Parameters for Irreversible Inactivation of Monoamine Oxidase

The irreversible inhibitors of monoamine oxidases (MAO) slow neurotransmitter metabolism in depression and neurodegenerative diseases. After oxidation by MAO, hydrazines, cyclopropylamines and propargylamines form a covalent adduct with the flavin cofactor. To assist the design of new compounds to combat neurodegeneration, we have updated the kinetic parameters defining the interaction of these established drugs with human MAO-A and MAO-B and analyzed the required features. The K_i values for binding to MAO-A and molecular models show that selectivity is determined by the initial reversible binding. Common to all the irreversible inhibitor classes, the non-covalent 3D-chemical interactions depend on a H-bond donor and hydrophobic-aromatic features within 5.7 angstroms apart and an ionizable amine. Increasing hydrophobic interactions with the aromatic cage through aryl halogenation is important for stabilizing ligands in the binding site for transformation. Good and poor inactivators were investigated using visible spectroscopy and molecular dynamics. The initial binding, close and correctly oriented to the FAD, is important for the oxidation, specifically at the carbon adjacent to the propargyl group. The molecular dynamics study also provides evidence that retention of the allenyl imine product oriented towards FADH⁻ influences the formation of the covalent adduct essential for effective inactivation of MAO.     

Reversible and Irreversible Deactivation of Supported Bimetallic Catalysts for the Dehydrogenation of Lower Paraffins

The reasons and nature of the deactivation of spinel-supported bimetallic Pt–Sn, Pt–In, and Pt–Cu catalysts in the steam dehydrogenation of C₄–C₅paraffins are analyzed. The deactivation can be reversible and irreversible. Reversible deactivation is associated with the effects of the composition of reaction medium and gaseous medium for preliminary treatment on the surface composition of supported bimetallic alloys and with the coking of catalysts. Irreversible deactivation can result from the irreproducibility of the phase composition and dispersion of supported alloys in dehydrogenation–regeneration cycles and from the carbon erosion of catalysts. This erosion consists in the removal of active metal particles to the bulk of carbon deposits. The formation of carbon deposits on the surfaces of spinel supports and bimetallic catalysts is considered. Recommendations for optimizing the composition and the operating conditions of catalysts are given in order to reduce the effects of factors that result in reversible and irreversible deactivation.     

Direct and Cytochrome c Mediated Electrochemistry of Bilirubin Oxidase on Gold

Direct electron transfer (DET) of bilirubin oxidase from Myrothecium verrucaria (BOD) was established on promoter‐modified gold electrodes. The electrochemical behavior of the enzyme in solution was studied by means of cyclic voltammetry evaluating the biocatalytic reduction of dioxygen. The reaction of BOD at Au electrodes was shown to be efficient only at low pH. In addition, a novel interaction between BOD and cytochrome c (cyt.c) was found. It was shown that BOD efficiently accepts cyt.c as an electron donor in both cases when cyt.c is in solution and electrostatically adsorbed. The results suggest that cyt.c can play the role of a mediator facilitating electron transfer in a pH range where no DET could be observed between the enzyme and the electrode. For the interaction between cyt.c and BOD in solution the reaction kinetics has been studied electrochemically and spectrophotometrically.     

Enzymatic Oxygen Microsensor Based on Bilirubin Oxidase Applied to Microbial Fuel Cells Analysis

A selective oxygen biosensor based on bilirubin oxidase (BOx) was developed. The sensor was used for determining oxygen profiles in a membraneless, single‐chamber microbial fuel cell (SCMFC), fed with raw wastewater. The linear response of the sensor was optimized by a diffusion layer of silica gel. A computer‐controlled stage was used to obtain accurate and precise measurements. Oxygen concentration in biofilms covering electrodes was measured, showing 3 mg L^?1 of O₂ in the bulk solution, decreasing to 0 mg L^?1 in the cathodic biofilm. The MFC generated power in the range of 0–0.08 mW, associated to the oxygen content.     

