Rate-limiting Processes Research Articles

Effect of the Compaction and the Size of DNA on the Nuclear Transfer Efficiency after Microinjection in Synchronized Cells.

The nuclear transfer process is one of the critical rate-limiting processes in transgene expression. In the present study, we report on the effect of compaction and the size of the DNA molecule on nuclear transfer efficiency by microinjection. A DNA/protamine complex- or variously-sized naked DNA molecules were injected into the cytoplasm or nucleus of synchronized HeLa cells. To evaluate the nuclear transfer process, a nuclear transfer score (NT score), calculated based on transgene expression after cytoplasmic microinjection divided by that after nuclear microinjection, was employed. The compaction of DNA with protamine decreased the NT score in comparison with the injection of naked DNA when the N/P ratio was increased to >2.0. Moreover, when naked DNA was microinjected, gene expression increased in parallel with the size of the DNA in the following order: minicircle DNA (MC07.CMV-EGFP; 2257 bp) > middle-sized plasmid DNA (pBS-EGFP; 3992 bp) > conventional plasmid DNA (pcDNA3.1-EGFP; 6172 bp), while the level of gene expression was quite comparable among them when the DNAs were injected into the nucleus. The above findings suggest that the intrinsic size of the DNA molecule is a major determinant for nuclear entry and that minicircle DNA has a great advantage in nuclear transfer.

An Elementary Kinetic Model for the Electrochemical Reduction of Oxygen on LSM/YSZ Composite Cathodes

A computational half-cell model was developed to study the coupled interactions of species, electron and ion transport through porous media in addition to surface and electro-chemistry. Surface and electro-chemistry were represented using an elementary kinetic model. The model was validated against LSM/8YSZ composite cathode measurements made by Barbucci et al. [Electrochimica Acta, 47(13-14), 2183–2188 (2002)] to obtain realistic electrochemical parameters. The versatility of the micro-kinetic oxygen reduction mechanism was evaluated by predicting the performance of a pure LSM cathode. The simulated results were found to be in good agreement with measured data. A sensitivity analysis revealed that at high temperatures, ohmic resistance in the electrolyte and surface ionization of adsorbed oxygen atoms were the rate-limiting processes. At lower temperatures, dissociation of molecular oxygen adsorbed on the LSM surface and ion transfer at the LSM/YSZ interface were also found to be rate-limiting.

Electrochemical Impedance Spectroscopy Data from Solid Oxide Cells Undergoing Co-Electrolysis: The Influence of Rig Inductance

High temperature solid oxide cells (SOCs) are used for co-electrolysis of carbon dioxide and steam for the production of synthetic fuels. SOC rate-limiting processes can be investigated using Electrochemical Impedance Spectroscopy (EIS), but first, test rig contributions must be subtracted: inductance in particular. Here we illustrate the effect of inductance correction L on interpretation methods, including: impedance Z* and electric modulus M* formalisms, the Analysis of Differences in Impedance Spectra (ADIS) and the determination of Distribution of Relaxation Times (DRT). For inductances in the range ~10-7 H, results were sensitive to small changes in L, giving a significant change in high frequency Z* and M* data. This causes underestimation of the SOC polarisation resistance. ADIS results are less affected, due to the nature of the subtraction method, whereas DRT is highly sensitive to inductance, hence care must be taken to optimise inductance correction, to obtain accurate results.

