Tesla (T) MRI scanner and analyzed to estimate the chemical exchange rates and r2ex relaxivities. The reaction of Tm-DO3A-oAA with NO and O2 caused a 6.4-fold increase in the r2 relaxivity of the agent, whereas r1 relaxivity remained unchanged, which demonstrated that Tm-DO3A-oAA is a responsive T2ex agent. The effects of pH and temperature on the r2 relaxivities of the reactant and product supported the conclusion that the product's benzimidazole ligand caused the agent to have a fast chemical exchange rate relative to the slow exchange rate of the reactant's ortho-aminoanilide ligand. T2ex MRI contrast agents are a new type of responsive agent that have good detection sensitivity and specificity for detecting a biomarker, which can serve as a new tool for molecular imaging. Magn Reson Med 77:1665-1670, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Advances in Magnetic Resonance Imaging Contrast Agents for Biomarker Detection.

Recent advances in magnetic resonance imaging (MRI) contrast agents have provided new capabilities for biomarker detection through molecular imaging. MRI contrast agents based on the T2 exchange mechanism have more recently expanded the armamentarium of agents for molecular imaging. Compared with T1 and T2* agents, T2 exchange agents have a slower chemical exchange rate, which improves the ability to design these MRI contrast agents with greater specificity for detecting the intended biomarker. MRI contrast agents that are detected through chemical exchange saturation transfer (CEST) have even slower chemical exchange rates. Another emerging class of MRI contrast agents uses hyperpolarized (13)C to detect the agent with outstanding sensitivity. These hyperpolarized (13)C agents can be used to track metabolism and monitor characteristics of the tissue microenvironment. Together, these various MRI contrast agents provide excellent opportunities to develop molecular imaging for biomarker detection.

Choosing the best molecular precursor to prepare Li4Ti5O12 by the sol-gel method using (1)H NMR: evidence of [Ti3(OEt)13](-) in solution.

(1)H NMR spectroscopy at 400 MHz in toluene-d8 of evaporated mixtures of lithium ethoxide and titanium(iv) isopropoxide in ethanol, used to prepare the spinel Li4Ti5O12 by the sol-gel method, may help clarify why the atomic ratio 5Li : 5Ti and not 4Li : 5Ti is the right choice to obtain the pure phase when performing hydrolysis at room temperature. The mixtures xLiOEt/yTi(OPr(i))4 in ethanol undergo alcohol exchange at room temperature, and the evaporated residues contain double lithium-titanium ethoxide [LiTi3(OEt)13] rather than simple mixtures of single metal alkoxides; this is of great relevance to truly understanding the chemistry and structural changes in the sol-gel process. Detailed inspection of the (1)H and (13)C VT NMR spectra of mixtures with different Li/Ti atomic ratios unequivocally shows the formation of [LiTi3(OEt)13] in a solution at low temperature. The methylene signals of free lithium ethoxide and Li[Ti3(OEt)13] coalesce at 20 °C when the atomic ratio is 5 : 5; however, the same coalescence is only observed above 60 °C when the atomic ratio is 4 : 5. We suggest that the highest chemical equivalence observed by (1)H NMR spectroscopy achieved through chemical exchange of ethoxide groups involves the highest microscopic structural homogeneity of the sol precursor and will lead to the best gel after hydrolysis. Variable temperature (1)H NMR spectra at 400 MHz of variable molar ratios of LiOEt/Ti(OPr(i))4 are discussed to understand the structural features of the sol precursor. While the precursor with the atomic ratio 5Li : 5Ti shows no signal of free LiOEt at 20 °C, both 4Li : 5Ti and 7Li : 5Ti show free LiOEt at 20 °C in their (1)H NMR spectra, indicating that the molar ratio 5Li : 5Ti gives the maximum rate of chemical exchange. DFT calculations have been performed to support the structure of the anion [Ti3(OEt)13](-) at room temperature.

Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery

It is known that the chemical forms of tritium released from a ceramic breeder blanket are hydrogen form and water form. To recover tritiated water vapor, adoption of dryer that is packed column of synthetic zeolite has been proposed. On the other hand, synthetic zeolite is often used as a support of precious metal catalyst. Such catalysts usually have a capability of hydrogen isotope exchange between gas and water vapor. If this catalyst is used to dryer, the dryer may obtain a preferable function for tritium recovery by isotopic exchange reaction. To assess such functions, reaction rate should be estimated. The results of water adsorption experiment on cation-exchanged mordenite-type zeolite suggested the possibility that state of adsorbed water varied by exchanged cation. So, in this work, precious metal catalyst based on cation-exchanged mordenite was prepared, and the reaction rate of chemical exchange between hydrogen and tritiated water was investigated under temperature range between 30°C and 80°C by the steady-state approximation. In the case of platinum on Na-mordenite, the reaction between gaseous hydrogen and tritiated water vapor was almost expressed as first-order reaction concerning tritiated water vapor concentration.

A single diamagnetic catalyCEST MRI contrast agent that detects cathepsin B enzyme activity by using a ratio of two CEST signals.

CatalyCEST MRI can detect enzyme activity by monitoring the change in chemical exchange with water after a contrast agent is cleaved by an enzyme. Often these molecules use paramagnetic metals and are delivered with an additional non-responsive reference molecule. To improve this approach for molecular imaging, a single diamagnetic agent with enzyme-responsive and enzyme-unresponsive CEST signals was synthesized and characterized. The CEST signal from the aryl amide disappeared after cleavage of a dipeptidyl ligand with cathepsin B, while a salicylic acid moiety was largely unresponsive to enzyme activity. The ratiometric comparison of the two CEST signals from the same agent allowed for concentration independent measurements of enzyme activity. The chemical exchange rate of the salicylic acid moiety was unchanged after enzyme catalysis, which further validated that this moiety was enzyme-unresponsive. The temperature dependence of the chemical exchange rate of the salicylic acid moiety was non-Arrhenius, suggesting a two-step chemical exchange mechanism for salicylic acid. The good detection sensitivity at low saturation power facilitates clinical translation, along with the potentially low toxicity of a non-metallic MRI contrast agent. The modular design of the agent constitutes a platform technology that expands the variety of agents that may be employed by catalyCEST MRI for molecular imaging.

Simple models for disequilibrium fractional melting and batch melting with application to REE fractionation in abyssal peridotites

Disequilibrium melting arises when the kinetics of chemical exchange between a residual mineral and partial melt is sluggish compare to the rate of melting. To better understand the role of a finite crystal–melt exchange rate on trace element fractionation during mantle melting, we have developed a disequilibrium melting model for partial melting in an upwelling steady-state column. We use linear kinetics to approximate crystal–melt mass exchange rate and obtain simple analytical solutions for cases of perfect fractional melting and batch melting. A key parameter determining the extent of chemical disequilibrium during partial melting is an element specific dimensionless ratio (ε) defined as the melting rate relative to the solid–melt chemical exchange rate for the trace element of interest. In the case of diffusion in mineral limited chemical exchange, ε is inversely proportional to diffusivity of the element of interest. Disequilibrium melting is important for the trace element when ε is comparable to or greater than the bulk solid–melt partition coefficient for the trace element (k). The disequilibrium fractional melting model is reduced to the equilibrium perfect fractional melting model when ε is much smaller than k. Hence highly incompatible trace elements with smaller mobilities in minerals are more susceptible to disequilibrium melting than moderately incompatible and compatible trace elements. Effect of chemical disequilibrium is to hinder the extent of fractionation between residual solid and partial melt, making the residual solid less depleted and the accumulated melt more depleted in incompatible trace element abundances relative the case of equilibrium melting.Application of the disequilibrium fractional melting model to REE and Y abundances in clinopyroxene in abyssal peridotites from the Central Indian Ridge and the Vema Lithospheric Section, Mid-Atlantic Ridge revealed a positive correlation between the disequilibrium parameter ε and the degree of melting, which can be explained by an increase in melting rate and a decrease in REE diffusion rate in the upper part of the melting column. Small extent of disequilibrium melting for LREE and equilibrium melting for HREE in the upper part of the melting column can explain the elevated LREE abundances or spoon-shaped REE patterns in clinopyroxene in more refractory abyssal peridotites. The latter has often been attributed to melt refertilization.

Refinement of the inversion-transfer NMR experiment for faster characterization of chemical exchange.

