Redox contrasts

In addition to plate boundary and plume-related hydrothermal systems, the chemistry of the prebiotic world would have had strong redox contrasts in the restricted areas that had tidal coasts, and perhaps within the oceans where differing water masses interacted, or under ice. These redox contrasts were ultimately driven by photolysis in the atmosphere/ocean (presumably... 

Early life most likely depended on exploiting the transient redox contrasts available from two sources within the inorganic geological system— especially at hydrothermal vents (Reysenbach and Shock, 2002) and secondly from inorganic light-driven reactions, such as the formation of transient oxidizing and reducing species in the atmosphere by incident radiation. 

These sources of redox contrast would have been limited. The hydrothermal contrasts depend on local thermally driven juxtaposition (e.g., in vent fluids) of chemical species from differing environments. From the vents would come H2, H2S, and probably CH4. The size and activity of the hydrothermal biosphere, and hence its impact, would have been considerable, as early Archean volcanism was probably much more common than today, with a higher heat flow out of the Earth. Nevertheless, the total potential productivity of an early hydrothermal biosphere would have been small on a global scale compared to the modem photosynthetically driven biosphere. Moreover, modem biota at hydrothermal vents depend on the supply of sulfate, oxidized in the photosynthetic biosphere before photosynthesis, the sulfate supply may have been limited. Thus, as a first guess, with a planetary heat flow higher than today... 

Photosynthesis is the source of the redox power that allowed life to escape from the very restricted early settings where inorganic redox contrast existed, and occupy the planet. Without access to light energy, life would have been permanently restricted to a few narrow settings, probably as thin biofilms, and as plankton near upwellings. 

Geological contacts between two units with strong redox contrast, such as carbonatite and granite. 

Possibly the earliest habitats were thin biofilms of bacteria and archaea that existed by processing redox contrast between hydrothermal products and the external environment (sea water and atmosphere). The productivity of these early microbial mats would have been severely limited by the inorganic sources of redox power from below. Volcanic and hydro-thermal processes would have ensured a small but steady supply of H2, H2S, CH4 and possibly HCN from below. Nitrate (from dissolved NO2), and sulphate are crucial. Some sulphate would have come from dissolved magmatically exhaled SO3. Some would have been supplied by disproportionation of sulphite in seawater, the sulphite having come from magmatic SO2. Some sulphate would have been made photo-chemically in air. 

Life is improbable, but under the favorable physicochemical conditions of the early Earth it did begin. These conditions included an appropriate surface temperature, the presence of liquid water, the availability of appropriate chemical ingredients, the presence of a magnetic field, and an environment in which there were redox contrasts, all set in the context of a dynamic planet experiencing periodic resurfacing. 

Finally, life needs a redox contrast in order to function. Prior to photosynthesis it is likely that organisms would have occupied habitats where naturally occurring redox contrasts were present. Such conditions are likely to have been available in the atmosphere, where there is and has been, a mixture of oxidized and reduced gases, in the oceans, and at atmosphere-rock and sea-water-rock interfaces... 

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

In contrast to the relative ease of reduction, oxidation of fullerenes requires more severe conditions [113, 114]. Not only does the resonance stabilization raise the level of the corresponding oxidation potential (1.26 V versus Fc/Fc ), but also the reversibility of the underlying redox process is affected [115]. 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

Some variables such as temperature, pH, nutrient medium, and redox potential are favorable to certain organisms while discouraging the growth of others. The major characteristics of microbial processes that contrast with those of ordinary chemical processing include the following [1] ... 

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

They present a large and reverse redox potential range, in contrast to the well-defined narrow peaks of the inorganic or organic redox couples... 

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

The Rieske protein II (SoxF) from Sulfolobus acidocaldarius, which is part, not of a bci or b f complex, but of the SoxM oxidase complex 18), could be expressed in E. coli, both in a full-length form containing the membrane anchor and in truncated water-soluble forms 111). In contrast to the results reported for the Rieske protein from Rhodobacter sphaeroides, the Rieske cluster was more efficiently inserted into the truncated soluble forms of the protein. Incorporation of the cluster was increased threefold when the E. coli cells were subject to a heat shock (42°C for 30 min) before induction of the expression of the Rieske protein, indicating that chaperonins facilitate the correct folding of the soluble form of SoxF. The iron content of the purified soluble SoxF variant was calculated as 1.5 mol Fe/mol protein the cluster showed g values very close to those observed in the SoxM complex and a redox potential of E° = +375 mV 111). 

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