Actinic light density dependence of the bacteriorhodopsin photocycle

The photocycle of bacteriorhodopsin (BR) was studied in the 0.3 mu s to 10 s time interval after excitation, using a wide range of actinic light intensities (10 ns half-duration, 0.06-60 mJ/cm super(2)), at neutral and alkaline pH values. The relative weights of the rapidly and the slowly decaying components of the M intermediate (M sub(f) and M sub(s), respectively) and the yield of the third millisecond component, N(R,P), are the function of the exciting light intensity (density), while their lifetimes are not. The relative weight of M sub(s) is found to be a linear function of the portion of the BR molecules undergoing the photocycle. This suggests the existence of a cooperative interaction of the BR molecules arranged in the crystalline purple membrane sheets. Another source of M sub(s) is also found, which results a nonvanishing relative weight of M sub(s) even at very weak actinic light density values. The explanation for this may be a branching, or the heterogeneity of BR itself or with its environment. It is shown that the relative weights of the rising and decaying components of the M form(s) do not correlate directly with each other.