Decay of the tryptophan fluorescence anisotropy in bacteriorhodopsin and its modified forms

In this work we study the decay of the polarization of the Trp fluorescence in native bacteriorhodopsin (bR), deionized bR (dlbR), and the retinal-free form of bR, bacterioopsin (bO), using picosecond laser/streak camera system. Two types of depolarization processes are observed, one around 250 ps, which is temperature independent around room temperature, and the other in the 1-3-ns range, which is sensitive to temperature and certain bR modifications. This suggests the presence of at least two different environments for the eight Trp molecules in bR. Native bR and deionized bR gave the same depolarization decay times, suggesting that the removal of metal cations does not change the microenvironment of the emitting Trp molecules. The slow component is faster in bO than in bR, suggesting a change in the environment of the Trp molecules upon the removal of the retinal chromophore. All these results are discussed in terms of the different mechanisms of Trp fluorescence depolarization. A comparison between the depolarization decay in rhodopsin and bR is made.