Redox states

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

Can the contaminant be made less hazardous by changing its redox state ... 

Used primarily to reduce the Fe202 to FeO to give the characteristic amber color, although the redox state of the glass melt also influences the fining... 

The electrochemistry of S-N and Se-N heterocycles has been reviewed comprehensively. The emphasis is on the information that electrochemical studies provide about the redox properties of potential neutral conductors. To be useful as a molecular conductor the 4-1, 0, and -1 redox states should be accessible and the neutral radical should lie close to the centre of the redox spectrum. The chalcogen-nitrogen heterocycles that have been studied in most detail from this viewpoint... 

FIGURE 18.19 The structures and redox states of the nicotinamide coenzymes. Hydride ion (H , a proton with two electrons) transfers to NAD to produce NADH. 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

FIGURE 18.22 The redox states of FAD and FMN. The boxes correspond to the colors of each of these forms. The atoms primarily involved in electron transfer are indicated by red shading in the oxidized form, white in the semiqninone form, and bine in the reduced form. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Williamson, D. H., Lund, P, and Krebs, H. A., 1967. The redox state of free nicodnamide-adenine dinucleotide in the cytoplasm and mitochondria of rat liver. Biochemical Journal 103 514-527. 

In the previous sections, epoxidation was accompanied by a net oxidation of the substrate. We would like to conclude our discussion of various mechanisms of epoxide formation by presenting one example in which the substrate does not undergo a change in redox state. 

One major reason for the diversity of views on the redox states of conducting polymers is the variety of po ible shapes and forms of potentiodynamic current-voltage curves, even when the materials are prepared under more or less similar conditions. 

Reducing and oxidizing conditions m a sediment determine the chemical stability of the solid compounds and the direction of spontaneous reactions. The redox state can be recognized as a voltage potential measured with a platinum electrode. This voltage potential is usually referred to as E or Eh defined by the Nemst equation, which was introduced in Chapter 5, Section 5.3.1 ... 

Fig. 14. Plot of the g values g,g ) and of the average g value g vs rhombicity (UJ of (a) wild type (open symbol) and variant forms (closed symbols) of the Rieske protein in yeast bci complex where the residues Ser 183 and Tyr 185 forming hydrogen bonds into the cluster have been replaced by site-directed mutagenesis [Denke et al. (35) Merbitz-Zahradnik, T. Link, T. A., manuscript in preparation] and of (b) the Rieske cluster in membranes of Rhodobacter capsulatus in different redox states of the quinone pool and with inhibitors added [data from Ding et al. (79)]. The solid lines represent linear fits to the data points the dashed lines reproduce the fits to the g values of all Rieske and Rieske-type proteins shown in Fig. 13.

Fig. 15. EPR spectra of the Rieske cluster in membranes of Paracoccus denitrificans in different redox states of the quinone pool and with inhibitors added. Q x, ascorbate reduced Qred) reduced with trimethylhydroquinone dissolved in dimethyl sulfoxide +EtOH, reduced with trimethylhydroquinone dissolved in 90% ethanol +Myxo, ascorbate reduced with myxothiazol added + Stigma, ascorbate reduced with stigmatellin added. Only the gy and signals are shown. The dotted line has been drawn at...

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

Until then, the purification of the Fepr protein had been a laborous job as a 240-L batch yielded only as little as 5 mg of protein. With the overexpression clones of the Fepr proteins, the range of proteinconsuming studies such as Mossbauer spectroscopy, EXAFS, and, last but not least, crystallization experiments was greatly extended. Thus, several groups set off to systematically investigate the spectroscopic properties of both Fepr proteins, poised at all four (proposed) redox states. 

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