Stopped flow apparatus for time-resolved Fourier transform infrared difference spectroscopy of biological macromolecules in 1H2O

Stopped flow spectroscopy is an established technique for acquiring kinetic data on dynamic processes in chemical and biochemical reactions, and Fourier transform infrared (FT-IR) techniques can provide particularly rich structural information on biological macromolecules. However, it is a considerable challenge to design an FT-IR stopped flow system with an optical path length low enough for work with aqueous (1H2O) solutions. The system presented here is designed for minimal sample volumes (approximately 5 microL) and allows simultaneous FT-IR rapid-scan and VIS measurements. The system employs a micro-structured diffusional mixer to achieve effective mixing on the millisecond time scale under moderate flow and pressure conditions, allowing measurements in a cell path length of less than 10 microns. This makes it possible to record spectra in 1H2O solutions over a wide spectral range. The system layout is also designed for a combination of kinetic and static measurements, in particular to obtain detailed information on the faster spectral changes occurring during the system dead time. A detailed characterization of the FT-IR stopped flow system is presented, including a demonstration of the alkaline conformational transition of cytochrome c as an example.     

Mixing crowded biological solutions in milliseconds

In vitro studies of biological reactions are rarely performed in conditions that reflect their native intracellular environments where macromolecular crowding can drastically change reaction rates. Kinetics experiments require reactants to be mixed on a time scale faster than that of the reaction. Unfortunately, highly concentrated solutions of crowding agents such as bovine serum albumin and hemoglobin that are viscous and sticky are extremely difficult to mix rapidly. We demonstrate a new droplet-based microfluidic mixer that induces chaotic mixing of crowded solutions in milliseconds due to protrusions of the microchannel walls that generate oscillating interfacial shear within the droplets. Mixing in the microfluidic mixer is characterized, mechanisms underlying mixing are discussed, and evidence of biocompatibility is presented. This microfluidic platform will allow for the first kinetic studies of biological reactions with millisecond time resolution under conditions of macromolecular crowding similar to those within cells.     

Kinetic and chemical characterization of thermal decomposition of dicumylperoxide in cumene

Dicumylperoxide (DCP) is one of the most used peroxides in the polymer industry. It has been reported that its thermal decomposition can result in runaway phenomena and thermal explosions with significant economic losses and injuries to people. In the present paper thermal behaviour of dicumylperoxide in cumene was investigated over the temperature range of 393-433 K under aerated and de-aerated conditions. The results indicated that when oxygen was present, the decomposition rate did not follow a simple pseudo-first order kinetic as previously reported in literature. A satisfactory fit of the experimental data was, in this case, achieved by means of kinetic expression derived under the assumption of an autocatalytic scheme of reaction. The reaction rate was, on the contrary, correctly described by a pseudo-first order kinetic in absence of oxygen. Under both aerated and de-aerated conditions, chemical analysis showed that the decomposition mainly resulted in the formation of acetophenone and dimethylphenylcarbinol with minor occurrence of 2,3-dimethyl-2,3-diphenylbutane. The formation of methane and ethane was also invariably observed while the appearance of cumylhydroperoxide as a reaction intermediate was detected under only aerated conditions. Therefore, two reaction schemes were proposed to explain system behaviour in the presence of oxygen and after its purging.     

Facile analysis of EC cyclic voltammograms

It has been found empirically that, for an E(rev)C(irrev) process, the forward/backward ratio of the peak height magnitudes in cyclic voltammetry equals 1 + ktau, where k is the rate constant of the chemical reaction and tau is the time required for the scan to travel between the half-wave and reversal potentials. The relationship is largely independent of the scan rate and the reversal potential, except insofar as these influence tau. Though not exact, the relationship is obeyed closely enough to provide accurate rate constants under favorable conditions. The utility of this simple formula in extracting homogeneous kinetic information is demonstrated using experimental data for the electron-transfer-induced isomerization of an octahedral manganese complex. An explanation of the relationship is presented, as is a more exact formula that reduces to 1 + ktau when k is small. A semiquantitative explanation of the relationship is provided.     

Evaluation of styrene-acrylonitrile copolymerization thermal stability and runaway behavior

Evaluation of thermal stability and runaway behavior of any exothermic chemical system is of great importance for the design and operation of a chemical process. The evaluation process should be based on a thorough investigation of the reaction chemistry including reaction pathways, thermodynamic, and kinetic parameters. When addressing the reactivity hazards of any reacting system, the dominant pathway(s) should be identified. Identifying the main reaction pathway under specific conditions will lead to a better thermodynamic and kinetic characterization of the reacting system.In this article, the thermal stability and runaway behavior of styrene-acrylonitrile copolymerization reaction system in bulk is evaluated. Traditional thermal analysis techniques (calorimetric analysis) are combined with computational quantum chemistry methods and empirical thermodynamic-energy correlations. Reaction pathways are identified from the theoretical approach and verified by experimental measurements. The results of this analysis are compared to literature data for this system.     

