# The Role of Metal Ions in the Electron Transport through Azurin-Based Junctions
## 1 Introduction  
The study of metal-protein-metal junctions has opened up new avenues in the field of molecular electronics as it paves the way to develop new hybrid devices capable of exploiting proteins' remarkable properties [1]. In particular, they have the inherent capability of transferring charge over long distances. Thus, one can foresee the production of sensors, flexible implants, solar cells and many other types of electrical devices incorporating proteins [2; 3; 4; 5; 6]. The significant amount of work accumulated in recent years on this topic has given rise to a new field, namely that of protein-based electronics, which has been given the name of proteotronics [7]. Research in this field has been carried out on various fronts. On the one hand, remarkable effort has been made to understand the exact nature of the charge transfer mechanism in protein-based junctions as well as to identify the components which play the dominant role in the transport process [8; 9; 10; 11]. On the other hand, the possibility of modifying the conductance properties by the insertion of mutations [3; 10; 11] has also attracted a lot of interest. Research on the transport properties of the building blocks of proteins, namely amino acids and peptides, has also been carried out [12; 13; 14]. Recent experimental results highlight the possibility of tuning the current density through azurin-based junctions via the replacement of the central copper ion with other metals [15]. These measurements were performed on proteins monolayers lying on a conducting substrate and contacted to a macroscopic electrode at the opposite side. At low temperatures, the difference in current between the Cu(II)-azurin and the Zn-azurin was found to be nearly two orders of magnitude. This was ascribed to the fact that each metal contributes to levels lying at different energies with respect to the Fermi level. Combiningorganic and metal components in the central area of molecular junctions is indeed a strategy which has been used quite often in molecular electronics [16; 17; 18]. In the case of azurins, this possibility becomes particularly intriguing since the aforementioned results of [PERSON] et al. [15] indicate that by modifying only one (metal) atom one can modify the conductance properties of a system of almost 2000 atoms. To shed light on this issue, we thus performed theoretical calculations based on density functional theory (DFT) and calculated the electron-transport properties of several metal-azurin-metal junctions based on different metallic ions. Numerous computational studies on the active site on metal-substituted azurins are reported in the literature [19; 20], but none involved the study of a whole junction comprising an entire protein held between two metal electrodes. Azurins are one of the proteins which have been studied the most in the field of protein-based electronics; additionally, they have long been the object of many computational studies because of their key role in biological functions [21]. Over the years, various experimental techniques have been adopted to measure the conductance of proteins [6; 8; 10; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. In this work, we focus on structures which are likely to be formed during the experiments based on STM (Scanning Tunneling Microscopy), which are generally designed to measure the conductance of a single protein. The exact nature of the electron transport through the blue-copper azurin has been the object of a long debate [10; 34], the degree of coherence being one of the issues at the heart of the debate. More specifically, what still needs clarifying is whether the process is fully coherent or whether it is better described by a sequential-tunneling process consisting of various steps which are individually coherent. In the case of the metal-substituted proteins studied in [15], the low-temperature results showed evidence for coherent transport; it was suggested that both tunneling and hopping are possible, however, depending on the temperature and the type of junction [35].