INFLUENCE OF EXCITATION-ENERGY ON THE BACTERIORHODOPSIN PHOTOCYCLE

Kinetic curves for the bacteriorhodopsin (BR) photocycle were obtained both at 570 and at 412 nm at a series of increasing levels of intensi ty of the exciting laser. Singular value decomposition (SVD) of these curves showed two transitions in the kinetic profiles that occurred at specific levels of actinic light. This means that the photocycle was influenced by photon density in two ways. In a separate application of SVD, time-resolved optical spectra were analyzed at each of many leve ls of exciting laser intensities. The studies showed that the transiti on at the low level of laser intensity was due principally to an incre ase in the amount of BR that was turning over. The transition at the h igher level of laser intensity showed a fundamental change in kinetics of the photocycle. At low intensity levels, the fast form of M (M(f)) predominated, whereas at high levels the slow form of M (M(s)) predom inated. A distinction was found between M(f) and M(s), in that the for mer decayed directly to the O intermediate whereas the latter decayed.