Cyclic Oxidation-Reduction Systems

Uchida, Suzuki, and Ichihara (878) isolated a soluble enzyme system (thereby possibly excluding mitochondrial participation) from rabbit liver, and partially purified it. Two enzymes were involved. The first of these converted p-hydroxyphenylpyruvic acid to 2,5-dihydroxyphenylpyruvic acid. If this enzyme was resolved, vitamin C alone did not restore the activity, but vitamin C and vitamin B12 did. The amount of B12 required was very low, and they suggested that the true enzyme was a Bw derivative, possibly aquocobalamin hydroxide bound to enzyme protein, and that the function of the ascorbic acid was solely to stabilize the reactive form of the coenzyme. This agrees with the work of La Du and Greenberg (524), who considered the role of ascorbic acid to be quite unspecific. Ascorbate increased the rate of tyrosine oxidation in liver preparations but the net consumption was zero, and moreover numerous ene-diols were just as effective on a molar basis. La Du and Greenberg considered that ascorbic acid participates in a cyclic oxidation-reduction and happens to be a substance of the correct oxidation-reduction potential either to participate directly or to protect some other participant. 

Higgins et al. (1999) developed a system for the detection of Staphylococcus enterotoxinB (SEB), ricin toxin. Yersinia pestisYl antigen and 5ac/7/MX anthracis PA antigen based on the principle of electrochemiluminescence (ECL). ECL is a process that involves the generation of light from a voltage-dependent, cyclic oxidation-reduction reaction of rathenium heavy metal chelate. The redox... 

By a comparison of the structure of the isomers shown in Fig. 11, it can be seen that carbon atom number 6 in the 5,6-dihydro compound is still asymmetric while it is not asymmetric in the 5,8-dihydro compound. Therefore, if the oxidation of inactive /-L-tetrahydrofolate proceeded through the 5,8-dihydro compound as an intermediate, and if this intermediate were reduced nonenzymically back to the tetrahydro level, this process of cyclic oxidation-reduction should lead to racemiza-tion of the original tetrahydrofolate, and the racemization could be detected by the appearance of activity in the phenylalanine hydroxy-lation system, i.e., these reactions should result in the conversion of inactive /-L-tetrahydrofolate to (partially) active other hand, if the intermediate in the oxidative reaction were the 5,6-dihydro compound, the original optical specificity of the tetrahydrofolate should be retained through the oxidation-reduction process and the inactive /-L-tetrahydrofolate should remain inactive. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

Cyclic artificial photosynthetic systems (Fig. 11) include an oxidation process complementary to the reduction reaction. For light-driven reductive syntheses of valuable chemicals or for the removal of environmental pollutants the concept of utilizing a sacrificial electron donor can be adapted. Yet, for the application of artificial photosynthetic systems as fuel generation devices, several basic criteria must be met by the complementary oxidation process ... 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

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