A Carbon Nanotube Layered Electrode for the Construction of the Wired Bilirubin Oxidase Oxygen Cathode

Oxidatively treated carbon nanotubes were coated on a glassy carbon surface to form a CNT‐layer. On the CNT‐layered GC surface, a redox hydrogel film of the copolymer, of polyacryamide and poly(N‐vinylimidazole) complexed with [Os(4,4′‐dichloro‐2,2′‐bipyridine)₂Cl]^+/2+ wiring bilirubin oxidase was immobilized. A good contact was achieved between the hydrogel film and the hydrophilic CNT‐layer with carboxylated CNTs. The prepared bilirubin oxidase cathode on the CNT‐layer was employed for the electrocatalytic reduction of O₂, and enhanced current and stability were observed. Electron transfers from the electrode surface O₂ molecules were analyzed. The optimal composition of the enzyme, redox polymer, and cross‐linker in the catalyst and the thickness of the CNT‐layer were determined.     

Bilirubin Oxidase‐Based Nanobiocathode Working in Serum‐Mimic Buffer for Implantable Biofuel Cell

Low‐density graphite electrodes have been covalently modified with gold nanoparticles and used as scaffold for the oriented immobilization of Myrothecium verrucaria bilirubin oxidase. The developed nanobioelectrodes have been tested as a biocathode in a human‐serum surrogate buffer that mimics physiological conditions. The nanostructured bioelectrode offered a ?140 µA cm^?2 current density under serum‐mimic conditions at 0.2 V, and was operational during at least 6 days.     

Tin underpotential deposition on platinum and its catalytic influence on the kinetics of molecular oxygen electroreduction

The formation and dissolution of tin ad-layers on polycrystalline platinum were analysed by cyclic voltammetry in aqueous 10^–4 M tin(II)/1 M sulfuric acid in the 0.05–0.70 V versus RHE range. At this concentration level it was possible to observe that platinum sites involving (110) planes are mainly related to tin underpotential deposition. In contrast to previous results, no irreversible adsorption was found in the course of the electrodeposition. Thermodynamic adsorption parameters were calculated from the potential dependence of tin surface coverage. Catalytic properties of this new surface were studied on the basis of oxygen electroreduction as a model. Kinetic runs were performed with rotating ring-disk electrodes on bare and tin-modified platinum surfaces. Molecular oxygen reduction on tin-modified platinum takes place through the production of both water and hydrogen peroxide. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electronic Publication     

The mechanism of the electroreduction of alkenyldimethylsulfonium salts

The potential of the electroreduction of alkenyldimethylsulfonium salts and the preferable direction of C-S bond cleavage are determined to a considerable extent by the position of the double bond in the alkenyl substituent.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2159–2162, December, 1994.     

Reversible Deactivation of Enzymes by Redox‐Responsive Nanogel Carriers

Novel redox‐responsive polymeric nanogels that allow highly efficient enzyme encapsulation and reversible modulation of enzyme activity are developed. The nanogel synthesis and encapsulation of enzyme are performed simultaneously via in situ crosslinking of pyridyldisulfide‐functionalized water‐soluble reactive copolymers, which are synthesized via reversible addition–fragmentation chain transfer copolymerization. Obtained nanogels with loaded cellulase demonstrate very good colloidal stability in aqueous solutions. The enzymatic activity of cellulase is greatly reduced when encapsulated in the nanogels and rapidly recovered in 10 × 10⁻³ m dithiothreitol solution. Fluorescence resonance energy transfer (FRET)‐based experiments indicate that the recovered enzymatic activity is mainly ascribed to the release of the enzyme due to the degradation of the disulfide crosslinking network after addition of dithiothreitol (DTT), instead of the enhanced substrate transport rate. The developed enzyme immobilization method opens new possibilities for reversible activation/deactivation of enzymes and opens up new directions for targeted protein therapy and biotechnology applications.

