Chemicals redox cycling

The shallow nature of Pond 3513 makes chemical processes occurring In the sediment extremely Important. More work will be needed, however, to elucidate the redox cycle. 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

Scheme 6 Redox cycles of 3,3,3-(CO)3-3,1,2-MC2BgH12 (M = Mo, W). Reproduced by permission of the American Chemical Society from J. Am. Chem. Soc. 2001, 123, 9054.

Cohen, G.M. and d Arcy Doherty, M., 1987, Free radical mediated toxicity by redox cycling chemicals. Br. J. Cancer 55 46-52... 

With paraquat, doxorubicin, and nitrofurantoin, once the electron is donated to oxygen, another can be acquired, and so the process can continue as long as there is a source of electrons and the chemicals are present. This is called "redox cycling" (see chap. 6). 

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

Figure 3.36 Destruction of a steady-state redox cycle by a coupled chemical reaction. [From Ref. 59. reprinted with permission.]...

Most importantly, the radionuclide and the stable nuclide must undergo isotopic exchange. In practice, this means that the tracer and the stable atom must be in the same redox state. By heating or using redox cycles, the experimenter must assure this to be true. Anomalous experimental results have frequently been traceable to the chemical form of the administered radiotracer. Since reactor production of radionuclides often results in side reactions (see Chapter 10), various oxidation states may be present when the sample is produced. In one case involving phosphate-32P uptake in plants, the unexpected experimental results were explained by the fact that a large percentage of the tracer dose was actually in the form of phosphite-32P. 

As indicated above, frequently the amount of material involved in a radiochemical procedure is quite small. To obviate some of the difficulties associated with this, a weighable amount ( mg) of inactive material, the carrier, is added to the procedure at an early stage. It is essential that this carrier and the radionuclide (tracer) be in the same chemical form. This is achieved usually by subjecting the carrier + tracer system to one or more redox cycles prior to initiating any chemical separations to ensure that the carrier and tracers are in the same oxidation state. 

The carbon cycle is illustrated in Fig. 5.2. All living systems require an external source of energy, either in the form of chemical bond energy, as chemical (redox) potential or as some form of electromagnetic radiation usually in or near the visible light region. 

The loss of control of endogenous oxidative events in the use of molecular oxygen by the cell is the major factor in oxidative stress injury. Such a process, which is known as chemical-induced oxidative stress, may occur to an extent that ranges from a minor to a major contribution to overall toxicity. For example, chemicals that are known to undergo redox cycling cause exogenous oxidative stress to such a degree that they play a major role in chemically induced cell injury. In addition, some of these chemicals are known to form adducts with cellular constituents, particularly... 

FIGURE 16.13 Partial scheme for the metabolism of diethylstilbestrol (DES). DES is administered as the trans isomer (E-DES), which, in solution, is in equilibrium with the cis isomer (Z-DES). Cytochrome P450 enzymes oxidize E-DES and Z-DES to a postulated chemically reactive semiquinone (1), which is further oxidized to a quinone (2), thereby generating reactive oxygen species (ROS) that oxidize cellular macromolecules. Redox cycling is perpetuated and ROS formation is amplified by two enzymes, cytochrome P450 or cytochrome bs reductase, which reduce the quinone back to the semiquinone. The unstable semiquinone and diol epoxide (3) metabolites are presumably those that bind to DNA to form adducts and initiate carcinogenesis. 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

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