New Aspects of Protein‐film Voltammetry of Redox Enzymes Coupled to Follow‐up Reversible Chemical Reaction in Square‐wave Voltammetry

Protein‐film square‐wave voltammetry of uniformly adsorbed molecules of redox lipophilic enzymes is applied to study their electrochemical properties, when a reversible follow‐up chemical reaction is coupled to the electrochemically generated product of enzyme's electrode reaction. Theoretical consideration of this so‐called “surface ECrev mechanism” under conditions of square‐wave voltammetry has revealed several new aspects, especially by enzymatic electrode reactions featuring fast electron transfer. We show that the rate of chemical removal/resupply of electrochemically generated Red(ads) enzymatic species, shows quite specific features to all current components of calculated square‐wave voltammograms and affects the electrode kinetics. The effects observed are specific for this particular redox mechanism (surface ECrev mechanism), and they got more pronounced at high electrode kinetics of enzymatic reaction. The features of phenomena of “split net‐SWV peak” and “quasireversible maximum”, which are typical for surface redox reactions studied in square‐wave voltammetry, are strongly affected by kinetics and thermodynamics of follow‐up chemical reaction. While we present plenty of relevant voltammetric situations useful for recognizing this particular mechanism in square‐wave voltammetry, we also propose a new approach to get access to kinetics and thermodynamics of follow‐up chemical reaction. Most of the results in this work throw new insight into the features of protein‐film systems that are coupled with chemical reactions.     

Quasi-Steady-State Laws in reversible model of enzyme kinetics

In enzyme kinetics, the Quasi-Steady-State Assumption (QSSA) has been proposed for over 80 years, which plays a very important role in simplifying systems of equations derived from chemical reactions with enzymes. Five years ago, we proved that the QSSA is always true in the simplest model with the second elementary reaction irreversible, and called them as Quasi-Steady-State Laws. Thus, all conclusions based on QSSA have a solid foundation in irreversible case. However, the chemical reactions are not always so simple in many life processes. The second elementary reaction should be reversible in general, and the irreversible case is actually only an approximation. So it is more important and interesting to study the reversible case, and it has already attracted enzymologists for a long time. The basic assumption, i.e. QSSA in this general case has appeared in 1930. We proved this lasting over 80 years assumption in this paper.     

绝热式热动力学的研究4: 可逆反应的热谱解析

本文建立了可逆反应在绝热系统中的热动力学研究法, 并用自制的绝热式自动热量计, 研究了三个可逆反应体系的热动力学, 验证了本文方法的正确性。     

The regularities of reversible chain reactions in the equilibrium state

The characteristics of dynamic equilibrium states in the experimentally studied reversible chain reactions of quinoneimines with hydroquinones and in some reversible chain reactions with similar mechanisms are discussed. The concentrations of radicals and non-radical participants were calculated. The equilibrium concentrations of the same reaction participants depend only on the initial reactant concentrations, being independent of the number of chain initiation-chain termination steps in the reaction mechanism. The results of mathematical modeling of reversible chain reactions using the experimentally determined rate constants for elementary steps of a reaction in the quinoneimine-hydroquinone system are presented. Expressions relating the equilibrium constants of elementary steps to each other and to the equilibrium constant of the total stoichiometric reaction are derived. Examples of other actual reversible chain reactions are presented, indicating that such reactions are widespread.     

Reaction Kinetics of 2-Propanol Dehydration in Supercritical Water

A study of the kinetics and mechanism of chemical reactions in supercritical fluids is considered. An experimental procedure was proposed for examining reversible chemical reactions in supercritical water. The reaction kinetics of 2-propanol dehydration in supercritical water was studied. It was found that the uncatalyzed reactions of olefin hydrogenation by hydrogen dissolved in supercritical water occur at high rates near the critical point of water. The experimental data on the dehydration of 2-propanol in supercritical water are adequately described by first-order reaction rate equations. The rate constants and activation energies of 2-propanol dehydration near the critical point of supercritical water were found.     

