  Recently, a growing interest has been addressed to the electrical properties
of bacteriorhodopsin (bR), a protein belonging to the transmembrane protein
family. 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.