Biochemical adaptation to ocean acidification

The change in oceanic carbonate chemistry due to increased atmospheric PCO2 has caused pH to decline in marine surface waters, a phenomenon known as ocean acidification (OA). The effects of OA on organisms have been shown to be widespread among diverse taxa from a wide range of habitats. The majority of studies of organismal response to OA are in short-term exposures to future levels of PCO2 . From such studies, much information has been gathered on plastic responses organisms may make in the future that are beneficial or harmful to fitness. Relatively few studies have examined whether organisms can adapt to negative-fitness consequences of plastic responses to OA. We outline major approaches that have been used to study the adaptive potential for organisms to OA, which include comparative studies and experimental evolution. Organisms that inhabit a range of pH environments (e.g. pH gradients at volcanic CO2 seeps or in upwelling zones) have great potential for studies that identify adaptive shifts that have occurred through evolution. Comparative studies have advanced our understanding of adaptation to OA by linking whole-organism responses with cellular mechanisms. Such optimization of function provides a link between genetic variation and adaptive evolution in tuning optimal function of rate-limiting cellular processes in different pH conditions. For example, in experimental evolution studies of organisms with short generation times (e.g. phytoplankton), hundreds of generations of growth under future conditions has resulted in fixed differences in gene expression related to acid-base regulation. However, biochemical mechanisms for adaptive responses to OA have yet to be fully characterized, and are likely to be more complex than simply changes in gene expression or protein modification. Finally, we present a hypothesis regarding an unexplored area for biochemical adaptation to ocean acidification. In this hypothesis, proteins and membranes exposed to the external environment, such as epithelial tissues, may be susceptible to changes in external pH. Such biochemical systems could be adapted to a reduced pH environment by adjustment of weak bonds in an analogous fashion to biochemical adaptation to temperature. Whether such biochemical adaptation to OA exists remains to be discovered.

Theoretical analysis of hydrogen production by variable volume membrane batch reactors with direct liquid fuel injection

A direct liquid fuel injection/variable volume reactor integrated with a hydrogen selective membrane (CHAMP-DDIR) has been recently shown to be a promising new concept for hydrogen production in portable and distributed applications. The CHAMP-DDIR reactor performance has been analyzed using a simplified transport model with which conditions for maximum performance, e.g. highest volumetric power density, were identified. A prototype reactor demonstrated the ability to realize performance improvement, while also indicating a need for more a rigorous model for accurate exploration of the design and operation space. In this paper, we present a comprehensive reactor model which carefully considers the effects of heat and mass transfer, including rigorous treatment of multi-component species transport. The model is validated against experimental results through comparison of predicted and measured hydrogen production rate, reactor pressure, and temperature. The experimentally validated model is used to identify the relationship between CHAMP-DDIR design and operating parameters and the rate-limiting processes that govern reactor output. In addition, effects of heat and mass transfer limitations on CHAMP-DDIR performance are investigated by comparing the predictions among multiple cases with increasing level of complexity used for modeling the transport phenomena within the reactor.

Mapping the Inhomogeneous Electrochemical Reaction Through Porous LiFePO4-Electrodes in a Standard Coin Cell Battery

Nanosized, carbon-coated LiFePO4 (LFP) is a promising cathode for Li-ion batteries. However, nano-particles are problematic for electrode design, optimized electrodes requiring high tap densities, good electronic wiring, and a low tortuosity for efficient Li diffusion in the electrolyte in between the solid particles, conditions that are difficult to achieve simultaneously. Using in situ energy-dispersive X-ray diffraction, we map the evolution of the inhomogeneous electrochemical reaction in LFP-electrodes. On the first cycle, the dynamics are limited by Li diffusion in the electrolyte at a cycle rate of C/7. On the second cycle, there appear to be two rate-limiting processes: Li diffusion in the electrolyte and electronic conductivity through the electrode. Three-dimensional modeling based on porous electrode theory shows that this change in dynamics can be reproduced by reducing the electronic conductivity of the composite electrode by a factor of 8 compared to the first cycle. The poorer electronic wi...

Theoretical studies on the aminolysis mechanism of propylene carbonate with ammonia

The possible mechanisms of the aminolysis reaction of propylene carbonate with ammonia were investigated by computational techniques. Optimized structures for all the stationary points were examined by employing the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) methods. The uncatalyzed, base-catalyzed and acid-catalyzed aminolysis reactions were taken into account. Two possible mechanisms of the concerted process and the addition/elimination stepwise pathway are investigated, and the first stages of the stepwise mechanism for all pathways are the rate-limiting processes. The results indicate that the concerted process of non-catalyzed aminolysis has lower energy barrier in comparison with the stepwise mechanism. Concerning the base-catalyzed and acid-catalyzed aminolysis, the neutral stepwise pathway is more favorable than the concerted mechanism and the energy barriers of all steps are lower than those of the uncatalyzed aminolysis. The solvent effect by a polarized continuum model does not significantly change the conclusion.

