Speaker
Description
Motivated by their efficient charge-transfer capabilities over long-range distances, redox-active proteins have been recently incorporated into various nanobioelectronic devices in view of interesting applications like accurate biocompatible sensors or multi-state biomemristors. However, rather non-expected physical phenomena were observed when the proteins were connected to metal contacts. While in a native aqueous environment, the electron flow through the system of redox sites proceeds by the thermally activated hopping mechanism, the temperature-independent currents of relatively high magnitudes were detected on protein/metal junctions.1,2 These data suggest that the electrons on the bio/metallic interfaces and junctions are transferred by the coherent tunneling mechanism, independently of the redox-active states. Yet, the current magnitudes do not decay exponentially with the protein-junction width, as one would expect in the case of tunneling. Moreover, it is unclear how stable conduction channels could be formed in flexible organic macromolecules. The mechanism details are thus unknown and poorly understood.
Here, we investigate these electron-transport phenomena by means of atomistic computer simulations. We use classical molecular dynamics (MD) techniques based on polarizable GolP-CHARMM force field to explore protein/metal interactions3-5 and determine the protein junction geometries. On those, we apply large-scale density function theory (DFT) to compute the current-voltage (I-V) curves. We employ the DFT+$\Sigma$ scheme for establishing the correct electronic-state alignment on the interfaces, while the electronic coupling elements between the protein and metallic states are obtained by the projector-operator-based diabatization method (POD). As a demonstration of this state-of-the-art procedure1, we have studied the conductivity of small tetraheme cytochrome (STC) and Azurin proteins between gold electrodes. We confirmed that the transport mechanism through both STC and Azurin junctions proceeds by the coherent tunneling facilitated by conduction-band states of the proteins. In contrast to their redox properties in solution, the presence of the metal cations in the protein structures is not essential for their conductivity on the metal interfaces. The reason for this drastically different behavior in solution and on the metal interfaces is the significant electronic-level misalignment between the protein and metallic states.6
References:
1. Garg, K. et al.: Direct Evidence for Heme-Assisted Solid-State Electronic Conduction in Multi-Heme c-Type Cytochromes. Chem. Sci. 9, 7304 (2018).
2. Futera, Z. et al.: Coherent Electron Transport across a 3 nm Bioelectronic Junction Made of Multi-Heme Proteins. J. Phys. Chem. Lett. 11, 9766 (2020).
3. Biriukov, D. and Futera, Z.: Adsorption of Amino Acids at the Gold/Aqueous Interface: Effect of an External Electric Field. J. Phys. Chem. C, 125, 7856 (2021).
4. Futera, Z.: Amino-Acid Interactions with the Au(111) Surface: Adsorption, Band Alignment, and Interfacial Electronic Coupling. Phys. Chem. Chem. Phys. 23, 10257 (2021).
5. Kontkanen, O. V., Biriukov, D., Futera, Z.: Reorganization Free Energy of Copper Proteins in Solution, in Vacuum, and on Metal Surfaces. J. Chem. Phys. 156, 175101 (2022).
6. Futera, Z., Wu, X., Blumberger, J.: On the Crossover from Tunneling to Hopping Conduction in Multiheme Cytochrome Bioelectronic Junctions. Small, submitted.
