Chemical Society: Dr. David Cahen - Biomolecular Electronics: Electron Transport across Peptides and Proteins

Electron transport (ETp), i.e., electronic conduction, across peptides and proteins in a solid state–like configuration is remarkably efficient, comparable to that via completely conjugated molecules. From work with modified proteins, and with homopeptides we find that both cofactors and the secondary structure matter for the efficiency of the ETp.

While ET and ETp are related, nature regulates ET via redox chemistry, where control over the process is achieved at a price in free energy, for ETp no redox process is required. This allows studying ETp via non-redox proteins, such as the rhodopsins and albumins.
For most proteins (studied as monolayers) transport is thermally activated at > ~ 150 K, but several proteins show temperature-independent transport from 15K till denaturation, including transport distances over 6 nm! The temperature-independent current is the highest that such protein or its variants can carry, as shown for Azurin, which exhibits clear Meyer-Neldel (compensation) behavior, with the compensation temperature at denaturation.[1] As temperature-independence above the glass transition (~ 150K) is unlikely to be due to ballistic transport, tunneling is the probable mechanism. However, the observed longer ETp distances are beyond those, known even for completely conjugated organic molecules that allow efficient tunneling. This suggests a resonant process, which raises the question of how a floppy system (protein, peptide) can resonate at RT. This, in turn fuels speculations on a role for coherence in electron transport (as distinguished from energy transfer), and (dis)proving this provides a challenge for experimentalists.

1 N. Amdursky et al., Adv. Mater. 42,7142-7161 (2014) Electronic Transport via Proteins 10.1002/adma.201402304 (progress report);

N. Amdursky et al., Adv. Sci., 2015, 2, 1400026 Electron transfer proteins as electronic conductors: Significance of the metal and its binding site in the blue Cu protein, Azurin,  doi.org/10.1002/advs.201400026