Observation of plastoquinone kinetics in photosystem II from delayed fluorescence measurements

Attempts to account for the variations in photosystem II (PSII) under general conditions result in nonlinear and cumbersome models that are difficult to validate and render few insights about the system kinetics. In this research, the authors experimentally show that under certain conditions, linear-system techniques could be applied to advantage for probing some basic kinetic characteristics of the plastoquinones (PQs). The PQ redox states of the reaction centres were represented in a conditionally linear model structure with delayed fluorescence (DF) as a measurable output. DF data were acquired for different plant samples and conditions. After least-squares parameter optimisation, not only could the model closely describe the measured DF, but more significantly, the estimated parameters correctly reflected the expected changes induced by drought or [3-(3,4-dichlorophenyl)-1,1-dimethylurea] (DCMU) stress. Analysis showed that for short-pulse illumination, the PQ kinetic states of the reaction centres in an initially dark-adapted plant leaf can be represented as a time-invariant bilinear system in a five-dimensional state space. The system becomes linear for constant illuminations, but the system matrix and the kinetic behaviour are illumination dependent. In particular, the system behaves differently between lights-on and lights-off conditions. The simplicity of the model structure, nonetheless, permits observation and analysis of the PQ kinetics of PSII reaction centres from DF measurements by using linear-system techniques.     

Achieving uniform mixing in a microfluidic device: hydrodynamic focusing prior to mixing

We describe a microfluidic mixer that is well-suited for kinetic studies of macromolecular conformational change under a broad range of experimental conditions. The mixer exploits hydrodynamic focusing to create a thin jet containing the macromolecules of interest. Kinetic reactions are triggered by molecular diffusion into the jet from adjacent flow layers. The ultimate time resolution of these devices can be restricted by premature contact between co-flowing solutions during the focusing process. Here, we describe the design and characterization of a mixer in which hydrodynamic focusing is decoupled from the diffusion of reactants, so that the focusing region is free from undesirable contact between the reactants. Uniform mixing on the microsecond time scale is demonstrated using a device fabricated by imprinting optical-grade plastic. Device characterization is carried out using fluorescence correlation spectroscopy (FCS) and two-photon microscopy to measure flow speeds and to quantify diffusive mixing by monitoring the collisional fluorescence quenching, respectively. Criteria for achieving microsecond time resolution are described and modeled.     

Experimental study on thermophysicochemical properties of the LaNi5-H2 system

Thermophysicochemical properties of the LaNi₅-H₂ system are investigated for the practical utilization of hydriding alloys. The equilibrium pressure-composition isotherms are measured in full consideration of the thermal history. It is made clear that they are susceptible to the maximum temperature in operation processes. The kinetic properties are measured under isobaric and isothermal conditions. The results indicate the difference in reaction mechanism between the absorption and the desorption processes.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Electrospray micromixer chip for on-line derivatization and kinetic studies

An electrospray microchip for mass spectrometry comprising an integrated passive mixer to carry out on-chip chemical derivatizations is described. The microchip fabricated using UV-photoablation is composed of two microchannels linked together by a liquid junction. Downstream of this liquid junction, a mixing unit made of parallel oblique grooves is integrated to the microchannel in order to create flow perturbations. Several mixer designs are evaluated. The mixer efficiency is investigated both by fluorescence study and mass spectrometric monitoring of the tagging reaction of cysteinyl peptides with 1,4-benzoquinone. The comparisons with a microchip without a mixing unit and a kinetic model are used to assess the efficiency of the mixer showing tagging kinetics close to that of bulk reactions in an ideally mixed reactor. As an ultimate application, the electrospray micromixer is implemented in a LC-MS workflow. On-line derivatization of albumin tryptic peptides after a reversed-phase separation and counting of their cysteines drastically enhance the protein identification.     

Hydrodynamics and mass transfer of the coaxial jet mixer in flow injection analysis

A coaxial jet mixer that was previously proposed for rapid and efficient mixing under laminar flow conditions has been studied both theoretically and experimentally. A mathematical model that consists of a set of Navie-Stokes equations that determine the flow velocities and three diffusion-convection reaction equations that determine the reactant and product concentrations has been developed. Equations are solved with the help of finite difference techniques for different flow conditions. The quality of sample and reagent mixing is characterized by the mean product concentration and the amount of product produced. Theoretical results are compared with experimental ones for the mixing of bromothymol blue (a pH indicator) in the outer capillary with NaOH in the inner capillary of the jet mixer.     

