Protein Binding and Orientation Matter: Bias-Induced Conductance Switching in a Mutated Azurin Junction.

Jerry A Fereiro,Israel Pecht,Mordechai Sheves,David Cahen,Tatyana Bendikov

doi:10.1021/jacs.0c08836

Abstract

We observe reversible, bias-induced switching of conductance via a blue copper protein azurin mutant, N42C Az, with a nearly 10-fold increase at |V| > 0.8 V than at lower bias. No such switching is found for wild-type azurin, WT Az, up to |1.2 V|, beyond which irreversible changes occur. The N42C Az mutant will, when positioned between electrodes in a solid-state Au–protein–Au junction, have an orientation opposite that of WT Az with respect to the electrodes. Current(s) via both proteins are temperature-independent, consistent with quantum mechanical tunneling as dominant transport mechanism. No noticeable difference is resolved between the two proteins in conductance and inelastic electron tunneling spectra at <|0.5 V| bias voltages. Switching behavior persists from 15 K up to room temperature. The conductance peak is consistent with the system switching in and out of resonance with the changing bias. With further input from UV photoemission measurements on Au–protein systems, these striking differences in conductance are rationalized by having the location of the Cu(II) coordination sphere in the N42C Az mutant, proximal to the (larger) substrate-electrode, to which the protein is chemically bound, while for the WT Az that coordination sphere is closest to the other Au electrode, with which only physical contact is made. Our results establish the key roles that a protein’s orientation and binding nature to the electrodes play in determining the electron transport tunnel barrier.

Highlights

Proteins integrated into nanoscale devices as charge transport material may provide a route to future bioelectronic applications.[1,2] The functions of such applications would rest fundamentally on charge transport via the proteins and across the interface between them and the electrodes.[3]
We have recently explored the possibility of using monolayers of redox proteins in an essentially dry state, to achieve transistor action[6] with azurin, or conductance switching[7] with a cytochrome c mutant
The observed temperature-independent electron transport (ETp) is similar to that observed for WT Az,[20,35] a behavior that we found recently to persist down to 4 K.34

Summary

Introduction

Proteins integrated into nanoscale devices as charge transport material may provide a route to future bioelectronic applications.[1,2] The functions of such applications would rest fundamentally on charge transport via the proteins and across the interface between them and the electrodes.[3]. We have recently explored the possibility of using monolayers of redox proteins in an essentially dry state, to achieve transistor action[6] with azurin, or conductance switching[7] with a cytochrome c mutant. We succeeded in observing switching with an azurin mutant, a step toward multifunctional protein electronics. Azurin (Az) is an electron transfer copper protein involved in the energy conversion system of the bacterium Pseudomonas aeruginosa.[8,9] The copper ion[10] is bound at one (“north”) end of the barrel-shaped protein, coordinated to three equatorial ligands (N of His[46] and His[117] and S of Cys112) and two weaker bonded axial ligands (S of Met[121] and the peptide backbone oxygen of Gly45), resulting in a distorted trigonal bipyramidal geometry[9,11] (cf Figure 1A). Az structure and function were found to be maintained upon adsorption on surfaces in an essentially dry state.[12] This, along

Results

Discussion

Conclusion