Redox-Dependent Processes

Closely related to the these hydrographical changes are redox-dependent processes. The 1980s is well known for a stagnation period, when saline water inflows and oxygen supply to the deep water were rather sparse. Observations with help of a special sediment video camera system in the central Arkona Basin in 1989 (Rumohr, AMBIO 1990) proved a dramatic oxygen deficiency at the seafloor, occurrence of sulfur bacteria. 

FIGURE 14.25 Grain-sized distribution in surface sediments of the Arkona Basin in 1988 (top), 2005 (middle), and the relative differences (changes) during that time (bottom). 

SEDIMENTARY RECORDS OF ENVIRONMENTAL CHANGES AND ANTHROPOGENIC IMPACTS 

FIGURE 14.26 Model situation for mean current velocities of 5.5 cm/s in the near-bottom layer of the Arkona Basin. The current directions and velocities are described by black arrows (The figure was provided by Seifert, lOW). 

Phosphorus is known to be released from the sediments during anoxic conditions, as most of the time in the deep Gotland Basin (Hille et al., 2005). Otherwise, a change toward oxic conditions at the sediment surface causes phosphorus precipitation, mainly in the form of ironphosphates (Matthiesen et al., 2001), nearby or at the redox border that turns a few centimeter into the sediment because of bioturbation effects after recolonization of the seafloor. 

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The third redox-dependent process involved in metallocenter assembly requires changes to the protein, typically involving cysteine residues. For example, disulfide bonds that are spontaneously generated or enzymatically formed in periplasmic proteins must be reduced to create metal-binding thiolates or for cytochrome c biosynthesis. The opposite reaction, oxidation of cysteines during metallocenter assembly appears to be required for SODl activation. 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Fig. 7.5. Schematic representation of some of the redox mediator processes at a whole cell biosensor. Lipohilic mediators may be reduced at redox active sites in the plasma membrane or at sites within the cytoplasm or both processes may occur—depending on the cell type and the mediator. Lipophobic mediators can only be reduced at sites on the outside edge of the plasma membrane. The oxidized form of the mediator. O, may be present in excess, but much of the reduced form. R, may need to diffuse between packed cells (dotted arrows) or through the cytoplasm (squiggly arrows). The subscripts aq, cyt, elec, and surf represent mediator in the aqueous phase, within the cytoplasm, at the electrode surface, and at the plasma membrane-aqueous interface, respectively.

The redox system does not depend on endosomal acidification but needs TfR. Fe2Tf first binds to TfR which is located in close proximity to the proton-and electron-pumping NADHiTf oxidoreductase. The Fe—Tf bond is destabilized by proton efflux, making Fe3+ susceptible to reduction. Fe2+ is trapped by a plasma membrane binder and can be transported by a translocator [4]. As Al is a simple trivalent cation incapable of redox changes, it may be theoretically impossible that Al bound to Tf is taken up by a redox mechanism. Actually, no reports on a redox-mediated process of Al bound to Tf have been made. 

Vitamin C Ascorbic acid is the most important redox substance of cell metabolism. The body content probably amounts to about 2-5 g, the major part being stored in the liver and muscles. Intestinal absorption (80-90%) is an active, sodium-dependent process. The transport of ascorbic acid in the blood probably takes place as an ascorbic acid-albumin complex. Cellular uptake is stimulated by insulin. 

Catalysis of the reaction brings in both cases the possibility of a decrease in the overvoltage for a given current density. Furthermore, for the redox-catyalyzed process, the change in the product distribution can be expected to be small, whereas in the case of chemical catalysis, the different possibilities for the formation and decomposition of the adduct (depending on the nature of the catalyst) may lead to a change in selectivity. 

Figure 1. Schematic illustration of the effect of a change in the oxidation potential of the reductant upon the barrier heights in an outer-sphere reaction where R and P are reactants and products, respectively A is the associated, collision, encounter complex of reactants and B that of products. The lines coming from the left diagram, a normal redox-controlled process, show the dependence of the free energy of products and the transition state for electron transfer upon a change in the oxidation of the reductant. The right diagram illustrates how the transition state for the reaction changes to one that describes dissociation of the encounter complex of products.

Gibson, G. E., Blass, J. R, 2007. Thiamine-dependent processes and treatment strategies in neurodegeneration. Antioxid Redox Signal. 9, 1605-1619. 

Transient spectroelectrochemical studies are possible with modem and fast spectroscopic probes, e.g. based on diode array spectrometers [55]. Compared to steady-state techniques, the design of transient spectroelectrochemical experiments is more challenging and the data analysis has to take into account the development of diffusion- and convection-driven concentration profiles [56]. However, these experiments open up the possibility to study time-dependent processes such as the development of a diffusion profile. For example, it is known, but usually ignored, that changing the redox state of a molecule changes its diffusivity. It has recently been shown [57] that, for the oxidation of AA A A -tetra-methylphenylenediamine (TMPD) in water, ethanol, and acetonitrile (Eq. II.6.1), the diffusion coefficients of the reduced form, TMPD, and the diffusion coefficient of the oxidised form. 

Electrochemistry provides a powerful tool for elucidating the pH-dependent redox mechanisms of coordination complexes. In principle, any electrochemically active chemical (or biological) system may exhibit pH-dependent reduction potentials, if the concomitant pH-dependent process occurs on the same timescale as electron transfer. While the pH-dependent process is ultimately chemical in nature (i.e., involves bond breaking and/or bond making), the phenomenon that perturbs the redox center and alters the reduction potential may be electronic, structural (e.g., a conformational change), or environmental (e.g., changes in solvation), and often will be some ill-defined combination of these factors. ... 

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