  Multi-heme cytochromes (MHC) are fascinating proteins used by bacterial
organisms to shuttle electrons within and between their cells. When placed in a
solid state electronic junction, they support temperature-independent currents
over several nanometers that are three orders of magnitude higher compared to
other redox proteins of comparable size. To gain microscopic insight into their
astonishingly high conductivities, we present herein the first current-voltage
calculations of its kind, for a MHC sandwiched between two Au(111) electrodes,
complemented by photo-emission spectroscopy experiments. We find that
conduction proceeds via off-resonant coherent tunneling mediated by a large
number of protein valence-band orbitals that are strongly delocalized over heme
and protein residues, effectively "gating" the current between the two
electrodes. This picture is profoundly different from the dominant electron
hopping mechanism supported by the same protein in aqueous solution. Our
results imply that current output in MHC junctions could be even further
increased in the resonant regime, e.g. by application of a gate voltage, making
these proteins extremely interesting for next-generation bionanoelectronic
devices.
