Calculating Electrochemical Activity of Regular-Structure Porous Electrodes with an Immobilized Enzyme

A porous electrode of regular structure with an immobilized enzyme is studied. The electrode carcass, which consists of substrate particles, is a system of two sets of mutually perpendicular planes crossing one another (cellular structure). A monomolecular layer of enzyme molecules is deployed on the inner surface of such a porous substrate. In the center of each cell of the substrate gas pores, which are cylinders of porous grains of a hydrophobizing agent one grain thick, are situated. The rest of the cell space is filled by a solid polymer electrolyte. The ultimate goal of calculations is to estimate the electrochemical activity of such an electrode. The estimation is done for an oxygen electrode with an enzyme whose characteristics are close to those of laccase. The calculation assumes that active centers of enzyme molecules undergo a direct, i.e. without participation of mediators, reduction. It is shown that at an overvoltage of 30 mV, it would be possible to obtain a current density of 0.44 A cm^–2 in an electrode 16 m thick.     

Computer Simulation of the Structure of Porous Electrodes with an Immobilized Enzyme and Nanosized Support Particles

The efficiency of the operation of a porous electrode with an immobilized enzyme is defined, in particular, by a lucky structure of its active layer, which can contain nanosized particles of the support. The composites of such a kind are prepared with the aid of methods of colloidal chemistry. The aim of this particular investigation is to perform a computer simulation of processes of coagulation of particles of the support and their possible heterocoagulation with molecules of the enzyme. Algorithms of the formation of nanocomposite structures in solution are suggested. Calculations show that the concentration of the enzyme molecules in the nanocomposite structures cannot exceed a certain critical value. On the other hand, at a fixed value of the concentration of the enzyme molecules, the concentration of the support particles must not fall below a certain threshold quantity, which provides for the passing of current through the active layer. In order for all the enzyme molecules, rather than for a fraction of these, in the composite to take part in the process of bioelectrocatalysis, the concentration of support particles must be increased even higher, to an optimum value.__________Translated from Elektrokhimiya, Vol. 41, No. 6, 2005, pp. 738–747.Original Russian Text Copyright © 2005 by Chirkov, Rostokin.     

Porous Electrodes with Immobilized Enzymes: The Fractal-Percolation Properties of Supports Manufactured from Particles of Finely Divided Colloidal Graphite

The development of a porous active layer with an immobilized enzyme of a sufficiently large thickness is one of the problems that unavoidably emerge when constructing biofuel cells with high characteristics. Mounting up the thickness can be obstructed not only by the ohmic and diffusion limitations, which have been studied well enough. One more possibility of limitations (supports manufactured from finely divided colloidal graphite, FDCG), namely a “ fractal-percolation effect,” which has recently been discovered experimentally, is discussed in the paper. The essence of the effect consists of that the particles that are constituting a porous support may gather in random fractal clusters, which are connected with one another (the percolation part of the problem) with a probability that is other than unity. As a result, the electrons that are required for performing bioelectrocatalysis are capable of penetrating into a porous support only to a limited depth. Computer simulation of the fractal and percolation processes is performed in this work. As a result, quantitative relationship of the bulk concentration of FDCG in solution with the size of random fractal clusters, with the probability of their contact with one another, and with the degree of providedness of the material of the support by electrons is established. It may happen that all this information can become useful for the development of porous electrodes with an immobilized enzyme of high activity.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 943–953.Original Russian Text Copyright © 2005 by Chirkov, Rostokin.     

Porous Electrodes with Immobilized Enzymes: The Problem of Development of Nanocomposites with High Concentrations of Molecules of Active Enzymes

The currents that are generated in a porous electrode with an immobilized enzyme increase with increasing concentration of molecules of an electrochemically active enzyme. However, a finely divided composite, which is manufactured from colloidal particles of a support that have nanodimensions and molecules of the enzyme with the aid of methods of colloid chemistry, has a peculiar structure: it consists of a set of fractal clusters, which are capable of adsorbing only a limited number of enzyme molecules. The paper is devoted to computer simulation of all the stages of immobilization of the enzyme, specifically, producing random fractal clusters of required dimensions and deploying molecules of the enzyme on them. An analysis of the link of the concentration of molecules of an active enzyme with the structure and characteristics of a porous composite makes it possible to give an interpretation to experimental facts obtained by other authors for an oxygen electrode consisting of finely divided colloidal graphite and laccase.     