It is shown theoretically that the inversion-transfer experiment used to estimate the value of unidirectional rate constants in chemical exchange systems can be performed faster via a reduction of the recovery delay. The chemical exchange rate constants can then be estimated accurately with a formula close to that of standard inversion transfer and easy to use, after a justified approximation. A function was developed to determine the optimal value of the recovery delay for an optimal inversion-transfer sequence. The validity of these theoretical results was checked experimentally with a solution of N,N-dimethylacetamide in which chemical exchange arises from internal hindered rotation.

Amide proton transfer of carnosine in aqueous solution studied in vitro by WEX and CEST experiments.

Amide protons of peptide bonds induce an important chemical exchange saturation transfer (CEST) contrast in vivo. As a simple in vitro model for a peptide amide proton CEST effect, we suggest herein the dipeptide carnosine. We show that the metabolite carnosine creates a CEST effect and we study the properties of the exchange of the amide proton (-NH) of the carnosine peptide bond (NHCPB) in model solutions for a pH range from 6 to 8.3 and a temperature range from T = 5 °C to 43 °C by means of CEST and water exchange spectroscopy (WEX) experiments on a 3 T whole-body MR tomograph. The dependence of the NHCPB chemical exchange rate k(sw) on pH and temperature T was determined using WEX. For physiological conditions (T = 37 °C, pH = 7.10) we obtained k(sw) = (47.07 ± 7.90)/s. With similar chemical shift and exchange properties to amide protons in vivo, carnosine forms a simple model system for optimization of CEST pulse sequences in vitro. The potential for direct detection of the metabolite carnosine in vivo is discussed.

Suppression Mechanism for Enol–Enol Isomerization of 2‐Substituted Dimedones

Abstract2‐Substituted dimedones, described as having a simple chemical structure, are a type of cyclic β‐diketone with enol–enol interconversion. The influence of the enol‐enol isomerism interferes with structure analysis of many dimedones in solution by NMR spectroscopy. It was demonstrated that the isomerism of the 2‐substituted dimedones depends on solvent, time, solute concentration, and temperature. Negative activation energy of the isomerization was determined by the NMR line shape and spin‐spin relaxation analyses, which is an extraordinary phenomenon found for the first time in the molecular dynamics of NMR. Suppression of the isomerization by heating, dilution of the solute, and time control in several low‐polarity solvents enabled analysis of the enol form in detail. It was clarified that deceleration of the intramolecular chemical exchange rate in the isomerization was responsible for the decrease in the intermolecular hydrogen‐bonding effect in the suppression mechanism.

Transfer Rate Edited experiment for the selective detection of Chemical Exchange via Saturation Transfer (TRE-CEST)

Chemical Exchange Saturation Transfer (CEST) magnetic resonance experiments have become valuable tools in magnetic resonance for the detection of low concentration solutes with far greater sensitivity than direct detection methods. Accurate measures of rates of chemical exchange provided by CEST are of particular interest to biomedical imaging communities where variations in chemical exchange can be related to subtle variations in biomarker concentration, temperature and pH within tissues using MRI. Despite their name, however, traditional CEST methods are not truly selective for chemical exchange and instead detect all forms of magnetization transfer including through-space NOE. This ambiguity crowds CEST spectra and greatly complicates subsequent data analysis. We have developed a Transfer Rate Edited CEST experiment (TRE-CEST) that uses two different types of solute labeling in order to selectively amplify signals of rapidly exchanging proton species while simultaneously suppressing ‘slower’ NOE-dominated magnetization transfer processes. This approach is demonstrated in the context of both NMR and MRI, where it is used to detect the labile amide protons of proteins undergoing chemical exchange (at rates⩾30s−1) while simultaneously eliminating signals originating from slower (∼5s−1) NOE-mediated magnetization transfer processes. TRE-CEST greatly expands the utility of CEST experiments in complex systems, and in-vivo, in particular, where it is expected to improve the quantification of chemical exchange and magnetization transfer rates while enabling new forms of imaging contrast.

Ultrasensitive anion detection by NMR spectroscopy: a supramolecular strategy based on modulation of chemical exchange rate.