Re-examination of reversibility in reaction models for the spontaneous emergence of homochirality

The concept of reversibility in complex chemical reaction networks has recently been introduced in discussions concerning the origin of biological homochirality. In computational studies drawing on an analogy to recent experimental studies involving reversible crystallization processes, recycling of reaction educts has been suggested to provide a driving force for the spontaneous emergence of homochirality. We demonstrate here that reversible reaction networks closed to mass flow lead inexorably to a racemic state for thermally driven reactions, which must adhere to the principle of microscopic reversibility. This conclusion was reached for analogous "triangle reaction" networks studied by Onsager in 1931. Special cases such as photochemical reactions offer an exception that may have prebiotic relevance. Fundamental differences between physical and chemical systems are discussed in order to clarify the role of reversibility in each case.     

Para-phenylenediamine oxidation at a platinum electrode in acetonitrile solutions

p-Phenylenediamine oxidation at platinum electrodes in acetonitrile solutions has been studied under a very wide range of experimental conditions. Chronopotentiometry, rotating disc electrode and cyclic voltammetry were used as electrochemical techniques. Coulometry at constant potential and product analysis were also performed.The electrochemical reaction appears as a fast and reversible one electron exchange per molecule of PPD. The electrode reaction is further complicated by follow-up chemical reactions giving unknown products in the bulk of the solution.The whole polarization curve under steady state conditions shows two waves, while under non-steady state conditions a small wave at intermediate potentials is also apparent.The reaction pathway for the first wave was interpreted as a non-conventional e.c.e. mechanism where the parent molecule acts as a base in the chemical step.These assumptions were confirmed through experiments performed with pyridine or water addition.     

Digital simulation of electrochemical processes involving very fast chemical reactions : Part 1. A simple general approach

A simple approach based on the steady-state assumption and linear concentration profiles in the reaction layer is proposed to simulate electrochemical systems involving very fast homogeneous chemical reactions. The flux equations are derived in detail and the method is shown to be suitable for catalytic e.c.e and dimerization reaction mechanism. The accuracy of the proposed method is checked by comparing the results obtained with those already available in the literature.     

Acid/base-regulated reversible electron transfer disproportionation of N–N linked bicarbazole and biacridine derivatives

Regulation of electron transfer on organic substances by external stimuli is a fundamental issue in science and technology, which affects organic materials, chemical synthesis, and biological metabolism. Nevertheless, acid/base-responsive organic materials that exhibit reversible electron transfer have not been well studied and developed, owing to the difficulty in inventing a mechanism to associate acid/base stimuli and electron transfer. We discovered a new phenomenon in which N–N linked bicarbazole (BC) and tetramethylbiacridine (TBA) derivatives undergo electron transfer disproportionation by acid stimulus, forming their stable radical cations and reduced species. The reaction occurs through a biradical intermediate generated by the acid-triggered N–N bond cleavage reaction of BC or TBA, which acts as a two electron acceptor to undergo electron transfer reactions with two equivalents of BC or TBA. In addition, in the case of TBA the disproportionation reaction is highly reversible through neutralization with NEt₃, which recovers TBA through back electron transfer and N–N bond formation reactions. This highly reversible electron transfer reaction is possible due to the association between the acid stimulus and electron transfer via the acid-regulated N–N bond cleavage/formation reactions which provide an efficient switching mechanism, the ability of the organic molecules to act as multi-electron donors and acceptors, the extraordinary stability of the radical species, the highly selective reactivity, and the balance of the redox potentials. This discovery provides new design concepts for acid/base-regulated organic electron transfer systems, chemical reagents, or organic materials.     

