(Invited) Long Range Coherent Transport in Proteins via Electron Pairing

Abstract

Recently, Lindsay et al. showed in a series of papers (summarized in Lindsay, S. Life 2020, 10, 72.) that single proteins coupled to metal electrodes have unexpectedly good conductance properties. It became clear that much of the resistance drop happens at the metallic contacts, and the internal conductivity of the protein itself can be very close to the quantum limit 2e^2/h for short proteins. The localization length can be as large as ten nanometers for longer ones. Cahen et al. showed that the conductance is temperature independent in the range of 30-300 Kelvins in a series of measurements. All this indicates that proteins support fast and at least partially coherent electron transport at room temperature. According to electronic structure calculations, proteins have significant HOMO-LUMO gaps surrounded by localized orbitals, a typical arrangement for insulators ruling out efficient single-electron quantum transport even in the presence of photon mediation. However, due to the repetitive nature of the basic peptide backbone of the proteins, there are states deeper below the HOMO energy, which are intermediary between extended states typical in metallic conductance bands and the localized ones. These so-called critical states (Vattay G. et al. 2015 J. Phys.: Conf. Ser. 626 012023) have fractal density distributions and show large electron density correlations between other orbitals in the same energy region. It has been shown that in the presence of electron-electron attraction, these states are especially suited for pair formation of electrons, and an insulator-superconductor transition can occur even at high temperatures. Here we argue that polarizable sidechains of amino acids can create an attractive force between electrons propagating along the backbone. We show that the typical size and polarization of amino acids make the proper magnitude of pairing energy to maintain superconducting patches even at room temperature. We calculate and display the geometry of the superconducting gap over the protein structure and propose this new mechanism as a possible explanation for the observed coherent transport.