Classical Treatment of the Nuclear Factor

If the temperature is high enough for all the pertinent nuclear motions to be treated classically, the electron transfer rate is given by expression (13). The activation energy Eg defined by  

We may illustrate this approach to the determination of the nuclear factor by the elegant studies performed by Gray and co-workers, who have determined the thermodynamic properties and the rate temperature dependence for the electron transfer between Ru(NH3) covalently bound to the histidine residues of some proteins, and the redox eenter of these proteins [110, 111, 112, 113]. The experimental results obtained for cytochrome c [110] and azurin [111, 112] are very similar. Using the thermodynamic data and the value or the upper limit of Ea reported in these studies, we deduce from Eq. (23)  

In the case of myoglobin [113], the only possible solution for X is equal to 2.3 eV. We now postulate an additive property for the reorganization energy, whieh seems reasonable for these systems  

Kinetic experiments were performed in other ruthenium-derivative proteins [115, 116, 117, 118], but it is difficult to compare their results with those previously reported as long as the temperature dependence of the rate has not been measured. 

---