     

单滴汞上可逆和不可逆过程电流～时间曲线方程式

本文提出了受扩散控制的i_t与tβ的关系式以及计算β值的正确表达式,讨论了同时受扩散和电极反应速率控制的极谱电流与时间的关系,利用所得的理论公式可方便地求得标准电极反应速率常数(k_s),理论结论与实验相符。     

Characterization and Potential Applications of Immobilized Glucose Oxidase and Polyphenol Oxidase

Pyrrole functionalized polystyrene (PStPy) was copolymerized with pyrrole to obtain a conducting copolymer, P(PStPy‐co‐Py) which is used as the immobilization matrix. Glucose oxidase and polyphenol oxidase enzymes were immobilized via the entrapment method by electrochemical polymerization. Enzyme electrodes were prepared by electrolysis at a constant potential using sodium dodecyl sulfate (SDS) as the supporting electrolyte during the copolymerization of PStPy with pyrrole. Maximum reaction rates (V_max) and enzyme affinities (Michaelis‐Menten constants, K_m) were determined for the enzyme entrapped both in polypyrrole (PPy) and P(PStPy‐co‐Py) matrices. Optimizations of enzyme electrodes were done by examining the effects of temperature and pH on enzymes' activities along with the shelf life and operational stability investigations. Glucose oxidase enzyme electrodes were used for human serum analysis and glucose determination in two brands of orange juices. Polyphenol oxidase enzyme electrodes were used for the determination of phenolics in red wines of Turkey.     

Reversible and Irreversible Chemisorption in Nonporous‐Crystalline Hybrids

The tools of synthetic chemistry allow us to fine‐tune the reactivity of molecules at a level of precision not yet accessible with inorganic solids. We have investigated hybrids that couple molecules to the superior thermal and mechanical properties of solids. Herein we present, to the best of our knowledge, the first demonstration of reactivity between hybrid perovskites and substrates. Reaction with iodine vapor results in a remarkable expansion of these materials (up to 36 % in volume) where new covalent C? I bonds are formed with retention of crystallinity. These hybrids also show unusual examples of reversible chemisorption. Here, solid‐state interactions extend the lifetime of molecules that cannot be isolated in solution. We have tuned the half‐lives of the iodinated structures from 3 h to 3 days. These nonporous hybrids drive substrate capture and controlled release through chemical reactivity. We illustrate the strengths of the hybrid by considering radioactive iodine capture.     

Reversible Deactivation Radical Polymerization of Monomers Containing Activated Aziridine Groups

Polymers bearing activated aziridine groups are attractive precursors to α‐substituted‐β‐amino‐functionalized materials due to the enhanced reactivity of the pendant aziridine functionalities toward ring‐opening by nucleophiles. Two aziridine‐containing styrenic monomers, 2‐(4‐vinylphenyl)aziridine (VPA) and N‐mesyl‐2‐(4‐vinylphenyl)aziridine (NMVPA), were polymerized under a variety of reversible deactivation radical polymerization conditions. Low‐catalyst‐concentration atom transfer radical polymerization (LCC‐ATRP) and reversible addition‐fragmentation chain‐transfer (RAFT) polymerization were ineffective at producing well‐defined polymers from VPA due to side reactions between the aziridine functionalities and the agents controlling the polymerizations (catalysts or chain transfer agents). PolyVPA produced under nitroxide‐mediated polymerization (NMP) conditions had narrow molecular weight distribution at low to moderate conversions of monomer, but branched and eventually cross‐linked polymers were formed at higher conversions due to ring‐opening reactions of the aziridine groups. Most of these undesirable side reactions were eliminated by attaching a methanesulfonyl (mesyl) group to the aziridine nitrogen atom, and well‐defined linear homopolymers with targeted molecular weights were realized from NMVPA under RAFT and NMP conditions; however, side reactions between the aziridine groups and the catalyst in LCC‐ATRP still occured and the polymerization was uncontrolled using this technique.

     

氧在振动球磨石墨上阴极还原动力学的研究

石墨具有很好的导电性和化学稳定性而被广泛地用作电极材料,细粉状石墨对氧的电化学还原表现出相当好的催化活性^[1],氧在碳上的还原机理已经有许多人进行过研究[2,3],本工作中利用在真空条件下高速振动制备了超细振动球磨石墨,并运用旋转盘环电极技术研究了氧在这种石墨上的电化学还原机理。     

甲壳胺-羧甲基纤维素钠高分子复合物固定酶的研究

本文采用天然高分子甲壳胺与羟甲基纤维素钠的高分子复合物作哉体,固定葡萄糖氧化酶和纤维素酶。得到的固定化酶活性较高、热稳定性好。固定化后的葡萄糖氧化酶比固定化前的最适反应温度提高50℃,固定化后的纤维素酶最适反应温度提高20℃。固定酶的贮存稳定性明显提高,并可反复使用。     

