Quantum efficiency of native and mutant bacteriorhodopsin obtained from blue light induced relaxation experiments.

Bacteriorhodopsin was continuously excited with green background light. In this way a steady state distribution of all intermediates of the photocycle was obtained. Then a perturbation of the system was induced by a blue laser flash and the resulting absorption changes were measured. The experiments were done with native bacteriorhodopsin and with the point mutant BR, in which aspartate 96 is changed to asparagine. Blue light induced relaxation experiments revealed a rate constant belonging to the excitation of bacteriorhodopsin by the green background. With this rate constant the quantum efficiency of native bacteriorhodopsin and of BR was determined to be 0.60 ± 0.10. Signals obtained with native bacteriorhodopsin could be explained with a simple model of the photocycle consisting of three consecutive intermediates BR, L and M. To describe the behavior of BR, a further photoactive intermediate after the M state had to be postulated. Properties of this intermediate are similar to those of the N state. [ABSTRACT FROM AUTHOR]