Dual generation, reducible oxides

Figure 16.22 Chemical way to generate dual reaction center sites using promotion by reducible oxides.

Flavin redox states in a dual flavin enzyme. (Left) Single-electron reduction of the isoalloxazine ring generates the semiquinone radical, while reduction by two electrons generates the fully reduced species. (Right) Five possible oxidation levels of a dual flavin enzyme, where the FMN reduction potential is held at a more positive value relative U) FAD. The flavins can theoretically accept a maximum of four electrons obtained from two NADPH. However, in NADPH-cytochrome P450, reductase, full reduction of the flavins is not normally reached when NADPH serves as the reductant. 

Dual-electrode LCEC is very useful for the selective detection of chemically reversible redox couples. In this case, two electrodes are placed in series (Fig. 27.1 OB). The first electrode acts as a generator to produce an electroactive species that is detected more selectively downstream at the second electrode, which is set at a more analytically useful potential. One excellent example of the use of a dual-electrode detector for electrochemical derivatization is the detection of disulfides [34]. In this case, the first electrode is used to reduce the disulfide to the corresponding thiol. The thiol is then detected by the catalytic oxidation of mercury, described earlier. Because of the favorable potential employed at the second electrode, the selectivity and sensitivity of this method are extremely high. In addition, thiols can be distinguished from disulfides by simply turning off the generator electrode. 

Point Defect Generation During Phosphorus Diffusion. At Concentrations above the Solid Solubility Limit. The mechanism for the diffusion of phosphorus in silicon is still a subject of interest. Hu et al. (46) reviewed the models of phosphorus diffusion in silicon and proposed a dual va-cancy-interstitialcy mechanism. This mechanism was previously applied by Hu (38) to explain oxidation-enhanced diffusion. Harris and Antoniadis (47) studied silicon self-interstitial supersaturation during phosphorus diffusion and observed an enhanced diffusion of the arsenic buried layer under the phosphorus diffusion layer and a retarded diffusion of the antimony buried layer. From these results they concluded that during the diffusion of predeposited phosphorus, the concentration of silicon self-interstitials was enhanced and the vacancy concentration was reduced. They ruled out the possibility that the increase in the concentration of silicon self-interstitials was due to the oxidation of silicon, which was concurrent with the phosphorus predeposition process. 

The role of centre 2, for which the potentiometric reduction potential, at — 320 mV, is too low to be involved directly in electron transport between hydroquinone and fumarate, is not clear [164,234,235]. In these electrochemical experiments we have so far detected neither its electrode response nor any change in the steady-state biocatalytic current as the potential is lowered beyond — 320 mV. Cammack and co-workers have proposed [234, 235] that this centre might be active in one-electron reduction and reoxidation of the more reducing FAD radical species that must be generated, albeit transiently, during turnover. Their mechanism of reduction of oxidized FAD in SDH is termed the dual-pathway model since the two single electrons flowing consecutively out of FAD do so by different routes. The lower potential pathway uses centre 2. An equivalent mechanism, with the direction of electron flow reversed, is applicable for FR. 

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