Regular wave impact onto an elastic plate

A computational analysis of an elastic plate dropped against regular long waves is presented. The problem is considered within the linear potential-flow theory. The liquid flow is two-dimensional and the plate is modelled by an Euler beam. The analysis is based on the normal-mode method with hydroelastic behavior of the plate being of main interest. Different impact conditions are considered to study the dependence of the total energy of the plate–liquid system on impact geometry and plate properties. The contributions of kinetic and potential energies to the total energy are analyzed. It is shown that the kinetic part of the system energy is small at the instant of time when bending stresses in the beam approach their maximum values. Estimations of both the total energy and the maximum of bending stresses are presented. Most of the calculations are performed for the conditions of experiments carried out in MARINTEK. A range of the problem parameters is also considered, to reveal peculiarities of the unsteady interaction between a falling elastic plate and surface waves.     

An exothermic chemical reaction with linear feedback control

This paper investigates the effect of linear control as applied to an exothermic Sal?nikov chemical reaction. It will demonstrate how the control reduces the amplitude of the naturally occurring oscillations in temperature. However, in different parameter regions, this is not the case; that is, the reaction has desirable behaviour without control. A mathematical and numerical analysis of the system will be presented. Steady-state solutions are examined, and the emergence of limit cycles will be shown via the Hopf condition. The conditions under which oscillatory behaviour is not observed will be identified. When oscillatory behaviour is detected, the region to which it is confined will be described. Finally, an example is given of a nonlinear control, which ensures that the system will only portray stable behaviour, and under certain parameter constraints can eliminate oscillations entirely.     

Optimization method for simultaneous kinetic analysis

Kinetic analysis is often carried out for simultaneous determinations; thus, a theory to establish its optimal conditions is necessary. A very simple and fast model to find conditions for optimum analytical performance and to predict the quality of simultaneous kinetic analysis has been developed. It is general and applicable to any reaction order or rate constant. The model has been based on the angle between the kinetic vectors and on their norm ratio, which are readily calculated for any kinetic scheme. Evaluation of the proposed model has been carried out by detailed simulations of numerous experimental conditions and analysis by full PCR calculations (when applicable) or nonlinear least-squares fitting. An important conclusion is that analytical performance is determined to a large extent by the stability of the space spanned by the relevant component vectors (in addition to experimental noise and other factors). The quality of the analysis is governed by the angle between the kinetic vectors, while the norm ratio determines the error distribution between components. Optimum conditions for simultaneous kinetic analysis have been studied in several representative examples, regarding the timing of the kinetic monitoring and the effects of concentrations and of rate constant ratios, as well as several other factors of experimental relevance.     

Method for determination of association and dissociation rate constants of reversible bimolecular reactions by isothermal titration calorimeters

The rate law equation for reversible bimolecular reactions, which are describable by association and dissociation rate constants (k1 and k-1), is not solvable to a plain formula under stoichiometric reaction conditions. Therefore, it is a general technique to observe such reactions under pseudo first-order conditions, which make the reactions a single-exponential process, and enable us to determine k1 and k-1 without any complicated iterative computations needed to analyze the same reactions under stoichiometric reaction conditions. However, the accelerated reaction rates under pseudo first-order conditions are not always favorable to the physicochemical tools employing a slow or medium response time, such as thermal analysis instruments. In this study, we have developed a simple non-iterative analytical method to determine k1 and k-1 of reversible bimolecular reactions under stoichiometric conditions on the basis of experimental data of isothermal titration calorimetry (ITC), which is generally used to determine thermodynamic parameters rather than kinetic constants. Our method is principally based on the general principle of chemical bindings caused along with the titration processes, that is, the chemical relaxation kinetics, which had been hitherto considered in the analysis on the ITC data.     

Scanning electrochemical microscopy 50. Kinetic study of electrode reactions by the tip generation-substrate collection mode

A scanning electrochemical microscopy (SECM) methodology for localized quantitative kinetic studies of electrode reactions based on the tip generation-substrate collection (TG-SC) operation mode is presented. This approach does not use the mediator feedback required in typical kinetic SECM experiments. The reactant is galvanostatically electrogenerated on a tip placed in proximity to the substrate. It diffuses through the tip-substrate gap and undergoes the reaction of interest on the substrate surface. The substrate current is monitored with time until it reaches an apparent steady-state value. The process was digitally simulated using an explicit finite difference method, for an irreversible first-order electrode reaction at the substrate. Transient responses, steady-state polarization curves, and TG-SC approach curves can be used to obtain substrate kinetics. The effects of the experimental parameters were analyzed. The possibility of easily changing the experimental conditions with the SECM is an attractive approach to obtain independent evidence that can be used for a strict test of reaction mechanisms. The technique was applied for a preliminary simplified kinetic examination of the oxygen reduction reaction in phosphoric acid.     