Complexity index for the linear mechanisms of chemical reactions

A complexity measure is proposed for the kinetic models of chemical reactions with linear mechanisms. The index is related to the structure of fractional-rational kinetic laws for chemical reactions, as well as to the structure of cyclic graphs used to describe them. The complexity index is shown to be closely related to the detailed hierarchical classification and to the code of linear reaction mechanisms, recently introduced. A number of index properties are proved for two- and three-reaction routes. They reflect the influence of the various classification criteria, such as the number of reaction routes and intermediates, the type, class and subclasses of the mechanism, and the number of intermediates in each reaction route. Hierarchical levels of mechanisms with the same complexity (isocomplex mechanisms) are specified. Standard tables are presented with complexity indices for all topologically distinct linear reaction mechanisms having one to three reaction routes, two to six intermediates, and reversible elementary steps.[/p]     

Mechanisms of electrochemically-induced retro-cyclopropanation reactions of fullerene derivatives using digital simulations

Three C(60) derivatives, 1, 2 and 3, have been studied by cyclic voltammetry (CV) under high vacuum in anhydrous tetrahydrofuran (THF). The CV behavior was essentially similar to that already observed for other cyclopropanated fullerene derivatives. After the second reduction processes all compounds undergo a chemical reaction that generates another electroactive species. This "new" chemical species is likely to be the compound with the cyclopropane ring open. Differences in CV behavior were observed for the different addends. Electrochemical data obtained at different scan rates for a given potential window, were fit with the BAS digital simulation program, DigiSim. The purpose of this study was to probe the proposed mechanisms and to obtain reliable estimations of the kinetic constants for the homogeneous chemical reactions taking place during the CV experiments. Calculations at the PM3 level lend additional support to the conclusions derived from digital simulations. The proposed mechanism is similar for all the compounds and involves two main chemical reactions in a reversible square scheme.     

Reversibility of electrode reactions involving organic compounds

A general empirical approach allowing one to describe the kinetics and evaluate the mechanism of the electrode electron transfer reactions is offered. The approach is based on the electrode potentials, the vertical ionization potentials (oxidation), and the affinity to electron (reduction). An equation linking kinetic and thermodynamic parameters is derived. Electrode reactions involving organic compounds are discussed in polarographic terms. The conclusion is drawn that most electron transfer reactions involving organic compounds are reversible, and that the irreversibility of the net electrode reaction is due to the irreversibility of subsequent chemical and electrochemical stages. An experimental observation of the slow electron transfer is possible in the cases of a substantial reorganization of molecules in the presence of fast subsequent chemical and electrochemical reactions.     

Peculiarities of Ethylene Polymerization Reactions with Heterogeneous Ziegler–Natta Catalysts: Kinetic Analysis

The previously developed kinetic scheme for olefin polymerization reactions with heterogeneous Ziegler–Natta catalysts states that the catalysts have several types of active centers which have different activities, different stabilities, produce different types of polymer materials, and respond differently to reaction conditions. In the case of ethylene polymerization reactions, each type of center exhibits an unusual chemical feature: a growing polymer chain containing one ethylene unit, Ti—C₂H₅, is unusually stable and can decompose with the formation of the Ti—H bond. This paper examines quantitative kinetic ramifications of this chemical mechanism. Modeling of the complex kinetics scheme described in the Scheme demonstrates that it correctly and quantitatively predicts three most significant peculiarities of ethylene polymerization reactions, the high reaction order with respect to the ethylene concentration, reversible poisoning with hydrogen, and activation in the presence of α‐olefins.     

Stability of solutions to a reaction diffusion system based upon chemical reaction kinetics

In this paper, we deal with the stability problem to some mathematical models that describe chemical reaction kinetics. One is a set of ordinary differential equations induced by one reversible chemical reaction mechanism containing three chemical species. The other is a set of reaction diffusion equations based on the same chemical reaction. We show that all solutions of the model are asymptotically stable by applying the Liapunov method. We thus find that the concentration of each species has certain limits as time proceeds.      

Electrochemistry of quinizarine adsorbed on a glassy carbon electrode in aqueous solutions

The electrochemistry of quinizanne in aqueous solutions, adsorbed on electrochemically pretreated glassy carbon electrodes, is described. The anodic and cathodic reversible reactions are followed by physical and chemical processes, respectively. The desorption of the fully oxidized species from the electrode was assigned to the follow-up process in the anodic region. The follow-up reaction at the more negative potentials is ascribed to tautomerization. in which the central hydroxyl group(s) participate. When the range of the reduction is expanded, two additional irreversible reduction waves are observed. A full mechanism of the electrode reactions is proposed. Intramolecular hydrogen bonds and the pH of the aqueous solutions play an important role in the rationalization of the overall mechanism. The “electro-chemical label” concept is used to study the progress of the electrode reactions, and to evaluate the intermediates and the products which are confined to the electrode surface.     

