Redox thermodynamics for oxygen

Redox Thermodynamics for Oxygen Species Electrode material effects... 

The reaction thermodynamics for oxygen species are influenced by the solution matrix and its acidity. Thus, the redox thermodynamics of O2 are directly dependent upon hydronium ion activity,... 

Let us now consider redox limits for the thermodynamic stability of aqueous solutions. Maximum oxidation is defined by the dissociation of water molecules, with the formation of hydrogen ions and gaseous oxygen—i.e.,... 

A quantitative description of oxidative phosphorylation within the cellular environment can be obtained on the basis of nonequilibrium thermodynamics. For this we consider the simple and purely phenomenological scheme depicted in Fig. 1. The input potential X0 applied to the converter is the redox potential of the respiratory substrates produced in intermediary metabolism. The input flow J0 conjugate to the input force X0 is the net rate of oxygen consumption. The input potential is converted into the output potential Xp which is the phosphate potential Xp = -[AG hoS -I- RT ln(ATP/ADP P,)]. The output flow Jp conjugate to the output force Xp is the net rate of ATP synthesis. The ATP produced by the converter is used to drive the ATP-utilizing reactions in the cell which are summarized by the load conductance L,. Since the net flows of ATP are large in comparison to the total adenine nucleotide pool to be turned over in the cell, the flow Jp is essentially conservative. 

Because of the nonreversibility of the biological mediation of the NO, N, conversion, the NOt -N2 couple cannot be used as a reliable redox indicator. For example, NOj may be reduced to N, in an aquatic system even if the bulk phase contains some dissolved oxygen. The reduction may occur in a microenvironment with a pe value lower than that of the bulk, for example, inside a floe or within the sediments the N2 released to the aerobic bulk phase is not reoxidized although reoxidation is thermodynamically feasible. 

Figure 11.3. Sketch of pe°H = 7 and pH = values for four-electron, two-electron, and one-electron redox processes of oxygen compounds. H2O2, O2, OH, and O3 are treated as if they were metastable. Thermodynamically, the OH radical (reduction to H2O) and O3 (reduction to O2) and H2O2 (reduction to H2O) are the strongest oxidants oxygen with regard to its reduction to H2O2 and with regard to its reduction to O2 is a weak oxidant.

Advances in nanostructured conducting materials for DET have resulted in impressive current densities for the ORR, and application of these three-dimensional materials to DET from MCOs other than CueO may provide biocathodes with the characteristics suitable for an implantable EFC. While a DET approach using MCOs can provide for ORR at potentials approaching the thermodynamic reduction potential for oxygen, the current density achievable in this approach still relies upon intimate contact, and correct orientation, ofthe MCO to a conducting surface. Use of a mediator, capable of close interaction with the TI site of the MCOs, and with a redox potential tailored to permit rapid electron transfer to the TI site, can eliminate the requirement for direct contact in the correct orientation between MCO and electrode, and offer the possibility of a three-dimensional biocatalytic reaction layer on electrodes for higher ORR current densities. 

Most reviewers5 8 now argue that photosynthetic oxygen evolution results from a sequential four-step electron-transfer process in which oxidizing equivalents from chi fl+- are accumulated in a "charge-storing" complex to accomplish the concerted four-electron oxidation of two H2O molecules to one O2 molecule. The photooxidant (chi + ) and reductant (pheo O are one-electron transfer agents, and the matrix is the lipoprotein thylakoid membrane. Hence, evaluation and consideration of the one-electron redox potentials for PS 11 components within a lipoprotein matrix are necessary in order to assess the thermodynamic feasibility of proposed mechanistic sequences. 

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