REORIENTATIONS IN THE BACTERIORHODOPSIN PHOTOCYCLE ARE PH DEPENDENT

Chromophore reorientations during the bacteriorhodopsin photocycle in the purple membrane of Halobacterium salinarium have been detected by time-resolved linear dichroism measurements of the optical anisotropy over the pH range from 4 to 10 and at ionic strengths from 10 mM to 1 M. The results show that reorientations in the L and M states of bacte riorhodopsin are pH dependent, reaching their largest amplitude when t he membrane is at pH 6-8. Reorientations on the millisecond time scale of unexcited spectator proteins in the native purple membrane also de pend on pH, consistent with the suggestion that spectator reorientatio ns are triggered by reorientation of the photoexcited protein. The res ults imply that a group with a pK(a) of 5 to 6 enables reorientations, and that the deprotonation of a site at pH values above 9 restricts r eorientational motion. This suggests that reorientations in M may be c orrelated with proton release.