NMR spectroscopy is a powerful tool for monitoring molecular interactions and is widely used to characterize supramolecular systems at the atomic level. NMR is limited for sensing purposes, however, due to low sensitivity. Dynamic processes such as conformational changes or binding events can induce drastic effects on NMR spectra in response to variations in chemical exchange (CE) rate, which can lead to new strategies in the design of supramolecular sensors through the control and monitoring of CE rate. Here, we present an indirect NMR anion sensing technique in which increased CE rate, due to anion-induced conformational flexibility of a relatively rigid structure of a novel sensor, allows ultrasensitive anion detection as low as 120 nM.

Quantitative description of radiofrequency (RF) power-based ratiometric chemical exchange saturation transfer (CEST) pH imaging.

Chemical exchange saturation transfer (CEST) MRI holds great promise for the imaging of pH. However, routine CEST measurement varies not only with the pH-dependent chemical exchange rate, but also with CEST agent concentration, providing pH-weighted information. Conventional ratiometric CEST imaging normalizes the confounding concentration factor by analyzing the relative CEST effect from different exchangeable groups, requiring CEST agents with multiple chemically distinguishable labile proton sites. Recently, a radiofrequency (RF) power-based ratiometric CEST MRI approach has been developed for concentration-independent pH MRI using CEST agents with a single exchangeable group. To facilitate quantification and optimization of the new ratiometric analysis, we quantified the RF power-based ratiometric CEST ratio (rCESTR) and derived its signal-to-noise and contrast-to-noise ratios. Using creatine as a representative CEST agent containing a single exchangeable site, our study demonstrated that optimized RF power-based ratiometric analysis provides good pH sensitivity. We showed that rCESTR follows a base-catalyzed exchange relationship with pH independent of creatine concentration. The pH accuracy of RF power-based ratiometric MRI was within 0.15-0.20 pH units. Furthermore, the absolute exchange rate can be obtained from the proposed ratiometric analysis. To summarize, RF power-based ratiometric CEST analysis provides concentration-independent pH-sensitive imaging and complements conventional multiple labile proton group-based ratiometric CEST analysis.

Role of Benzyl Alcohol in the Unfolding and Aggregation of Interferon α-2a

Benzyl alcohol (BA) is the most widely used antimicrobial preservative in multidose protein formulations, and has been shown to cause protein aggregation. Our previous work on a model protein cytochrome c demonstrated that this phenomenon occurs via partial unfolding. Here, we examine the validity of these results by investigating the effect of BA on a pharmaceutically relevant protein, interferon α-2a (IFNA2). IFNA2 therapeutic formulations available on the pharmaceutical market contain BA as a preservative. Isothermal aggregation kinetics and temperature scanning demonstrated that BA induced IFNA2 aggregation in a concentration-dependent manner. With increasing concentration of BA, the apparent aggregation temperature of IFNA2 linearly decreased. Denaturant melts measured using protein intrinsic fluorescence and that of the 1-anilinonaphthalene-8-sulfonic acid dye indicated that IFNA2 stability decreased with increasing BA concentration, populating a partially unfolded intermediate. Changes in nuclear magnetic resonance chemical shifts and hydrogen exchange rates identified the structural nature of this intermediate, which correlated with an aggregation "hot-spot" predicted by computational methods. These results indicate that BA induces IFNA2 aggregation by partial unfolding rather than global unfolding of the entire protein, and is consistent with our earlier conclusions from model protein studies.

Oxygen diffusion in Sr3YCo4O10.5: An electrical conductivity relaxation and thermogravimetric analysis approach

Chemical oxygen diffusion coefficients and chemical surface exchange rates along with oxygen self-diffusion coefficients in dense samples of Sr3YCo4O10.5+δ were studied by using a combination of electrical conductivity relaxation (ECR) measurements and thermogravimetric analysis (TGA) data in the temperature range of 650°C–900°C. Arrhenius behavior, A=A0exp−EART, was observed with activation energies of EADchem=1.16±0.01eV, EADO=1.56±0.01eV and EAkchem=0.76±0.05eV for the chemical oxygen diffusion coefficient, the oxygen self-diffusion coefficient, and the chemical surface exchange rate, respectively. The thermodynamic factor changes with temperature from 611 at 650°C to 177 at 900°C.