Solid-State Electron Transport Through Proteins Beyond Known Tunneling or Hopping Limits

A central puzzle of bio-molecular electronics is how electron transport (ETp) through more than several nm thick solid-state junctions, can be so efficient, often without thermal activation. Such behavior does not fit known transport mechanisms. As proteins lack extended conjugation and thus are low charge carrier density molecules (with low DOS^#), the experimental ETp results are, well, weird. Starting from a wide range of ETp results within the QM tunnel limit (~2, ~5 or ~7 nm for saturated, conjugated, or multiheme molecules), currents should decrease exponentially with junction width. This decrease should limit detection for non-conjugated polymers, such as proteins, beyond the above (junction) widths. But we measure conduction across up to 60 nm wide protein films, with weak width, and near-complete lack of temperature independence. While the results can be modeled, they exclude known transport mechanisms, adding to ETp results via bacterial nanowires (mm-s) and cable bacteria ( £1 cm). We can <i>rationalize</i> the data, assuming ETp is limited by injection (via tunnelling) through one of the contacts, followed by much more efficient charge propagation across the proteins, the <i>mechanism of which is as yet unknown</i>. Discussion will also include, as unintended consequence, insight in biological electron transfer.<br/><br/>* <i>Weizmann Institute of Science</i>, Rehovot, Israel. Work with Mordechai <i>Sheves</i>, Israel <i>Pecht</i> ++++<br/>^# density of states