Ultra-fast aqueous Li-ion redox energy storage from vanadium oxide-carbon nanotube yarn electrodes

Half-cell electrochemical characterizations were conducted on carbon nanotube-vanadium oxide (CNT-VOx) yarn electrodes in an 8 M LiCl aqueous electrolyte. A supercritical fluid deposition and in-situ oxidation process was utilized to deposit nanoscale coatings of vanadium oxide on carbon nanotube (CNT) surfaces throughout the porous structure of CNT yarns. The high surface area, interconnected pore structure and high electrical conductivity of the CNT yarn enabled extraordinary rate capabilities from the high capacity Li/VOx system. High-rate cyclic voltammetry scans, requiring current densities of hundreds of amperes per gram of electrode mass, produced rectangular voltammograms with distinguishable redox peaks from Li-ion intercalation/deintercalation. Capacitances of over 150 F g−1 were achieved at a scan rate of 5 V s−1 over a 1.2 V potential window resulting in an energy density of >32 Wh kg−1 (>30 Wh L−1) for the yarn electrode. The charge storage also showed good reversibility when cycled over this large potential window, maintaining 90% of the capacitance after 100 cycles at a scan rate of 2 V s−1. Electrochemical impedance spectroscopy shows the frequency dependent behavior is distinctly lacking of the characteristic responses from the rate-limiting processes associated with faradaic charge storage in VOx.

Understanding Direct Borohydride – Hydrogen Peroxide Fuel Cell Performance with a Calibrated Numerical Model

A numerical model is developed for a hydrogen peroxide – direct borohydride fuel cell (H2O2-DBFC) to provide insight into cell processes which are obscured in experiments by the complexity of electrode reactions and transport in the reactant flow channels. In the model, transport is modeled using a 2D finite volume approach. Multi-step electrode reactions, including competing parasitic reactions, are represented by global rate expressions, with parameters calibrated to single-cell experiments. The results are calibrated to a H2O2-DBFC in which fuel (1-50 mM NaBH4 / 2 M NaOH) is oxidized at a Au anode and oxidizer (10-40 mM H2O2 / 1 M H2SO4) is reduced at a Pd:Ir cathode. This modeling approach provides critical insight into the rate-limiting processes at different operating conditions of a H2O2-DBFC.

A reassessment of the redshift distribution and physical properties of luminous (sub-)millimetre galaxies

Motivated by the current controversy over the redshift distribution and physical properties of luminous (sub-)mm sources, we have undertaken a new study of the brightest sample of unlensed (sub-)mm sources with pre-ALMA interferometric follow-up in the COSMOS field. Exploiting the very latest multi-frequency supporting data, we find that this sample displays a redshift distribution indistinguishable from that of the lensed sources uncovered with the South Pole Telescope (SPT), with z_median ~ 3.5. We also find that, over the redshift range z ~ 2-6 the median stellar mass of the most luminous (sub-)mm sources is M_* ~ 3 * 10^11 M_odot, yielding a typical specific star-formation rate sSFR ~ 3 Gyr^-1. Consistent with recent ALMA and SMA studies, we confirm that source blending is not a serious issue in the study of luminous (sub-)mm sources uncovered by ground-based, single-dish surveys; only ~ 10-15% of bright (S_850 ~ 5 - 10 mJy) (sub-)mm sources arise from significant (i.e. >20%) blends, and so our conclusions are largely unaffected by whether we adopt the original single-dish mm/sub-mm flux-densities/positions, or the interferometric data. Our results suggest that apparent disagreements over the redshift distribution of (sub-)mm sources are a result of "down-sizing" in dust-enshrouded star-formation, consistent with existing knowledge of the star-formation histories of massive galaxies. They also indicate that extreme star-forming galaxies at high redshift are, on average, subject to the same star-formation rate-limiting processes as less luminous objects, and lie on the "main sequence" of star-forming galaxies at z>3.

