  New science and new technology need new materials and new concepts. In this
respect, biological matter can play a primary role because it is a material
with interesting and innovative features that have found several applications
in technology, from highly sensitive sensors for medical treatments to devices
for energy harvesting. Furthermore, most of its phenomenology remains unclear
thus giving new hints for speculative investigations. In this letter, we
explore the possibility to use a well-known photosensitive protein, the
Reaction Center of Rhodobacter Sphaeroides, to build up an electrical pH
sensor, i.e., a device able to change its resistance depending on the pH of the
solution in which it crystalizes. By using a microscopic model successfully
tested on analogue proteins, we investigate the electrical response of the
Reaction Center single protein under different conditions of applied bias,
showing the feasibility of the bio-rheostat hypothesis. As a matter of facts,
the calculated resistance of this protein grows of about 100% when going from a
pH = 10 to a pH = 6.5. Moreover, calculations of the current voltage
characteristics well agree with available experiments performed with a current
atomic force microscopy under neutral conditions. All findings are in
qualitative agreement with the known role of pH in biochemical activities of
Reaction Center and similar proteins, therefore supporting a proof-of-concept
for the development of new electron devices based on biomaterials.
