  Electron transfer (ET) through proteins, a fundamental element of many
biochemical reactions, has been studied intensively in solution. We report the
results of electron transport (ETp) measurements across proteins, sandwiched
between two solid electrodes with a long-range goal of understanding in how far
protein properties are expressed (and can be utilized) in such a configuration.
While most such studies to date were conducted with one or just a few molecules
in the junction, we present the high yield, reproducible preparation of large
area monolayer junctions of proteins from three different families: Azurin
(Az), a blue-copper ET protein, Bacteriorhodopsin (bR), a membrane
protein-chromophore complex with a proton pumping function, and Bovine Serum
Albumin (BSA). Surprisingly, the current-voltage (I-V) measurements on such
junctions, which are highly reproducible, show relatively minor differences
between Az and bR, even though the latter lacks a known ET function. ETp across
both Az and bR is much more efficient than across BSA, but also for the latter
the currents are still high, and the decay coefficients too low to be
consistent with coherent tunneling. Rather, inelastic hopping is proposed to
dominate ETp in these junctions. Other features such as asymmetrical I-V curves
and distinct behavior of different proteins can be viewed as molecular
signatures in the solid-state conductance.