Electron and Ionic Transport in Microbial Anode Respiration and Their Importance in Microbial Electrochemical Cell Applications

Anode-respiring bacteria (ARB) catalyze the complete oxidation of organic compounds (e.g. acetate, glucose) into electrical current and carbon dioxide. ARB produce a biofilm at the electrode surface, where even cells on the outer part of the biofilm are participating in current production. Our team uses a variety of electrochemical techniques in order to characterize electron transport responses from various ARB. Through these experiments, we have observed a complex response to anode potential that allows ARB, such as G. sulfurreducens, to optimize their efficiency in electron transport. Identifying this complex behavior allows us to better understand and predict the response of ARB to different conditions in microbial electrochemical cells (MXCs). While the topic of electron transport is the focus of most ARB research, ionic transport is the most important factor in determining rate-limiting and potential loss processes. ARB require near-neutral pH in the medium to grow, differing from chemical fuel cells commonly employed, which run under acidic or alkaline conditions. This pH requirement results in a major transport limitation, as H+ ions (now in mM range) should be transported from anode to cathode to achieve electron neutrality. In an MXC anode, H+ ions accumulate in the ARB biofilm, creating an acidification that limits current generation. We have identified and characterized ARB that work outside the neutral pH range, including Geoalkalibacter ferrihydriticus and Thermoanaerobacter pseudethanolicus, that allow us to operate MXCs at either acidic or basic conditions. Meanwhile, at the cathode, local gradients leading to pH > 12 is typical in MXC operation. As a consequence, the pH gradient results in Nernstian concentration overpotential of > 300 mV. Thus, understanding and controlling ionic transport in MXCs is essential to ensure an efficient operation. I will discuss our current efforts to characterize and overcome ionic transport limitations in order to develop efficient MXC processes.

Strain rate sensitivity of nanolayered Cu/X (X=Cr, Zr) micropillars: Effects of heterophase interface/twin boundary

Manipulation of internal features, e.g., layer/twin interfaces, is standard practice in engineering structural materials to tailor their properties. In the present work, we fabricated nanotwinned (NT)- and nanocrystalline (NC)–Cu/X (X=Cr, Zr) nanolayered micropillars (NLs) with equal layer thickness (h) spanning from 5 to 125nm. The microcompression methodology was employed to investigate the layer/twin interfaces effects on plastic characteristics of nanolayered materials at different strain rates. Experimental results clearly unveil that the Cu/X NTNLs exhibit a significant increase in strength and in strain rate sensitivity (SRS) by introducing nanotwins, in comparison with the Cu/X NCNLs. The non-monotonic evolution of SRS with h observed in the Cu/X NTNLs is explained by a competition between the monotonically increased interfaces density and the decreased twin boundaries density with reduction in feature size h. Unlike the monotonically enhanced SRS of Cu/X NCNLs, the SRS of Cu/X NTNLs first decreases and subsequently increases with reducing h. A phenomenological model is proposed to rationalize these experimental findings and highlight the microstructural feature size effects on the rate-limiting processes of metallic materials.

Impedance As a Diagnostic Tool to Characterize Mixed-Potential Sensor Response

Commercial tape casting and screen-printing methods have been used to fabricate a planar, self-heated, mixed-potential NOx sensor for application in vehicle on-board emission control systems. The device consists of dense La0.8Sr0.2CrO3 (LSCrO) and Pt electrodes and a porous YSZ electrolyte on one side of a dense ceramic substrate and a Pt-heater with independent leads on the backside of the substrate. While these sensors have demonstrated high sensitivity and selectivity to NOx when operated at a positive bias, optimization of the sensor device geometry remains an open question. In this work, we used circular cells of dense YSZ with LSCrO working and Pt counter electrodes in order to identify the impedance response of each individual sensor component. The impedance response measured on the cell was then used to identify the rate-limiting processes underlying the response of the planar sensor device.