In-situ polymerization behaviour of bone cements

The polymerization behaviour of bone cements during total hipreplacements is characterized by a fast and highly non-isothermal bulk reaction.In the first part of this paper the reaction kinetics are analysed bycalorimetric analysis in order to determine the rates of polymerization inisothermal and non-isothermal conditions. A phenomenological kinetic model,accounting for the effects of autoacceleration and vitrification, is presented.This model, integrated with an energy balance, is capable of predicting thetemperature across the prosthesis, the cement and the bone and the degree ofreaction in the cement, during in situ polymerization. The temperatureand the degree of reaction profiles are calculated, as a function of the settingtime, taking into account the system geometry, the thermal diffusivity of bone,prosthesis and cement, and the heat rate generated by the reaction according tothe kinetic model. Material properties, boundary and initial cond!itions are the input data of the heat transfer model. Kinetic and heat transfermodels are coupled and a numerical solution method is used. The model is appliedin order to study the effects of different application procedures on temperatureand degree of reaction profiles across the bone–cement–prosthesissystem.     

Scale-up feasibility in high-shear mixers: determination through statistical procedures

The wet granulation scale-up of a formulation exhibiting plastic deformation behavior under compression was examined. Through experimental factorial design, the effect of solution level, mixing time, and mixer speed on granulation properties was investigated. Measurements of mean particle size, tapped density, bulk density, Carr's index, coarse-to-fine ratio, cumulative percentage, and flow rate were taken and compared among granulations. In addition, comparisons were done on the hardness of tablets made from the formulations. It was shown that the characteristics of the granulations made under different conditions were highly reproducible. The excipient system of microcrystalline cellulose and pregel starch was shown to be a very robust formulation that is resistant to changes in the scaling-up process in high-shear mixers.     

Generalized kinetic model of creep and long-term strength of metals under conditions of combined creep and non-stationary loading. Report no. 1. General case

A system of kinetic relationships describing the creep and long-term strength of metallic materials is presented. It takes into consideration a wide range of effects observed under conditions of step-wise, short-term, repeated and cyclic changes in the loading. A correspondence of the model with the results of classical experimental investigations is shown.Translated from Problemy Prochnosti, No. 4, pp. 49–55, April, 1990.     

840S环氧树脂体系固化反应特性

用差示扫描量热法(DSC) 在动态条件下对840S 环氧树脂体系的固化反应动力学进行了研究。根据所测量的不同升温速率的DSC 曲线, 运用温度升温速率( T-β) 图外推法得到该环氧树脂体系的固化工艺参数, 即凝胶化温度、固化温度、后处理温度, 这些温度参数为制定合理的固化工艺提供了理论基础。采用Kissinger 方程和Crane 方程计算该840S 环氧树脂体系的动力学参数, 即表观活化能E_a 、表观频率因子A 和反应级数n 。根据所计算的动力学参数, 建立了该840S 环氧树脂体系的固化动力学模型。利用所建立的固化动力学模型分别预测了等温和动态条件下840S 环氧树脂体系的固化反应特性。      

Thermo-oxidative decomposition of multi-walled carbon nanotubes: Kinetics and thermodynamics

Abstract

Multi-walled carbon nanotubes (MWCNTs) are familiar well owing to their capability of finding wide-reaching applications based on their fascinating properties. Kinetics of thermally activated processes in MWCNTs can help in understanding and controlling those processes which might eventually lead to develop materials with optimized efficiencies. Even though, inadequate information on the kinetics of thermal decomposition of WMCNTs has been reported in the literature, and its thermodynamics is yet to be addressed. In this regard, the present study deals with a detailed kinetic investigation on the thermo-oxidative decomposition of MWCNTs by employing advanced kinetic approaches. Kinetic analysis of MWCNTs decomposition reveals that although the kinetic triplets remain comparable, reaction model under isothermal condition is not the same as under non-isothermal conditions. It alters from contracting cylinder (non-isothermal) to random nucleation followed by isotropic growth of particles (isothermal). Thermodynamics of MWCNTs thermal decomposition points out that the process is non-spontaneous with enhanced endothermicity. In addition, the structure of activated complex is found to be relatively more organized in comparison with the reactant. An account of the interpretations of the obtained kinetic and thermodynamic parameters is also given and discussed in this study.