Mediated Redox Reactions

Table 2. Analytical scale mediated redox reactions with participation of a significant layer of polymer...

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

Interestingly, titanium dioxide can also act as a photocatalyst [87]. In some investigations into these phenomena a moisture-mediated redox reaction has been postulated. 

BIOPLUME III is a public domain transport code that is based on the MOC (and, therefore, is 2-D). The code was developed to simulate the natural attenuation of a hydrocarbon contaminant under both aerobic and anaerobic conditions. Hydrocarbon degradation is assumed due to biologically mediated redox reactions, with the hydrocarbon as the electron donor, and oxygen, nitrate, ferric iron, sulfate, and carbon dioxide, sequentially, as the electron acceptors. Biodegradation kinetics can be modeled as either a first-order, instantaneous, or Monod process. Like the MOC upon which it is based, BIOPLUME III also models advection, dispersion, and linear equilibrium sorption [67]. 

Aerobic life is surprisingly fast O2 reduction most likely does not take place through slow one-electron steps. Macromolecular electron transfer catalysts (enzymes) with fixed steric positions are presumably able to catalyze the more-or-less synchronous four-electron reduction of O2. Thus, for most biologically mediated redox reactions, the O2-H2O system appears often to be the operative redox couple. 

This type of reaction has been well documented in the case of the peroxidase-mediated redox reactions of phenols [38,39], This reaction may have important physiological connotations for lignin biosynthesis since peroxidases unable to oxidize the syringyl moiety present in sinapyl... 

The subsurface environment also shares many processes with surface waters. Microbially mediated redox reactions and biodegradation processes are significant in each medium, and much of what was discussed in Chapter 2 on these topics applies directly to the subsurface as well. The presence of particles, and their potential to absorb chemicals, also is common to both surface waters and groundwaters when modeling subsurface transport, the high ratio of solid material to water requires special recognition of even moderate sorbing tendencies. 

Figure 2. Examples of the types of chemical reactions promoted by aquatic microbes, including reductant excretion (1), transplasma membrane electron transport (2), redox reactions involving extracellular solutes (3) and reductants excreted by the cell (3/>), hydrolysis reactions (4), ami nonenzymatically mediated redox reactions (5). The cell wall and membrane are represented by the curved parallel lines and extracellular enzymes by ovals.

Attack points are metal ion centers and specific cysteine residues of proteins. The mechanisms by which cysteine nitrosylation regulates protein functions can be broadly described in allosteric terms similar to protein phosphorylation. Often, 02-mediated redox reactions cooperate in the allosteric control by NO of protein functions. S-nitrosylation of target proteins is a redox-based signal with exquisite specificity based on the selective modification of single cysteine residues. The selectivity of S-nitrosylation has been shown to be provided by both the subcellular distribution of NOS enzymes and the sequence context of cysteine residues in target proteins. Two nitrosylation motifs have been identified. In one motif, the target cysteine is located between an acidic and a basic amino acid, as revealed in either the primary sequence or the tertiary structure. In the other motif, the cysteine is contained in a hydrophobic region. 

The amino acid side chains in the active site of enzymes catalyze proton transfers and nucleophilic substitutions. Other reactions require a group of nonprotein cofactors, that is, metal cations and the coenzymes. Metal ions are effective electrophiles, and they help orient the substrate within the active site. In addition, certain metal cations mediate redox reactions. Coenzymes are organic molecules that have a variety of functions in enzyme catalysis. 

Kreevoy and his coworkers (and others such as Lee et al. [43] and Wiirthwein et al. [44]) have applied this formalism to various models for nicotinamide-mediated redox reactions. An immediate result was that the conclusions about transition-state structure based only on the traditional interpretation of a, i.e., ascribing to it the properties of x, can lead to substantial error. 

Transfer coefficient for the mediator redox reaction at the electrode surface (unitless)... 

In order to exploit the efficiency and sensitivity of DNA-mediated redox reactions in biosensing applications, further improvements in the methodology were required to demonstrate the feasibility of this approach. A practical adaptation of the system would have three features (1) the ability to use a non-cross-linked redox probe, (2) the ability to detect point mutations within oligonucleotides of varying sequence composition, and (3) the ability to achieve in situ hybridization at the electrode surface. The success obtained in each of these areas [25] indicates that this approach may hold promising applications. 

The chemical production of hydroxyl radical, either by metal-mediated redox reactions or as a result of radiolysis products, can induce strand breaks in DNA. An example of a chemotherapeutic agent whose chemical production of activated oxygen is believed to be the critical step in its action is bleomycin, a naturally occurring antibiotic. The binding of metals to this and other antibiotics has been summarized in detail [1—3] and the material presented here is based on these reviews. 

This equation can be applied to any microbially-mediated redox reaction to determine its feasibility under standard conditions. For example, Geobacter metallireducens [54] reduces ferric iron, such as iron(lll) hydroxide, by acquiring electrons from organic compounds, such as acetate ... 

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