Picosecond fluorescence spectroscopy of the purple membrane of Halobacterium halobium in alkaline suspension

The fluorescence lifetime of the light-adapted purple membrane of Halobacterium halobium in alkaline suspension was measured with a femtosecond-pulse laser-synchroscan streak camera system (< 10 ps resolution time, 620 nm excitation) at 22°C. The lifetimes of slow, fast and very fast decay components were found to be 62 ± 2, 9 ± 2 and < 1 ps, respectively. The fastest component is attributed to bacteriorhodopsin (bR568). We examined the effect of pH and excitation power on the amplitude of the slow component and concluded that its origin is attributable to the Q intermediate, which is a photoproduct of the N intermediate formed in the later stage of the photocycle of bR568. We found that Q is the same species as pseudo-bacteriorhodopsin (p-bR).