A comparison of literature models for the oxidation of normal heptane

The development of detailed chemical kinetic models has proceeded unabated since the pioneering work of Dixon-Lewis and coworkers forty-five years ago. In that time, computational power has increased ten million-fold, and yet, we do not have a consensus on a complete description for the pyrolysis and oxidation of fuels as simple as hydrogen. This paper presents a comparison of three detailed models and one lumped, semi-empirical model, all from literature, for the oxidation of normal heptane, a fuel of practical interest because of its use in gasoline surrogates. Specifically, the ignition delay times and species concentration histories predicted by the models are compared. We find that most of the models give predictions that agree with each other within the propagated uncertainty from the rate constant estimates, and also show “satisfactory agreement” with experimental measurements. Differences among the models can be attributed to peculiarities in how the models treat the initial decomposition of the fuel. The rate coefficients for these reactions do not agree within the uncertainty assigned for them, however, which means that the models are fundamentally, quantitatively different. The model’s predictions agree with each other and with experiment, however. The predictive ability of the models is the only way we can fairly compare them, and by that measure all of the models are equally good. Given the predictive capability of the semi-empirical model, we suggest that future combustion research should concentrate on developing a model that treats only the key, rate-limiting processes in a rigorous manner, rather than continuing the current trend of attempting to describe all possible elementary reactions exactly.

Simplifying mass production of PEM fuel cell technology

While current PEM fuel cell research is mainly focused on reducing system costs and improving efficiency, less attention is being paid to the possibilities offered by mass-production and economies of scale. PA Consulting Group has identified over-complicated components, fragmented supply chains, and a lack of significant national organisation as rate-limiting processes in the development of the UK fuel cell industry.

Moving from Basic Toward Systems Pharmacodynamic Models

Building upon many classical foundations of pharmacology, a diverse array of mechanistic pharmacokinetic-pharmacodynamic (PK/PD) models have emerged based on mechanisms of drug action and primary rate-limiting or turnover processes in physiology. An array of basic models can be extended to handle various complexities including tolerance and can readily be employed as building blocks in assembling enhanced PK/PD or small systems models. Our corticosteroid models demonstrate these concepts as well as elements of horizontal and vertical integration of molecular to whole-body processes. The potential advantages and challenges in moving PK/PD toward systems models are described.

Use of a reactive transport model to describe reductive dechlorination (RD) as a remediation design tool: application at a CAH-contaminated site

In this paper, a numerical model is presented that is capable of describing the complex set of biochemical processes that occur in chlorinated aliphatic hydrocarbon (CAH)-contaminated groundwater when an exogenous electron donor is added. The reactive pattern is based on the degradation pathways of both chlorinated ethanes and ethenes, and it includes electron donor production (H2 and acetate) from the fermentation of an organic substrate as well as rate-limiting processes related to electron acceptor competition. Coupling of the kinetic model to a convection-dispersion module is described. The calibration phase was carried out using data obtained at a real CAH-contaminated site in the north of Italy. Model simulations of different application scenarios are presented to draw general conclusions on the effectiveness of reductive dechlorination (RD) as a possible cleanup strategy. Early outcomes indicate that cleanup targets can only be achieved if source longevity is reduced. Therefore, metabolic RD is expected to produce beneficial effects because it is known to induce bioenhanced degradation and transformation of CAHs.