Calculation of Effective Electroconductivity of Porous Electrodes with Pores Partially Filled by a Conductive Liquid

A method for calculating the effective electroconductivity of porous media filled in part by a conductive liquid is suggested. Its principal advantage is the possibility to automate calculations with the aid of a personal computer. Porous electrodes are simulated by a simple cubic lattice of links. A calculation of the dependence of the effective electroconductivity of such structures on the share of conductive links (with each link having a unit resistance) is performed. The calculation stages are as follows. (1) Links of percolation clusters are divided into dead-end links that make no contribution to electroconduction and through links. (2) A percolation cluster is freed of dead-end links and there remains a trunk that consist of through links. (3) A trunk is viewed as a bulk equivalent electric circuit, for which a computer performs the recording of all the Kirchhoff equations. (4) A computer-aided calculation of a set of Kirchhoff equations is performed, which permits the determination of the distribution of potential and current over the thickness of a porous electrode and, in particular, the magnitude of the effective electroconductivity of the porous electrode.     

Calculating Electrochemical Activity of Porous Electrodes of a Filled-up Type with an Immobilized Enzyme and Regular Gas Pores

A model for a porous electrode of a filled-up type with an immobilized enzyme, in which a system of regular gas pores is formed, is studied. The system of regular gas pores is in fact a collection of equidistant cylinders whose thicknesses are equal to one grain of the hydrophobizing agent. Usually, the gas reactant passes into a porous electrode via a number of chains that comprise porous grains of the hydrophobizing agent. The latter is distributed in the electrode bulk in a random fashion. In this case, channels for supplying gas are formed at high concentrations of the hydrophobizing agent. As a result, the number of molecules of a catalyst in a porous electrode, and along with it the current, are not great. Should we pass to electrodes with regular gas pores, the required amount of a hydrophobizing agent would decrease. As a result, the number of molecules of the enzyme in the electrode and the current would increase. The calculation of the last quantity is the subject matter to which the paper pays a major attention. The calculation is performed for two versions of distribution of a hydrophobizing agent over the electrode bulk, specifically, a random distribution and a regular distribution. Concrete calculations are carried out for an oxygen porous electrode with an enzyme whose electrochemical characteristics are close to those obtained on laccase. It was assumed in the calculations that the enzyme operates without mediators. It is established that a porous electrode with a quasi-regular structure has a considerable advantage. With an electrode thickness of 13 m one can manage to obtain currents of nearly 0.74 A cm^–2 as early as at an overvoltage of about 30 mV.     

A Dedicated Instrument for the Analysis of Blood Urea Nitrogen Using an Immobilized Enzyme Reagent

Abstract

A dedicated instrument for the quantitative determination of urea in blood serum or plasma has been developed. The instrument uses urease immobilized on a porous alumina support and a gas-detecting electrode to measure ammonia resulting from hydrolysis of urea. Reproducibility and recovery studies show a high degree of accuracy, sensitivity, and specificity. Comparison of patient samples (plasma and serum) show no statistical difference when compared to the diacetyl monoxime, Auto Analyzer method.     

Calculating the Electrochemical Activity of Porous Electrodes of a Filled-up Type with an Immobilized Enzyme

The electrochemical activity of porous electrodes of a filled-up type with an immobilized enzyme is calculated with the aid of computer-aided modeling. The percolation properties of two-component (a mixture of dispersed particles of the substrate and the enzyme) and three-component (a carrier of a gas reactant is added) models of a porous electrode are investigated. Taking into account specific features inherent in the forming macroscopic clusters (collections of particles of one sort or another connected with one another), i.e. an electron cluster and a gas cluster, makes it possible to determine the concentration of an active catalyst which is capable of taking part in the electrochemical process. The calculation of the electrochemical activity is performed in two-component structures, where the process of the current generation is limited by diffusion limitations, and in three-component structures, where the process of the current generation is limited by ohmic limitations. Estimates of the current are performed using an oxygen porous electrode with Nafion and an enzyme as an example. The electrochemical characteristics of this model electrode are close to those obtained on laccase. Introducing a hydrophobizing agent into the active layer of the porous electrode (passing from two-component structures to three-component structures) produces a positive effect. Although the number of active molecules of the enzyme drops in this case by an order of magnitude, the liquidation of diffusion limitations eventually raises the magnitude of the current by approximately threefold. Calculations show that it is quite feasible to obtain currents of 0.2 A cm^–2 on a porous electrode 16 m thick at an overvoltage of 30 mV.     

