Conformational motion in bacteriorhodopsin: the K to L transition

By comparison of the time dependence of linear dichroism and transient absorption in light-adapted bacteriorhodopsin over the first 10 microseconds following excitation, conformational motion in the protein has been detected. Time-resolved linear dichroism and transient absorption scans are reported for several wavelengths that probe the K610 and L550 intermediates in the bacteriorhodopsin photocycle. The transient absorption scans are insensitive to conformational motion and yield the lifetimes of the K610 and L550 intermediates. In contrast, the time-resolved linear dichroism scans demonstrate orientational motion of the chromophore with a 1.7-microsecond rotational time. The wavelength dependence of the least-squares fitting parameters establishes that this motion is associated with L550. This motion is discussed in relation to a protein conformational change in the course of the bacteriorhodopsin photocycle. No evidence is observed for orientational motion on the time scale of the L550----M410 transition.