Experiment and Hope—Evidence for Quantum Solid-State Electronic Conduction Across Proteins

Electron Transport (ETp), <i>electronic</i> conduction, across ultra-thin protein films as active layer in a solid-state junction, can be surprisingly efficient. Length-normalized ETp efficiency can be similar to, or even exceed that of conjugated molecules; moreover, it can be temperature-independent down to at least 4K. If intra-protein transport dominates (contacts are not limiting; a challenge as proteins are poly-electrolytes), no significant transport barrier is measured, challenging the simplest explanation for this transport, hopping conduction.<br/>Such results have, nowadays, the banner “quantum” all over them, as they seem consistent with quantum tunneling; one of the problems with such an explanation. While recently the first golden standard evidence, with alkyl-thiol molecule films, for such, interference, was reported, it is still lacking for proteins. In truth, coherent transport through proteins, non-periodic systems that are dynamic, even at low temperatures, is not at all obvious. Still there are some results that are<br/>surprising in this context:<br/>- the above-noted temperature independence, which we have now found for up to ~ 20 nm (multilayers of bacteriorhodopsin, bR)<br/>- the inability to fit, small, short proteins (Azurin, Az) data to any reasonable hopping model<br/>- the possibility to bring short proteins (Az, Cytochrome C) in and out of resonance by applied bias<br/>- clear inelastic electron tunneling spectra for Az (and their absence for larger proteins).<br/>Unraveling the transport mechanism is, apart from scientific curiosity, of interest, as understanding ETp may have relevance for ET (replace coupling to contacts by electron injection / extraction). I will discuss our recent experimental data and their analyses, and try to put them in perspective also with reports from other groups. The aim is to separate expectation and anticipation from experimental observables, to define the puzzle that we try to solve3, even if it starts to resemble the ‘duck test”.<br/><br/>* work done with <b>Mordechai Sheves and Israel Pecht</b>, students & PD fellows, all @theWeizmann Institute; further collaborations are with A Vilan, J Blumberger, G Vattay, E Papp, C Cuevas, L Zotti; M Tornow, +++<br/>References: 1 C. Bostick et al. Rep. Prog. Phys (2018); 2 N. Amdursky et al., Adv. Mater. (2014); 3 D.Cahen, I.Pecht, M. Sheves, J. Phys.Chem. Lett. (2021)