Dynamics of basaltic glass dissolution – Capturing microscopic effects in continuum scale models

The method of ‘multiple interacting continua’ (MINC) was applied to include microscopic rate-limiting processes in continuum scale reactive transport models of basaltic glass dissolution. The MINC method involves dividing the system up to ambient fluid and grains, using a specific surface area to describe the interface between the two. The various grains and regions within grains can then be described by dividing them into continua separated by dividing surfaces. Millions of grains can thus be considered within the method without the need to explicity discretizing them. Four continua were used for describing a dissolving basaltic glass grain; the first one describes the ambient fluid around the grain, while the second, third and fourth continuum refer to a diffusive leached layer, the dissolving part of the grain and the inert part of the grain, respectively.The model was validated using the TOUGHREACT simulator and data from column flow through experiments of basaltic glass dissolution at low, neutral and high pH values. Successful reactive transport simulations of the experiments and overall adequate agreement between measured and simulated values provides validation that the MINC approach can be applied for incorporating microscopic effects in continuum scale basaltic glass dissolution models. Equivalent models can be used when simulating dissolution and alteration of other minerals.The study provides an example of how numerical modeling and experimental work can be combined to enhance understanding of mechanisms associated with basaltic glass dissolution. Column outlet concentrations indicated basaltic glass to dissolve stoichiometrically at pH 3. Predictive simulations with the developed MINC model indicated significant precipitation of secondary minerals within the column at neutral and high pH, explaining observed non-stoichiometric outlet concentrations at these pH levels. Clay, zeolite and hydroxide precipitation was predicted to be most abundant within the column.

Carbonaceous resin capsule for vapor-phase monitoring of volatile hydrocarbons in soil: partitioning and kinetic model verification

The resin capsule system (RCS) was tested as a means of providing data on the presence and forms of volatile hydrocarbons. Results indicated that resin capsules provided data showing sensitivity to soil variables (texture and moisture content) and time. The objectives of this paper are to evaluate the RCS methodology and to determine whether carbonaceous resin capsules provide results that can be described by fundamental chemical partitioning and kinetic principles. Findings revealed a significant relationship between quantities of benzene, toluene, ethylbenzene, and xylene adsorbed on the capsule and quantities partitioned into the vapor phase. Kinetic evaluation indicated that the vapor adsorption by the resin capsule is regulated by diffusion processes. No verification of rate-limiting processes was possible due to limitations imposed by the experimental design, but it appears that during early stages, adsorption rate was limited by vapor diffusion through the soil. The resin capsule data also reflected differences that would be expected due to properties of the organic liquids present. These results provide further evidence that the RCS could be developed to suggest direct in situ monitoring to reveal quantities and nature of organic substances in soils.

Fabrication and characterization of all-ceramic solid oxide fuel cells based on composite oxide anode

All-ceramic solid oxide fuel cells (SOFCs), which offer advantages in carbon tolerance, sulfur resistance and redox stability, are fabricated and evaluated. The electrolyte-supported cells are composed of a La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM)–Ce0.9Gd0.1O1.95−δ (GDC) anode, an Y2O3-stabilized ZrO2 (YSZ) electrolyte, a GDC interdiffusion barrier layer, and a La0.8Sr0.2Co0.2Fe0.8O3−δ (LSCF)–GDC cathode. A particle-dispersed glycine-nitrate process is developed to synthesize extremely fine and homogeneous LSCM–GDC ceramic composite powders. The electrochemical performance of the LSCM–GDC anode is comparable to that of conventional Ni-based anodes. The impedance spectra of the all-ceramic SOFCs are successfully interpreted by the independent characterization of the individual electrodes via half-cell measurements. The impedance of the LSCM–GDC anode is dominated by a low-frequency arc originating from the “chemical capacitance”, which is associated with the variation of the oxygen nonstoichiometry in the mixed conducting ceramic electrode. In addition, the impedance arc associated with the electrode–gas interaction is observed in the LSCM–GDC anode. The rate-limiting processes for the LSCF–GDC cathode are observed to be solid-state oxygen diffusion and surface chemical exchange. Herein, the reaction mechanisms and rate-limiting processes of the all-ceramic SOFCs are discussed in detail and compared with those of conventional Ni-based SOFCs.