Resonance raman and optical transient studies on the light-induced proton pump of bacteriorhodopsin reveal parallel photocycles.

The photocycle of bacteriorhodopsin (bR) was studied at ambient temperature in aqueous suspensions of purple membranes using time-resolved resonance Raman (RR) and optical transient spectroscopy (OTS). The samples were photolyzed, and the fractional concentrations of the retinylidene chromophore in its parent state, BR 570 , and in the intermediate states L 550 , M 412 , N 560 and O 640 were determined in the time domain 20 μs-1 s and in the pH range 4-10.5. Two kinetically different L components could be identified. At pH 7 one fraction of L (∼65%) decays in 80 μs to M (deprotonation of the Schiff base), whereas the residual part is converted in ∼0.5 ms to N