Proteorhodopsin characterization based on metal–insulator–metal structure technique

We investigated the light-induced conductivity changes of dry green light absorbing proteorhodopsin (GPR) using a metal–insulator–metal system (MIM). The steady-state and time-dependent current–voltage (I–V) characteristics were measured as a function of GPR concentration for different illumination conditions. As the photoconductivity of GPR in the dry film was varied due to the incident light intensity and the wavelength, we could directly observe the photoconductivity changes inside the GPR by measuring the I–V of the MIM system. In addition to these steady-state measurements, applied bias-dependent properties were studied. The optical absorption spectrum of the GPR film between 450 nm and 650 nm correlates with the action spectrum of the MIM system. The GPR maintained higher photoactivity under green light illumination, which is correlated with the formation of an M-like intermediate.