Thermodynamic sequences potential change

With the standard state for each component chosen as the pure component in the phase of interest and at the temperature of interest, Chang et al. (4-) have discussed three thermodynamic sequences for the calculation of the reduced standard state chemical potentials. The pathways for each sequence are shown in Figure 2. 

Figure 2. Three thermodynamic sequences for evaluating the reduced standard state chemical potential change.

Electrode potentials are determined by the affinities of the electrode reactions. As the affinities are changes in thermodynamic functions of state, they are additive. The affinity of a given reaction can be obtained by linear combination of the affinities for a sequence of reactions proceeding from the same initial to the same final state as the direct reaction. Thus, the principle of linear combination must also be valid for electrode potentials. The electrode oxidation of metal Me to a higher oxidation state z+>2 can be separated into oxidation to a lower oxidation state z+>1 and subsequent oxidation to the oxidation state z+>2. The affinities of the particular oxidation processes are equivalent to the electrode potentials 2 0, i-o> and E2-. ... 

The first experimental evidence that Met in j8-APl-42 is more easily oxidized than in other peptides and proteins comes from one-electron oxidation of /3AP1-40 using azide radicals (Nj) produced by pulse radiolysis.Thermodynamic considerations indicate that Nj should not oxidize Met residues unless the one-electron reduction potential of Met is lowered because of favorable environment. It was shown that Met is the target in /3-AP(l-40) oxidation. Conversely the oxidation of /3-AP(40-l) with a reversed sequence of amino acids has shown that Tyr is the target of Nj radicals. These observations are the first experimental evidences that (i) Met in AP(l-40) is more easily oxidized than in other peptides or proteins, and (ii) a change in a primary sequence drastically affects the one-electron reduction potential of Met, even in a small peptides. 

The titration curves of the three polypeptides can be analyzed by adapting the Zimm-Bragg theory for the helix-coil transition " to the cooperative binding of the surfactant to the polypeptide, provided that the thermodynamic contribution of any conformational change accompanying the binding is comparatively small and that the electrical potential on the surface of the polypeptide molecule can be regarded to be constant. The theory defines two parameters the equilibrium constant s and an initiator a. If the polypeptide sequence is schematically written as... 

The thermodynamic approach in the prediction of the sequence of occurrence of the different phases was also used by Lee et al. [24], The authors have taken into account the change of the driving force due to the formation of a metastable solution. Still, they have disregarded the influence of concentration and chemical potential gradients on the driving force. 

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