TUNNELING PATHWAY AND REDOX-STATE-DEPENDENT ELECTRONIC COUPLINGS AT NEARLY FIXED DISTANCE IN ELECTRON-TRANSFER PROTEINS

The tunneling pathway model for electron transfer, which accounts for the unique covalent, hydrogen-bonded, and van der Waals contacts linking donor and acceptor in a protein, gives a consistent description of electron-transfer rates in ruthenated proteins (cytochrome c, myoglobin, and cytochrome b5), while simpler exponential decay models are not fully adequate. We report several new testable predictions of the pathway model relating electron-transfer rates to protein structure. The analysis predicts qualitative differences in the distance dependence of protein electron transfer at short (< 5 angstrom) vs long distance, differences in the nature of the coupling through alpha-helix vs beta-sheet, and the possibility of switching electronic coupling upon oxidation/reduction or ligation/deligation in metalloproteins.