Electron transfer processes free energy change

The potential energy surfaces on which the electron-transfer process occurs can be represented by simple two-dimensional intersecting parabolic curves (Figure 6.23). These quantitatively relate the rate of electron transfer to the reorganisation energy (A.) and the free-energy changes for the electron-transfer process (AG°) and activation (AG ). 

The electrical contact of redox proteins is one of the most fundamental concepts of bioelectronics. Redox proteins usually lack direct electrical communication with electrodes. This can be explained by the Marcus theory16 that formulates the electron transfer (ET) rate, ket, between a donor-acceptor pair (Eq. 12.1), where d0 and d are the van der Waals and actual distances separating the donor-acceptor pair, respectively, and AG° and X correspond to the free energy change and the reorganization enery accompanying the electron transfer process, respectively. 

The free energy change AG involved in the actual electron transfer process encounter complex - - ion-pair, can be calculated according to the expression 6.29. 

Due to the complicated kinetics for both processes no attempt was made in ref. 83 to treat the data quantitatively. It was estimated, however, that the back electron transfer reaction is slower by about 3 orders of magnitude than that of the forward electron transfer. At the same time, the free energy change for the forward reaction (AG° = - 0.4 eV) is smaller than that for the back electron transfer (AG° = — 1.7 eV). This decrease of the reaction rate at large exothermicity was attributed [83] to the decrease of the Franck-Condon factors with increasing J in the situation when J > Er (see Chap. 3, Sect. 5). 

Radical ion species (D+, A ) are usually produced by one-electron transfer from D to A or D to A in polar solvents. The calculated values for the free-energy change using the Rehm-Weller equation (Eq. 1) predict a photoinduced electron-transfer process between D and A [38]... 

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