对锰离子参与类Fenton反应机理的研究

锰主要以Mn2 形式存在,有人发现其具有与其他过渡性金属离子截然相反的抗氧化活性,采用自旋捕捉-ESR技术、芳环羟基化反应-高效液相色谱(HPLC)法和琼脂糖电泳法三种方法研究Mn2 参与类Fenton反应的情况时,均检测到Mn2 与H2O2反应产生.OH,Mn2 与H2O2反应可以发生类Fenton反应,产生.OH。这一现象的产生可能是Mn2 引起生物体内氧化损伤之故。同时显示,Mn2 的类Fenton反应是否产生.OH与反应过程Mn2 以及其他成分浓度有关(如高浓度抑制,低浓度促进),为诸多文献中Mn2 作为促氧化剂还是抗氧化剂的争论提供了可能解释。同时Mn2 能引起.OH持续低量的产生为一些慢性疾病的发生提供了合理的解释。     

Physico-chemical mechanism of the argyrophil III reaction

Summary Because there are several points of physico-chemical similarity between the argyrophil I reaction (formation of metallic silver grains by reducing groups of the tissue) and the argyrophil III reaction (formation of metallic silver grains by reducing groups existing in a dissolved state) a similarity between their mechanisms is also assumed. The electrochemical half processes of the argyrophil III reaction (i.e. the transformation of tissue-adsorbed reducing molecules into their oxidized form, and the reduction of silver ions to silver atoms) take place separately in space, while the electrons released in the former half reaction are transported by the semiconduction bands of the tissue to the catalytic points where the metallic silver grains are forming.     

Thermodynamics of the mechanism of the nitrogenase reaction

The fixation of molecular nitrogen by nitrogenase requires a lot of energy because 16 mol of ATP are hydrolyzed per mole of nitrogen converted to ammonia. Kim and Dees determined the crystallograpic structure of nitrogenase and this has led to a three-step mechanism that involves Feprotein and MoFeprotein in addition to ferredoxin. Each of these steps can be interpreted in terms of two half reactions that are connected through their transfer of electrons. Estimates can be made of the standard apparent reduction potentials of these three steps and their dependencies on pH and ionic strength. This mechanism is compared with the same type of analysis of an alternative three-step mechanism in which the hydrolysis of ATP is coupled with the reduction of molecular nitrogen, rather than the reduction of Feprotein. The problem with the first mechanism is that the second step produces 12 mol of hydrogen ions per mole of nitrogen fixed and the third step consumes 10 mol of hydrogen ions per mole of nitrogen fixed. The alternative mechanism does not have this problem.     

Physico-chemical mechanism of the argyrophil I reaction

Summary Kinetic experiments have shown that the argyrophil I reaction (the formation of metallic from ionic silver by reducing groups of the tissues) is a catalytic process. Topochemical considerations, and several reaction kinetic observations, suggest that the semi-conductor properties and the favourable chemical structure of certain sites (catalytic points) of the tissue structure play a fundamental role in the catalysis. The electrochemical half processes in the argyrophil I reaction (i.e., the transformation of tissue-bound reducing groups into their oxidized form and the reduction of silver ions into silver atoms) take place separately in space, while the electrons released in the former half reaction are transported by the semi-conduction bands of the tissue to the catalytic points where the metallic silver grains are formed.     

Physico-chemical mechanism of the argyrophil I reaction

Kinetic experiments have shown that the argyrophil I reaction (the formation of metallic from ionic silver by reducing groups of the tissues) is a catalytic process. Topochemical considerations, and several reaction kinetic observations, suggest that the semi-conductor properties and the favourable chemical structure of certain sites (catalytic points) of the tissue structure play a fundamental role in the catalysis. The electrochemical half processes in the argyrophil I reaction (i.e., the transformation of tissue-bound reducing groups into their oxidized form and the reduction of silver ions into silver atoms) take place separately in space, while the electrons released in the former half reaction are transported by the semi-conduction bands of the tissue to the catalytic points where the metallic silver grains are formed.     

On the mechanism of the peroxidase-catalyzed oxygen-transfer reaction

We reported evidence that horseradish peroxidase (HRP) and chloroperoxidase (CPO) catalyze oxygen transfer from H2O2 to thioanisoles [Kobayashi, S., Nakano, M., Goto, T., Kimura, T., & Schaap, A. P. (1986) Biochem. Biophys. Res. Commun. 135, 166-171]. In the present paper, the reaction mechanism of this oxygen transfer is discussed. The oxidation of para-substituted thioanisoles by HRP compound II showed a large negative rho value of -1.46 vs. the sigma + parameter in a Hammett plot. These results are in accord with the formation of a cation radical intermediate in the rate-determining step. Hammett treatments for HRP- and CPO-dependent S-oxygenations did not provide unequivocal proofs to judge the reaction mechanism, because of the poor correlations for sigma + and sigma p parameters. Different behavior was found in kinetics and stereoselectivity between the two enzymes. Results in the present study and recent studies strongly suggested the formation of a cation radical intermediate. The oxygen atom would transfer by reaction of compound II and the cation radical intermediate. Although involvement of the cation radical was not confirmed in the CPO system, a similar mechanism was proposed for CPO.     

Prenyltransferase: the mechanism of the reaction.

The enzyme, prenyltransferase, which normally catalyzes the addition of an allylic pyrophosphate to isopentenyl pyrophosphate, has been found to catalyze the hydrolysis of its allylic substrate. The rate of this hydrolysis is markedly stimulated by inorganic pyrophosphate. Competition experiments with 2-fluoroisopentenyl pyrophosphate and inorganic pyrophosphate demonstrated that inorganic pyrophosphate stimulated hydrolysis by binding at the isopentenyl pyrophosphate site. Hydrolysis carried out in H218O or with (1S)-[1-3H]geranyl pyrophosphate show the C-O bond is broken and the C1 carbon of geranyl pyrophosphate is inverted in the process. These results are interpreted to favor a carbonium ion mechanism for the prenyltransferase reaction.     

Study on the reaction mechanism and the static injection chemiluminescence method for detection of acetaminophen

Acetaminophen, also called paracetamol, is found in Tylenol, Excedrin and other products as over–the‐counter medicines. In this study, acetaminophen as a luminol signal enhancer was used in the chemiluminescence (CL) substrate solution of horseradish peroxidase (HRP) for the first time. The use of acetaminophen in the luminol–HRP–H₂O₂ system affected not only the intensity of the obtained signal, but also its kinetics. It was shown that acetaminophen was to be a potent enhancer of the luminol–HRP–H₂O₂ system. A putative enhancement mechanism for the luminol–H₂O₂–HRP–acetaminophen system is presented. The resonance of the nucleophilic amide group and the benzene ring of acetaminophen structure have a great effect on O‐H bond dissociation energy of the phenol group and therefore on phenoxyl radical stabilization. These radicals act as mediators between HRP and luminol in an electron transfer reaction that generates luminol radicals and subsequently light emission, in which the intensity of CL is enhanced in the presence of acetaminophen. In addition, a simple method was developed to detect acetaminophen by static injection CL based on the enhanced CL system of luminol–H₂O₂–HRP by acetaminophen. Experimental conditions, such as pH and concentrations of substrates, have been examined and optimized. The proposed method exhibited good performance, the linear range was from 0.30 to 7.5 mM, the relative standard deviation was 1.86% (n = 10), limit of detection was 0.16 mM and recovery was 99 ± 4%. Copyright © 2013 John Wiley & Sons, Ltd.