Chromophore reorientations in the early photolysis intermediates of bacteriorhodopsin

The photoselection-induced time-resolved linear dichroism of a bacteriorhodopsin suspension of purple membrane from 350 to 750 nm is measured by a new pseudo-null measurement technique. In combination with time-resolved absorption measurements, these linear dichroism measurements are used to determine the reorientation of the retinal chromophore of bacteriorhodopsin from 50 ns to 50 mu s after photolysis. This time range covers the times when the K photointermediate decays to form L, as well as the early times during the formation of the M intermediate in the photocycle. An analysis of the photoselection-induced linear dichroism measured directly, along with the absorbance changes; polarized parallel to the linearly polarized excitation, shows that the anisotropy is invariant over this time period, implying that the photolyzed chromophore rotates less than 8 degrees C with respect to unphotolyzed chromophores during this part of the photocycle.