Determination of Oxalate with Immobilized Oxalate Oxidase in an Enzyme Thermistor

Abstract

This paper describes a rapid enzymic procedure, based on calorimetry, for specific determination of oxalic acid. Oxalate is oxidized by immobilized oxalate oxidase to hydrogen peroxide and carbon dioxide. The heat generated by this reaction is measured in a calorimetric device, the enzyme thermistor. Oxalate concentrations as low as 0.02 mM can be determined. Also described is purification and immobilization of the enzyme, as well as the effect of some of its inhibitors. The urinary oxalate content of 16 persons was determined using the enzyme thermistor. For samples containing a very low concentration of oxalate (less than 0.2 mM) an extraction step with tributylphosphate can be introduced to purify and concentrate the oxalate. The oxalate content in different kinds of food was also determined.     

Substrate specificity of camphor-induced cytochrome P-450 Immobilized on an electrode

The substrate specificity of a camphor-induced cytochrome P-450 (P-450_cam) was measured by using a new assay system: electrochemical control of P-450_cam activity by protein immobilization on an electrode. Immobilized P-450_cam showed the obvious substrate specificity for hydroxylation of the substrate, suggesting that the simple assay system is applicable for the study of the effect of the other components of the electron transfer system on activity. Copyright © 1998 John Wiley & Sons, Ltd.     

Al-MOF基多级孔Al2O3固定化酶反应器的构筑及构效关系研究

针对生物酶在固相载体负载后存在的催化活性与稳定性之间“此消彼长”的问题, 本工作采用“自牺牲模板”策略以铝基金属有机骨架材料(Al-MOF)为前驱体设计制备多级孔Al₂O₃ (MHAl₂O₃)材料, 再以“聚多巴胺(PDA)”仿生膜对材料表面进行功能化修饰后用以固载辣根过氧化物酶(HRP). 通过调节前驱体的煅烧温度来实现载体孔径大小的调控, 探讨了载体的孔道限域效应对固定化酶反应器催化活性的影响, 所得固定化酶反应器的热稳定性和重复使用性显著提高. 为了解析固定化酶反应器的构效关系, 采用酶动力学和热动力学参数研究了固定化酶反应器催化过程中酶与底物的相互作用, 结果表明固载后酶分子对底物的亲和性和专一性得到提升. 将固定化酶反应器用于模拟废水中苯胺黑药的催化降解时, 表现出非常高效的催化效率.     

Efficiency of Three-Layered Porous Electrodes with Variable Conductance or Variable Specific Areas of Layers: A Comparative Analysis

Efficiency of action of variable conductance and specific area of porous matrix on the current distribution is compared in identical conditions with the aid of numerical calculations based on a one-dimensional model for a porous electrode. Versions of three-layered flow-through porous electrodes with different and identical parameters are considered at different reactant conversion degrees.     

高效液相色谱-柱后固定化酶反应器-电化学检测法分析大鼠脑微透析液中的乙酰胆碱和胆碱

建立了用高效液相色谱分离－柱后固定化酶反应器酶解－电化学检测器检测酶解最终产物Ｈ２Ｏ２的方法，分析了麻醉和自由活动大鼠脑微透析液中乙酰胆碱（ＡＣｈ）和胆碱（Ｃｈ）的含量。至少在０．２～１００μｍｏｌ／Ｌ范围内ＡＣｈ和Ｃｈ的浓度与其响应的线性关系良好，它们的检测极限都可达５０ｆｍｏｌ。对高效液相色谱结合固定化酶反应器的分析方法作了简要的讨论。     

