Actinic light density dependence of the O intermediate of the photocycle of bacteriorhodopsin

The O intermediate of the photocycle of bacteriorhodopsin (BR) was studied by absorption kinetic measurements at different actinic light densities. With increasing exciting flash intensity, the relative yield of O slightly increases, while that of M f strongly decreases at the expense of M s. Kinetic calculations and the optical anisotropy of O show that O can be formed only from M f although M f and O have different light intensity dependences. In order to resolve the apparent contradiction, a phenomenologically new cooperative regulatory mechanism seems to be necessary.