Nanosecond photolytic interruption of bacteriorhodopsin photocycle: K-590 --> BR-570 reaction.

The molecular processes comprising the room temperature bacteriorhodopsin (BR) photocycle are examined through the properties of the photo-induced reverse reaction, K-590 + hnu --> BR-570 (K --> BR). Two sequential pumping pulses, each of 10-ns duration, are used, respectively, to initiate the photocycle via the forward BR-570 + hnu --> K-590 (BR --> K) reaction (532 nm) and to photolytically interrupt the thermal BR photocycle after a 20-ns delay via K --> BR (620-700 nm). The ground-state BR-570 population, monitored by 633-nm absorption 200 mus after the photocycle begins, provides a quantitative measure of the efficiency with which K --> BR interrupts the photocycle to reform BR-570. The quantum yield (Phi) for K --> BR is found to be 1.6 +/- 0.1 times larger than that for BR --> K which, when compared to a Phi of 0.64 for BR --> K, suggests that Phi for K --> BR is approximately 1.0. The significance of such a high efficiency K --> BR reaction with respect to mechanistic descriptions of the BR photocycle is discussed.