Determination of Cr(VI) Using a Pulse Amperometric Method with an Ionophore‐Immobilized Membrane Electrode

A pulse amperometric method for the determination of Cr(VI) in aqueous solution has been investigated with an ionophore‐immobilized membrane electrode. The double potential step chronoamperometry was applied to measure the current for the facilitated transfer of Cr(VI) by the membrane with N,N,N′,N′‐tetrakis(3‐aminopropyl)‐1,4‐butanediamine (DABAm4) coated on a silver electrode. The current responses were stably detected by the pulse amperometry while voltage pulses, ΔE=?240 mV, of short duration of 50 ms were employed at the interval of 5 s. In the range of 1.65–81.3 μM, Cr(VI) in water could be selectively determined in the presence of various interfering anions below 100 μM without removal of Cr(VI) from the interferences using an ion exchange column.     

Multistage Electrochemical Reactions with the Transfer of Intermediates between Near-Electrode Layer and Bulk Solution: Analysis of a Kinetic Model and Computer-Aided Modeling

Kinetic equations for a multistage electrode process complicated by the transfer of intermediates between the near-electrode layer and bulk solution are derived and analyzed. Computer-aided modeling of the kinetics of processes of this kind is performed. Conditions under which the share of diffused intermediates is no longer vanishingly small are determined. Effect of various factors on the individual stages is established. Techniques of processing experimental data for determining kinetic parameters of separate stages are suggested.     

Cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte: The effect of the flooding of pores by water on the characteristics of the active layer

A computer model of the active layer of the cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte is studied. The active mass of the electrode consists of equidimensional grains of the substrate (agglomerates of carbon particles with platinum particles embedded in them) and a solid polymer electrolyte (Nafion). The flooding by water can be experienced by both the pores in the substrate grains, which facilitate the oxygen penetration into the active layer of the electrode, and the voids between the grains. All possible versions of the flooding of these pores by water are considered. A calculation of the optimum, at a given polarization of the electrode, value of electrochemical activity, the thickness of the active layer, and the weight of platinum is performed. The major parameters of the system are the concentrations of grains of the substrate and solid polymer electrolyte, the size of these grains, the platinum concentration in the substrate grains, the average diameter of pores in the substrate grains, and the polarization of electrodes. The ultimate aim of the work is to estimate how the flooding of pores of the active layer of the cathode by water affects the magnitude of the optimum current, the effective thickness of the active layer, and the weight of platinum.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 35–47.Original Russian Text Copyright © 2005 by Chirkov, Rostokin.     

Antivitamin B12 Inhibition of the Human B12-Processing Enzyme CblC: Crystal Structure of an Inactive Ternary Complex with Glutathione as the Cosubstrate

B₁₂ antivitamins are important and robust tools for investigating the biological roles of vitamin B₁₂. Here, the potential antivitamin B₁₂ 2,4-difluorophenylethynylcobalamin (F2PhEtyCbl) was prepared, and its 3D structure was studied in solution and in the crystal. Chemically inert F2PhEtyCbl resisted thermolysis of its Co−C bond at 100 °C, was stable in bright daylight, and also remained intact upon prolonged storage in aqueous solution at room temperature. It binds to the human B₁₂-processing enzyme CblC with high affinity (K_D=130 nm ) in the presence of the cosubstrate glutathione (GSH). F2PhEtyCbl withstood tailoring by CblC, and it also stabilized the ternary complex with GSH. The crystal structure of this inactivated assembly provides first insight into the binding interactions between an antivitamin B₁₂ and CblC, as well as into the organization of GSH and a base-off cobalamin in the active site of this enzyme.     

ChemInform Abstract: Syntheses and Properties of a Family of New Compounds RE3Sb3Co2O14 (RE: La,Pr, Nd,Sm—Ho) with an Ordered Pyrochlore Structure.

As an example of the isostructural Ln₃Sb₃Co₂O₁₄ (Ln: La, Pr, Nd, Sm—Ho) series with an ordered pyrochlore structure, the La variant is prepared by a citrate complex method employing stoichiometric amounts of La(NO₃)₃, Co(NO₃)₂, and Sb tartrate together with citric acid with a metal/citrate molar ratio of 1:2 (1.