Electron tunneling in proteins: Role of the intervening medium

Analysis of electron-transfer (ET) kinetics data obtained from experiments on Ru-modified proteins (azurin, cytochrome c, myoglobin) and the bacterial photosynthetic reaction center reveals that distant donor-acceptor electronic couplings depend upon the secondary structure of the intervening polypeptide matrix. The beta-sheet azurin structure efficiently and isotropically mediates coupling with an exponential distance-decay constant of 1.1 Angstrom(-1). The experimentally derived distance-decay constant of 1.4 Angstrom(-1) for long-range ET in myoglobin and the reaction center suggests that hydrogen-bond couplings are weaker through alpha helices than across beta sheets. The donor-acceptor interactions of systems with comparable tunneling energies fall into two coupling zones: the beta zone (bounded by distance-decay constants of 0.9 and 1.15 Angstrom(-1)) includes all the beta-sheet (azurin) couplings and all but one coupling in cytochrome c; the alpha zone (boundaries: 1.25 and 1.6 Angstrom(-1)) includes less strongly coupled donor-acceptor pairs in myoglobin and the reaction center as well as a relatively weakly coupled pair in cytochrome c.