On the action spectrum of the photoelectric transients of bacteriorhodopsin in solid-state films

Abstract

We have measured the time-resolved charge displacement currents of bacteriorhodopsin (BR) from Halobacterium halobium in multilayer arrays of well-oriented purple membranes in photoelectric cells with the sandwich electrode configuration: SnO 2 |BR|Ag. The purple membrane cells were irradiated by dye laser pulses entering the cells through the light-transmitting SnO 2 electrode. The photochemical cycle of BR was found to be reversible in these solid films, and we observed a major decay component for the M intermediate with a decay time of 46 ± 3 ms as measured by the transient optical absorption at 412 nm. We also observed a minor component (≤5% of the total amplitude) with a decay time of 0.97 ± 0.15 s. We measured the early action spectrum for the instrument limited amplitudes of the photoelectric behaviour: this action spectrum differed from the absorption spectrum of the absorbing BR pigment. However, the action spectrum measured 500 μs after flash excitation was in agreement with the absorption spectrum of BR. We attribute the nature of the early action spectrum to the presence of red and blue absorbing species which are not capable of pumping protons across the purple membrane bilayer. We also measured the charge displacement currents when the purple membrane cells were perturbed by small applied voltages with a specific polarity: for conduction of negative charges in the same direction as vectorial proton transport, enhanced transient photocurrents resulted. We interpret these enhanced photocurrents in terms of an enhancement of observed charge separation resulting from a reduced back reaction from the K intermediate of the photocycle.