Recent Advances in Understanding the Electron Transport Through Metal-Azurin-Metal Junctions

Abstract

Azurin proteins are the workhorse of protein electronics. This is a branch of biomolecular electronics, a recent research field which investigates electronics based on biomolecules such as proteins, peptides, amino acids, bacterial nanowires or DNA. In general, the possibility of including biosystems in solid-state junctions has opened the way to the development of novel electrical devices, and proteins have attracted enormous attention thanks to their many interesting properties. In the particular case of metal-azurin-metal junctions, experimental measurements have revealed extremely efficient electron transport over large distances, showing conductance values which are higher than certain conjugated molecules of similar lengths. Moreover, the electrical current has often been found to be temperature-independent, which has been used as an evidence of coherent transport or quantum tunneling. Interesting effects have been observed, moreover, upon insertion of single amino-acid mutations. In spite of a huge amount of work, the exact mechanism for the charge flow through these systems is still under debate. In this review, we will revise the recent advances made in the electron-transport measurements of azurin-based junctions as well as the corresponding theoretical modelling. We will discuss the interpretation of the currently-available experimental results as well as the open issues which still remain to be clarified.

Single-molecular protein-based bioelectronicsviaelectronic transport: fundamentals, devices and applications

“…26 For instance, regarding the electron transport mechanism through azurin-based junctions, there is still a debate over the mechanism of electron transport through azurin-based junctions, including the types of transport mechanism (coherent vs. incoherent) and the role of the metal ion that takes place in the system. 23,121 Experimental studies have proposed either a 2-step tunneling or a fully-coherent transport mechanism, while theoretical studies have ruled out the possibility of fully-coherent transport and suggested the need to consider other types of transport. Theoretical investigation such as DFT calculation has been used to shed light on the interactions between the charge transport properties and the internal structural changes in protein molecules.…”

“…21,22 On a more fundamental level, previous studies have demonstrated that the electronic charge transport through a peptide matrix is essential for proteins to perform their functions in many biological processes such as respiration, photosynthesis, enzymatic processes, and cellular signaling, although the exact mechanism is still under debate. 16,[23][24][25][26] This has led to a myriad of theoretical and experimental studies of the electronic properties of individual protein molecules and their assimilation as active components in electronic circuits, sparking interest in the past decades and giving rise to a new field of protein-based bioelectronics, also referred to as ''proteotronics''. 3,[15][16][17][18][19]26,27 In general, protein-based bioelectronic devices rely on capturing single proteins or their monolayer onto different configurations of electrodes and exploring the electronic conductivity by measuring the current response of devices as a function of bias voltage.…”

Understanding molecular and electrochemical charge transfer: theory and computations

“…These first reported studies of Az as a primary single-molecule metalloprotein probe have, however, still left challenges, taken up by other groups. 60 Open questions are, for example:…”

“…• High current densities have inspired rationales associated with: (i) “Coherent single-step” vs. “incoherent two-step” ET as above; 60 (ii) molecular probe orientation as for cyt b 562 , Fig. 16; 4,45 (iii) protein conformational dynamics and ET “gating”, for example as for the two-centre metalloproteins, cyt c 4 and copper nitrite reductase.…”

“…“abnormally” long-range ET, high conductivity of both redox and non-redox protein complexes, or temperature dependence and environmental phase transitions, overviews e.g. 48,60…”