On the mechanisms responsible of photocurrent in bacteriorhodopsin (Bacteriorhodopsin's sense of light)

Recently, a growing interest has been addressed to the electrical properties of bacteriorhodopsin (bR), a protein belonging to the transmembrane protein family. Several experiments pointed out the role of green light in enhancing the current flow in nanolayers of bR, thus confirming potential applications of this protein in the field of optoelectronics . By contrast, the mechanisms underlying the charge transfer and the associated photocurrent are still far from being understood at a microscopic level. To take into account the structure-dependent nature of the current, in a previous set of papers we suggested a mechanism of sequential tunneling among neighbouring amino acids. As a matter of fact, it is well accepted that, when irradiated with green light, bR undergoes a conformational change at a molecular level. Thus, the role played by the protein tertiary-structure in modeling the charge transfer cannot be neglected. The aim of this paper is to go beyond previous models, in the framework of a new branch of electronics, we called proteotronics, which exploits the ability to use proteins as reliable, well understood materials, for the development of novel bioelectronic devices. In particular, the present approach assumes that the conformational change is not the unique transformation that the protein undergoes when irradiated by light. Instead, the light can also promote a free-energy increase of the protein state that, in turn, should modify its internal degree of connectivity, here described by the change in the value of an interaction radius associated with the physical interactions among amino acids. The implemented model enables us to achieve a better agreement between theory and experiments in the region of a low applied bias by preserving the level of agreement at high values of applied bias. Furthermore, results provide new insights on the mechanisms responsible for bR photoresponse.