Natural systems redox

In natural systems, redox proteins such as cytochrome c (cyt c) function not only to transfer electrons, but to transfer electrons specifically to a particular redox partner, usually another macromolecule. Transfer of electrons between subunits of modified hemoglobins and within complexes of cyt c with cyt b5 and cyt c with cyt c peroxidase (Cep) have therefore been studied extensively (41,42). These studies have revealed the fundamental requirements for the recognition process leading to the formation of the protein-protein complex as well as the thermodynamic features of the electron-transfer reaction itself. This reaction, outlined in equation 6, consists of three fundamental processes recognition to form a complex (Ki), electron transfer within the complex, and dissociation of the redox-altered complex (K2). For the cyt c-Ccp complex, Fe(II) cyt c corresponds to P2red and oxidized Cep corresponds to P °x. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

The Cycling of Iron in Natural Systems Some Aspects Based on Heterogeneous Redox Processes... 

The objectives of this section are to illustrate the relevance of some of the heterogeneous redox processes (that were discussed in Chapter 9) and of the photochemical processes occurring on some inorganic Fe-bearing minerals (this Chapter) to the cycling of iron in natural systems. 

The interpretation of Eh-pH diagrams implies assumption of complete equilibrium among the various solutes and condensed forms. Although this assumption is plausible in a compositionally simple system such as that represented in figure 8.20, it cannot safely be extended to more complex natural systems, where the various redox couples are often in apparent disequilibrium. It is therefore necessary to be cautious when dealing with the concept of the system Eh and the various redox parameters. 

Upon adsorption of Fe " at a solid surface, the standard redox potential of the Fe /Fe pair is reduced substantially from 0.77 V to 0.35-0.45 V (Wehrli, 1990) thereby facilitating the electron transfer. Buerge and Hug (1999) have demonstrated that this higher reactivity may be responsible for the fact that solid phases (Fe oxides, Si02, and clay minerals) in natural systems accelerate Cr reduction and that goethite and lepidocrocite are by far more active in this respect than the rest of the solid phases, because these two FeOOH forms adsorb much more Fe ". The authors attribute this to better overlap and charge delocalization at the surface of the Fe oxides. 

Naturally the redox properties of the general structures A-C ould strongly be influenced by different heteroatoms X. Since this report deals mainly with Weitz type systems, X is incorporated into a heterocyclic ring. 

The cavity of diphenylglycoluril derivative 3 is well suited to partially encapsulate a [4Fe-4S] cluster. Compound 29 which contains four arms terminating with thiol groups was synthesized and treated with (n-Bu)4N 2 Fe4S4Cl4) in dimethylformamide to give cluster complex 30 [31]. The product was characterized by a number of techniques, including cyclic voltammetry and differential pulse polarography. The current response of 30 was very small, but improved upon addition of a modulator, e.g. Ba or Na" ions. This behavior is similar to that observed for certain redox active enzymes [32]. As in the natural systems, a maximum response is observed when the Ba concentration is... 

In conclusion we should stress that quantification of rates of redox reactions in natural systems is difficult. Numerous compound- and system-specific factors may influence the overall reaction rate. Evaluation of the relative reactivities of a series of structurally related compounds that are likely to react by the same reaction mechanism(s), may, however, provide important insight into the processes determining a given reaction in a given system. Such information may allow at least order-of-magnitude estimates of how fast a given compound will undergo oxidation or reduction in that system. 

Using stoichiometric model systems, it can be shown that some naturally occurring redox processes have a pronounced pH-controlling action, even in the presence of substances that act as buffers. High pH values can be reached particularly in systems where higher metal oxides act as oxidizers whereas an acid condition often develops when free oxygen is the oxidizer. However, in most natural systems carbonates and silicates have a more pronounced pH controlling effect than redox processes. 

Such specificity is often possible with systems engineered for contaminant remediation. However, natural systems frequently involve complex mixtures of redox-active substances that cannot be characterized readily. The characterization of redox conditions in complex environmental media is a long-standing challenge to environmental scientists that continues to be an active area of research (Section 3.3). 

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