Environment, redox potentials

Interaction with the luminal content may be a chemical or enzymatic/microbial degradation. The chemical degradation is usually related to pH changes in the gut lumen but can also be a result of the reductive environment (redox potential negative) produced by the presence of anaerobic bacteria (Shamat 1993). 

Changes in the environment of the redox site can lead to changes in the redox potential via alteration of the interaction energy of the redox site with the outer shell. In many... 

J Li, MR Nelson, CY Peng, D Bashford, L Noodleman. Incorporating protein environments in density functional theory A self-consistent reaction field calculation of redox potentials of [2Ee2S] clusters in feiTedoxm and phthalate dioxygenase reductase. J Phys Chem A 102 6311-6324, 1998. 

VS Shenoy. Contribution of Protein Environment to Redox Potentials of Rubredoxm and Cytochrome c. M.S. Thesis. Pullman, WA Washington State University, 1992. 

Anaerobic organisOTj—Flourish in the absence of oxygen in environment with low redox potential. 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

Environment (aqueous) Lower the redox potential of the solution, i.e. lower Increase the potential of the M /M equilibrium, i.e. increase, Lower a by raising pH, remove dissolved O2 or other oxidising species Increase / + by removing complexants (e.g. CN ions) from solution... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

The same pifa,ox values as for the water-soluhle Rieske protein have been determined for the Rieske protein in bovine heart mitochondrial bci complex (102) this is consistent with the fact that the redox potential of the Rieske cluster is unperturbed within the bci complex and indicates that the environment of the Rieske cluster must be accessible within the complex. However, in the bci complex from Para-coccus denitrificans, the redox potential at pH 6.0 was found to be 45 mV lower than at pH 7, indicating the presence of a third group with a redox-dependent pi a value below 7 (36). No redox potential difference between pH 6 and 7 was found for the water-soluble Rieske... 

The redox potentials of iron-sulfur centers vary considerably, depending on the type of center (242) and its environment in the protein (243), ranging from about -700 mV in the case of some [4Fe-4S] centers (244, 245) to about +450 mV in the case of some [4Fe-4S] centers (246). Iron-sulfur centers therefore act as redox centers in numerous electron transfer systems, including the respiratory and photo synthetic bioenergetic chains as well as a wide variety of redox enzymes. Among the many studies that have dealt with these systems, some have yielded structural information of the kind described in Section III. Most of them were designed, however, to measure the... 

In view of the low Nin/Niin redox potential of [NiFe] hydrogenase, the search for low-potential Nin/Niin complexes not only with pure S-donor but also with mixed N/S environment has been of major interest, and significant advances in this field have been reported.298,299 Various mononuclear systems are listed in Figure 1 according to their redox potentials (usually in DMF). Homodinuclear Ni2 and heterodinuclear NiM complexes with N/S donor ligands, in particular... 

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