Donor-acceptor electronic coupling in ruthenium-modified heme proteins

The rates of electron transfer (ET) in six Ru-modified cytochrome c derivatives were analyzed in terms of four theoretical models describing donor-acceptor electronic coupling. The simplest model, which treats the protein as a homogeneous medium, fails to describe the variations in ET rates with changes in donor-acceptor separation. The three other models explicitly account for the inhomogeneity of the polypeptide matrix and are more successful in describing the electronic couplings. Calculations of relative coupling strengths give results within an order of magnitude of experimentally determined values for cytochrome c. The homogenous-medium model is more successful in describing ET in Ru modified myoglobin, and two of the inhomogeneous-medium models suggest that mutltiple pathways are important in mediating the electronic coupling.