Cooperativity-induced optical anisotropy changes during the photocycle of bacteriorhodopsin.

Previous photoselection measurements showed that if the excitation is weak no optical anisotropy changes appear in immobilized purple membranes during the photocycle. The present study demonstrates that surprisingly at stronger excitations the anisotropy changes versus time. At 412 nm the dichroic ratio decreases after a few milliseconds, while at 570 nm the similar decrease is followed by an increase. The phenomenon cannot be described by tiltings of the retinal chromophore. It is the consequence of the cooperative interaction among the photocycling bacteriorhodopsin molecules that regulates the yields of more than one (expectedly two main) parallel pathways existing in the millisecond time domain of the photocycle.