TIME-RESOLVED POLARIZATION STUDY OF ANISOTROPY IN BACTERIORHODOPSIN

Time-resolved polarization spectroscopy is sensitive to the orientational dynamics of chromophores, and as a result it can be applied to study internal motion in restrictive environments. This paper describes the application of polarization spectroscopy to the photoactive protein bacteriorhodopsin on time scales from picoseconds to hundreds of microseconds. Anisotropy persists in both the ground-state bacteriorhodopsin population and in the photocycle intermediates due to a population bottleneck in the bacteriorhodopsin photocycle. The time dependence of the polarization signal expected for a sequence of intermediates is described and is shown to be sensitive both to population kinetics and to internal motion. The observed time dependence of the polarization signal reveals internal motion in the purple membrane fragments on the time scale of the K → L transition ( ∼ 1 μs). The results are consistent with the known kinetics of the bacteriorhodopsin photocycle. Evidence is also presented and discussed for a decay component on a time scale of <50 ps. © 1990 American Chemical Society.