Cell-free systems redox reactions

For a redox reaction in an electrochemical cell the decrease in free enthalpy (- AG) is in accordance with the energy delivered by the transfer of electrons through an external circuit if this takes place in a reversible way, i.e., at a rate slow enough to allow complete attainment of equilibrium, the conversion of 1 gram mole will deliver an electrical energy of - AG = z FE. In total cell reaction mred, + n ox2 m ox, + nred2, where m81 = nS2 electrons are transfered (<5, and S2 represent the respective valence differences of the two redox systems), we have... 

Biocatalytic redox reactions offer great synthetic utility to organic chemists. The majority of oxidase-catalyzed preparative bioconversions are still performed using a whole-cell technique, despite the fact that the presence of more than one oxidoreductase in cells often leads to product degradation and lower selectivity. Fortunately, several efficient cofactor regeneration systems have been developed (160), making some cell-free enzymatic bioconversions economically feasible (161,162). 

The solution phase of an electrochemical cell does not contain free electrons, but it does generally contain a redox couple which can equilibrate with the free electrons in the electrode. This allows us to extend the concept of the Fermi level to the solution. Consider the redox couple 0,R in contact with an electrode el at which the redox reaction 0 H- nt R occurs. When the system is at equilibrium, the work of transferring an electron across the electrode/solution interface to transform (l/n)0 to (l/n)R is zero, i.e. 

The decrease in free energy of the system in a spontaneous redox reaction is equal to the electrical work done by the system on the surroundings, or AG = nFE. The equilibrium constant for a redox reaction can be found from the standard electromotive force of a cell. 10. The Nernst equation gives the relationship between the cell emf and the concentrations of the reactants and products under non-standard-state conditions. Batteries, which consist of one or more galvanic cells, are used widely as self-contained power sources. Some of the better-known batteries are the dry cell, such as the Leclanche cell, the mercury battery, and the lead storage battery used in automobiles. Fuel cells produce electrical energy from a continuous supply of reactants. 

F is the Faraday constant (96,485 CVmol), n the charge number, and Erev Qie reversible potential or equilibrium potential of the cell reaction. By convention, the work supplied by a system is negative, which explains the negative sign of equation (2.35). For a given electrochemical redox reaction (2.1), the free energy of reaction is equal to ... 

We saw in Section 2.2 that if a chemical transformation is spontaneous, and pressure and temperature are constant, the Gibbs free energy (G) of the system will decrease. We have seen in the above section that a redox reaction will proceed spontaneously under standard conditions if its standard cell potential (Ejcii) is positive. Therefore, there should be a quantitative relationship between AG under standard... 

Thermodynamics has countless applications other than the expansion of gases to run steam engines. In fact, probably within your reach right now are a laptop computer, IVIP3 player, cell phone, and/or wristwatch, which represent one type of application. Metal-plated jewelry and silverware represent the other. These are a few of the objects you use every day that rely on a major field in applied thermodynamics— electrochemistry, the study of the relationship between chemical change and electrical work. We typically study this relationship with electrochemical cells, systems that incorporate a redox reaction to produce or utilize electrical energy. In this chapter, we examine the essential features of the two types of electrochemical cells as well as the quantitative relationship between free energy and electrical work. 

A cell-free isobutene-forming system from disrupted R. minuta cells was also created (Fujii et al. 1988). Isobutene was produced from a-ketoisocaproate, isovaleryl-CoA and isovalerate where isovalerate gave the best isobutene formation rate of these three with 9.1 nL/L/h. Adding NADPH to the reaction mixture proportionally increased the rate of isobutene production, until a concentration of around 0.1 mM, as did increasing the concentratiOTi of isovalerate until 30 mM. EDTA had no inhibitory effect (Fujii et al. 1988). The formation of isobutene was inhibited by some redox reagents and completely eliminated by the presence of carbon monoxide (Fujii et al. 1989b). 

In all the preceding evaluations, it is prudent to first employ a redox system that is known to be reversible and free from the complications of coupled chemical reactions and adsorption. Deficiencies in the electronics and/or cell design will thus be revealed before the critical experiments are begun. Typical... 

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