Intramolecular electron transfer driving force dependence

Figure 3. Driving force-dependence of intramolecular electron transfer rates in Ru-ammine-His33 modified Zn-substituted cytochrome c ( ), and Ru-bpy-His33 modified Fe-cytochrome c ( ). Solid lines were generated using Eq. 1 and the following parameters Ru-ammine,, i=1.15 eV, Hab = 0.10 cm Ru-bpy, X = 0.74 eV, Hab = 0.095 cm". ...

Although electron transfer rates within myoglobin appear to follow an exponential dependence on distance, the derived rate expression is not directly transferable to other electron transfer proteins. A particularly striking comparison is between the c cytochromes and the copper proteins plastocyanin and azurin. Intramolecular electron transfer rates are at least 10-100 times slower in the copper proteins compared to the c cytochromes, even though the distances and driving forces for the reactions are comparable. The origin of this behavior is unclear, but it does suggest caution in the quantitative transfer of rate expressions between different systems. 

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