A COMPARISON OF THE PHOTOELECTRIC CURRENT RESPONSES RESULTING FROM THE PROTON-PUMPING PROCESS OF BACTERIORHODOPSIN UNDER PULSED AND CW LASER EXCITATIONS

When excited with a pulsed laser, an electric field-oriented bacterior hodopsin (bR) film on an indium-tin oxide (ITO) conductive electrode g enerates a photocurrent composed of at least three different component s: B1 (<100 ps), B2 (similar to 60 mu s), and B3 (similar to ms). When excited with an electronic shutter modulated CW light pulse (>200 ms in duration), a differential photocurrent (components D1 and D2 with d ecay times in milliseconds) is observed from the bR film. D1 is observ ed when the CW light is turned on, and D2 is observed when the CW ligh t is turned off. In this paper, we compare the amplitudes and lifetime s of B2, B3, and D1 at various values of pH and ionic strength of the electrolyte solution in which the photocurrent is measured. It is foun d that changing the film orientation changes the polarity (sign) of B1 and B2, while it does not affect the polarity of B3 and D1. It is als o found that B3 and D1 change their polarity upon changing the pH of e lectrolyte solution, whereas B1 and B2 do not. These results suggest t hat the origin of B3 and D1 is different from that of B1 and B2. Our r esults suggest that B3 and D1 are due to the formation of a transient proton capacitor between the two ITO electrodes resulting from the pro ton pumping in bR. The magnitude and sign of B3 and D1 are determined by the transient proton concentration change (accumulation or disappea rance) occurring near the bR-modified ITO electrode interface on the m illisecond time scale. The change of sign in B3 and D1 as a function o f pH is due to the sequence of proton release/uptake in the bR photocy cle: It first releases protons into the aqueous solution at high pH, w hile it first takes up protons from the aqueous solution at low pH. Th e effects of buffer and ionic strength on B3 and D1 are discussed in t erms of the kinetics of proton release/uptake and of the transportatio n of positive and negative ions in the electrolyte solution.