聚吡咯葡萄糖氧化酶电极的生物电化学响应

采用分步骤合过程，制备了以聚吡咯膜为载体的葡萄糖氧化酶电极，探讨了其生物电化学响应特性，计算了酶催化反应的有关动力学参数。与溶解态酶相比，该电极表现出良好的生物电化学特征，而且酶蛋白对溶液温度的稳定性有显著提高。     

Comparison of Critical Exponents Determined by Rheology and Dynamic Light Scattering on Irreversible and Reversible Gelling Systems

Summary: The sol-gel transition of a radical chain cross-linking copoly-merization system [N-vinylcaprolactam/2-hydroxylethyl methacrylate/allyl-methacrylate] and various thermoreversible gelling systems (mixtures made of xanthan gum and locust bean gum as well as gelatin) have been studied using in-situ time-resolved dynamic light scattering (DLS) and in-situ rheology. A critical dynamical behavior was observed near the sol-gel transition, which is characterized by the presence of a power-law spectra in the time-intensity correlation function g₂(t)−1 ∝ t^−µ and in the low-amplitude oscillatory shear experiment G′(ω) ∝ G″(ω) ∝ ωⁿ. A comparison of the obtained critical dynamical exponents µ and n were made according to the theory by Doi and Onuki. This theory predicts a relation between these exponents, but up to now no detailed experimental comparison was done in the past. It was found that for all investigated systems n > µ.     

Study on the United Glucose Oxidase Electrode Constructed by Composite Electrode

A novel type of united glucose oxidase (GOD) electrode was designed. Glucose oxidase and ferrocene (Fc), which was a mediator, were added into the composite electrode that was constructed by graphite powder, acetylene black, and epoxy resin. These three materials in composite electrode kept constant proportion in weight. And the optimum amounts of GOD and Fc among united enzyme electrode were 5% and 2%, respectively. The glucose was detected linearly in the concentration range 0.01–9.0 mM with a 20-s steady-state response time and 36 nA/mM of the sensitivity at 0.15 V applied potential. And electrode fouling problem and the response current from the interferents were avoided. The response current of the united GOD electrode had no obvious deterioration within 80 days when stored at 4°C in a refrigerator. The detecting results of human serum by the united GOD electrode had good consistency with that by standard enzyme method. The maximum deviation between these two detecting values was 5%. It might be used for detecting the blood sugar in clinical assay.     

Electrochemical Pretreatment of Amino‐Carbon Nanotubes on Graphene Support as a Novel Platform for Bilirubin Oxidase with Improved Bioelectrocatalytic Activity towards Oxygen Reduction

The electrochemical conditioning of amino‐carbon nanotubes (CNTs) on a graphene support in an alkaline solution is used to produce ?NHOH as hydrophilic functional groups for the efficient immobilization of bilirubin oxidase enzyme. The application of the immobilized enzyme for the direct electrocatalytic reduction of O₂ is investigated. The onset potential of 0.81 V versus NHE and peak current density of 2.3 mA cm^?2 for rotating modified electrode at 1250 rpm, indicate improved biocatalytic activity of the proposed system for O₂ reduction.     

Comparison of Oxalate Oxidase Enzyme Electrodes for Urinary Oxalate Determinations

Abstract

Enzyme electrodes for oxalate were constructed by immobilizing oxalate oxidase enzyme (E, C, 1,2,3,4) on both potentiometric carbon dioxide and amperometric hydrogen peroxide sensors. These systems were optimized, and the response characteristics of the two biosensors examined in a comparison study. Areas investigated include linear range, calibration curve slope, response time, limit of detection, and lifetime. Selectivity studies were carried out with the system and showed no measurable response to millimolar levels of various L-amino acids, metal ions or ascorbic acid.

This comparison was extended into the choice of an optimal sensor for urinary oxalate determinations with minimal sample pretreatment. The peroxide sensor performed best in urine samples, allowing for the 1:40 dilution necessary to minimize the effect of any interferents present. At this or greater dilution, the recovery of standard additions of oxalate averaged 95.5%, with an average relative standard deviation of 4.9%. The system retained its activity throughout two weeks of continued use.