Understanding Electron Transport in Proteins—Insights from Poly-Ionomer Studies

There are now multiple reports of puzzling efficient electron transport, ETp, across (layers of) various proteins in different experimental setups, with weak or no temperature dependence and weak length, and voltage dependence over ”too” large distances. The absence of protein energy levels within kT of the electrode Fermi level makes understanding such ETp challenging. Within a broader view, proteins are poly-ionomers, containing a high fraction (tens of percent) of ionic, i.e., charged repeating units. Thus, a clear control experiment is to measure ETp of poly-ionomers. Our current-voltage-temperature and impedance measurements on multi-layers of poly-ionomers indicate ETp across up to ~90 nm. i.e., like with protein films, ETp over widths well beyond accepted tunneling limits. In another sanity check, a junction with ~30 nm <i>uncharged</i> polymer, PMMA, behaves as expected, as an insulator. Thus, we should understand poly-ionomer ETp, at least as a point of reference, to identify protein-specific ETp mechanisms. Possibly, the repeating charge distribution in poly-ionomers may form a channel that enables efficient transport guided by internal electric fields. Elucidating the transport model is particularly important, as it may shed light on ETp through cable bacteria.<br/><br/>* work done with Mordechai Sheves, Israel Pecht, Sudipta Bera, all from the Weizmann Inst. of Science