Free Energy and Temperature Dependence of Tunneling

Classical Free Energy and Temperature Dependence of Tunneling 

FIGURE 1. The maximum electron tunneling rate depends on the distance between donor and acceptor and the effective height of the tuimeling barrier, decaying rapidly with distance (exponential slope 5). 

classical Marcus theory predicts an electron transfer rate that has a Gaussian dependence on the free energy of the reaction (Marcus, 1956 Marcus and Sutin, 1985). 

FIGURE 3. Marcus theory predicts a Gaussian dependence of the electron transfer rate ket on free energy AG, which appears as a parabolic dependence on a log plot. The maximum rate is found when the driving force matches the reorganization energy, AG = fiX. 

In principle it should be possible to determine the reorganization energy simply by examining the temperature dependence of the reaction to determine the activation energy, without the need to replace natural redox centers with exotic cofactors, or mutational changes to modulate midpoint potentials. However, this method is fraught with traps for the unwary. First, it may be more appropriate to use the semi-classical version of the 

---