Reaction Center of Rhodobacter Sphaeroides, a Photoactive Protein for pH Sensing: A Theoretical Investigation of Charge Transport Properties

In the perspective of an increasing attention to ecological aspects of science and technology, it is of interest to design devices based on architectures of modular, low cost, and low-pollutant elements, each of them able to perform simple duties. Elemental devices may be themselves green as, for example, proteins able to make simple actions, like sensing. To this aim, photosensitive proteins are often considered because of the possibility of transferring their specific reaction to visible light into electronic signals. Here, we investigate the expected electrical response of the photoactive protein Reaction Center (bRC) of Rhodobacter Sphaeroides within the proteotronics, a recent branch of molecular electronics that evaluates the electrical properties of a protein by using an impedance network protein analog based on the protein tertiary structure and the degree of electrical connectivity between neighboring amino acids. To this purpose, the linear and nonlinear regimes of the electrical response to an applied bias are studied when the protein is in its native state or in an active state. In the linear response regime, results evidence a significant difference in the electrical properties of bRC when the pH value of the solution in which the protein is embedded changes from acid to basic. In the non-linear response regime, the current-voltage characteristics experimentally reported in the recent literature are interpreted in terms of a sequential tunneling mechanism of charge transfer. The qualitative agreement of present findings with available experiments strongly suggests the use of this protein as a bio-rheostat or a pH sensor.