An analytical approach to solutions of master equations for stochastic nonlinear reactions

In this paper we consider a class of nonlinear reactions which are important in stochastic reaction networks. We find the exact solution of the chemical master equation for a class of irreversible and reversible nonlinear reactions. We also present the explicit form of the equilibrium probability solution of the reactions. The results can be used for analyzing stochastic dynamics of important reactions such as binding/unbinding reaction and protein dimerization.     

Slow manifold for a bimolecular association mechanism

Finding the slow manifold for two-variable ordinary differential equation (ODE) models of chemical reactions with a single equilibrium is generally simple. In such planar ODEs the slow manifold is the unique trajectory corresponding to the slow relaxation of the system as it moves towards the equilibrium point. One method of finding the slow manifold is to use direct iteration of a functional equation; another method is to obtain a series solution of the trajectory differential equation of the system. In some cases these two methods agree order-by-order in the singular perturbation parameter controlling the fast relaxation of the intermediate (complex). However, de la Llave has found a model ODE where the series method always diverges. Bimolecular association is an example of a chemical reaction where the series method for finding the slow manifold diverges but the iterative method converges. In this mechanism a complex is formed which can then undergo unimolecular decay, i.e., [reaction: see text]. The kinetics of this reaction are investigated and its properties compared with two other two-step mechanisms where series expansion and iteration methods are equivalent: the Michaelis-Menten mechanism for enzyme kinetics, and the Lindemann-Christiansen mechanism of unimolecular decay in gas kinetics.     

Variational sensitivity analysis of a photochemical smog mechanism

The variational method of sensitivity analysis is applied to a recent chemical kinetic mechanism for photochemical smog. It is demonstrated that the variational method is capable of readily handling a large mechanism. The analysis can be applied to a single chemical species or to an arbitrarily large group of species contained in an objective function. Examples of the use of different objective functions are given. The variational method provides temporally distributed sensitivity coefficients. Therefore, the results are presented as a time-dependent ranking of reactions in orderof decreasing importance with respect to the objective function species. This ranking permits the time-dependent relative influence of each reaction to be readily determined by inspection. Particular attention is paid to reactions governing daytime O₃ formation and nighttime NO₃ and HONO formation.     

Monoesterification of styrene–maleic anhydride copolymers with alcohols in ethyl benzene: Catalysis and kinetics

A kinetic investigation on the monoesterification reaction of the maleic anhydride residue (MA) in styrene-maleic anhydride copolymers with aliphatic alcohols was carried out in ethyl benzene solution. By comparison to classic catalysts such as tributylamine (TBA) and pyridine, 4-dimethylaminopyridine (4DMAP) is by far the most effective catalyst for this reaction. While both general base and nucleophilic mechanisms contribute to the reaction catalyzed by TBA or pyridine, a nucleophilic mechanism prevails with 4DMAP. This reaction is reversible, and its chemical equilibrium constant decreases significantly with increasing temperature. Both kinetic and thermodynamic results showed that in the presence of 4DMAP, the forward and reverse reactions are second and first order, respectively. The existence of side reactions, reactivity of two styrene-maleic anhydride copolymers of different MA contents as well as two aliphatic alcohols of different lengths are also addressed. © 1993 John Wiley & Sons, Inc.     

A theoretical study of the gas-phase chemi-ionization reaction between uranium and oxygen atoms

The U+O chemi-ionization reaction has been investigated by quantum chemical methods. Potential-energy curves have been calculated for several electronic states of UO and UO(+). Comparison with the available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemi-ionization reaction U+O-->UO(+)+e(-) is proposed. The U+O and Sm+O chemi-ionization reactions are the first two metal-plus-oxidant chemi-ionization reactions to be studied theoretically in this way.