ACTINIC LIGHT DENSITY-DEPENDENCE OF THE BACTERIORHODOPSIN PHOTOCYCLE

The photocycle of bacteriorhodopsin (BR) was studied in the 0.3 mus to 10 s time interval after excitation, using a wide range of actinic li ght intensities (1 0 ns half-duration, 0.06-60 mJ/cm2), at neutral and alkaline pH values. The relative weights of the rapidly and the slowl y decaying components of the M intermediate (M(f) and M(s), respective ly) and the yield of the third millisecond component, N(RP), are the f unction of the exciting light intensity (density), while their lifetim es are not. The relative weight of M(s) is found to be a linear functi on of the portion of the BR molecules undergoing the photocycle. This suggests the existence of a cooperative interaction of the BR molecule s arranged in the crystalline purple membrane sheets. Another source o f M(s) is also found, which results a nonvanishing relative weight of M(s) even at very weak actinic light density values. The explanation f or 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 a nd decaying components of the M form(s) do not correlate directly with each other.